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The expansion of short rotation eucalypt plantations in low soil organic matter (SOM) sandy soils may offer an alternative to improve soil C sequestration. The goal of this study was to estimate the changes in C stocks and emissions in different SOM fractions following conversion of the native Cerrado to pasture and then to eucalypt plantation. Therefore, we studied soils under native Cerrado, planted pasture (cultivated for 34years following the clearing of the Cerrado) and eucalypt plantation (4years). The C and N stocks in particulate organic matter and mineral‐associated organic matter (MAOM) were determined 4years after eucalypt planting. Soil CO₂‐C, CH₄‐C fluxes and CO₂‐C concentrations in soil profile were measured in different seasons over 4years. Variation in the natural abundance of ¹³C was used to partition the SOM‐C. The soil CO₂‐C and CH₄‐C fluxes were influenced by soil surface moisture (r= 0.185ᵒ and r= 0.430**, respectively), whereas only the soil CH₄‐C fluxes correlated with soil surface temperature (r= 0.355**). The highest soil CO₂‐C flux in soil under eucalypt occurred after 4years of eucalypt planting (2.5 kg ha⁻¹h⁻¹, approximately 70%). The pasture soil acted as a CH₄‐C source to the atmosphere. The pasture MAOM‐C losses in the 0.0–1.0‐m soil layers were not compensated by the new eucalypt C inputs (MAOM‐C lost ~9.6 Mg ha⁻¹). In summary, the recent worldwide expansion of short rotation eucalypt plantations should be carefully considered, particularly under pasture degraded soil sandy soils, because land uses able to increase SOM are priorities.

Certain plant combinations can stimulate the mineralization of soil organic matter (SOM), thereby changing the storage of soil organic carbon and affecting the physical and chemical soil properties...

Accidents related to the rupture of iron ore tailings dams have dramatically impacted on the Brazilian natural ecosystem. So, the development of strategies to recover soil organic matter levels and build-up Technosols are required. This study aimed to evaluate the effect of N-mineral and -organic fertilization and the rhizospheric effect on soil organic matter mineralization from Technosols built under iron ore tailings. The experiment was carried out in a greenhouse; we used a factorial scheme 2 × 2 × 3: without and with the plant Crotalaria juncea; without and with mineral fertilization; and three different organic fertilization (without organic fertilization, sewage sludge, and household waste composting), with four randomized blocks. The experimental units were PVC columns installed with a top layer of iron ore tailing from a dam, where gases were sampled to determine the CO 2 -C, CH 4 -C concentrations, and CO 2 -δ 13 C. The organic fertilization promoted a positive priming effect of 184 %. However, with mineral fertilization, no effect was observed in soil derived-CO 2 -C accumulated and priming effect. Similarly to non-planted soil, the planted soils with mineral and organic fertilization promoted a steady reduction in soil derived-CO 2 -C accumulated and rhizosphere priming effect. The soil with mineral fertilization improved in 5.52 and 0.45 g per column the shoot and root dry matter of Crotalaria when compared to the soil without mineral fertilization. In summary, the organic fertilization in Technosols built over iron ore tailings dams during revegetation was not enough to ensure the plant development and preservation of soil organic matter. However, the combination of mineral and organic fertilization promoted negative rhizosphere priming effect and a significant increase in plant biomass.

Planting trees is one of the most effective activities in recovering soil organic carbon (SOC) stocks of degraded areas, but we still lack information on how different tree species can influence soil respiration, one of the main sources of dioxide carbon (CO2) to the atmosphere. This study aimed to explore the influence of different forest species on the autotrophic and heterotrophic components of the total soil respiration in a bauxite mining area under reclamation. We analysed the soil CO2 efflux under five treatments: i) monoculture of clonal Eucalyptus; ii) monoculture of Anadenanthera peregrina (L.); iii) a mixed plantation of 16 native forest species (Nat); iv) a mined area without vegetation cover; and v) a natural forest cover. This design allowed exploring the soil CO2 dynamics in a gradient of recovery, from a degraded area to natural vegetation. Additionally, we measured soil temperature, moisture and soil characteristics. Soil CO2 efflux increased with increasing forest species cover in the rainy months. There was no significant change in CO2 efflux among the tree species. Heterotrophic soil respiration contributed to 64% of total soil CO2 efflux and was associated with litter decomposition. Amongst the abiotic variables, increases in soil moisture had the most influence on CO2 efflux. Therefore, these results help to understand the factors that underpin the loss of SOC and can orient management practices to improve soil organic matter and restore soil quality in degraded areas.

In this review, we present the state of art regarding rhizosphere effects on eucalypt plantations. It provides a greater understanding of carbon (C) and nitrogen (N) turnover in forest soils. P organic hydrolysis, soil mineral solubilization, indoleacetic acid, gibberellin, resistance factors, and production of siderophores by rhizosphere microbial populations help to explain the tolerance of Eucalyptus plants to biotic and abiotic stresses and the apparent steady-state condition of C and N soil stocks in many planted forests. This work aims to present the main findings on Eucalyptus rhizosphere processes and highlights their importance for trees nutrition, especially for N mineralization triggered by microbial activation or microbial community structure changes regarding the so-called rhizosphere priming effect and N fixation. Furthermore, we present an explanatory conceptual model of the steady-state condition for soil organic matter (SOM) stocks and its relation with fertilization based on a nutrient balance model. This review also considers the main experimental and modeling studies that demonstrate the quantitative importance of rhizosphere processes to Eucalyptus genus and their shortcomings. This provides a framework for process modeling under scenarios of global climate change. A better understanding of rhizosphere microbiological processes may allow improvements in Eucalyptus nutrition and production, as well as in accurate long-term estimates of SOM stocks and C-CO2 exchanges between forest soils and the atmosphere.

Soil organic matter (SOM) originates predominantly from above and belowground OM inputs derived from plants. Although the formation of SOM is well studied, it still remains unclear how the biochemical composition of litter affects the formation of new SOM as well as the degradation of “native” SOM. In the present study we aimed to disentangle the effect of plant litter composition on C transference from different plant tissues into specific SOM fractions and to determine the magnitude of priming effect on native SOC caused by litter amendments. To this end, we individually incubated 13C enriched Eucalyptus spp. major litter types (bark, leaves, twigs and roots) in soil (0–20 cm) of a sandy-clay loam (Haplic Ferralsol - Brazil). Additionally, a soil sample without plant residue addition was incubated as a control. The samples were incubated at 80% of their water-holding capacity at 25 ºC for 200 days. Soil respiration was assessed along the incubation period through headspace gas sampling and 13C/12C–CO2 analysis in a cavity ring-down spectrometer. After the incubation, soil subsamples were physically fractionated using a combined density-particle size separation method. The total C and the δ 13C of each soil organic matter fraction were measured and the litter-C contribution for each SOM fraction was assessed using a two-end member isotope mixing model. The molecular composition of the incubated plant material and SOM fractions were determined by solid-state 13C-CPMAS-NMR spectroscopy. Interestingly, we found no significant differences for total SOM contents among the different treatments. Conversely, incubation without litter amendment (control treatment) resulted in lower total SOM contents, indicating mineralization of “native” SOM along the incubation period. The partitioning of litter-derived C into SOM fractions indicated that leaves litter were preferentially transferred to mineral associated organic matter (MAOM), while roots contributed more to particulate organic matter (POM). Cumulative C-CO2 evolution from the treatments over the incubation period increased in the following order: twigs > leaves > bark > roots > controls. Incubation with twigs, bark and roots significantly increased “native” SOM respiration, while the treatment with leaves addition did not differ from the control. When tracing the source of “native” SOM-derived CO2, we observed a similar amount of C being respired from MAOM, regardless the treatment, while incubation with twigs, bark and roots resulted in higher respiration of “native” SOM-derived C from POM. Our data demonstrates that the biochemical composition of plant litter determines the fate of newly formed organic matter (MAOM or POM) and controls the degradation of “native” SOM. Therefore, plant residues enriched in more easily degradable compounds (leaves) are preferentially transferred to MAOM and causes less native SOC priming. On the other hand, plant residues enriched in structural compounds (twigs, bark and roots), are preferentially respired or allocated into the POM, also resulting in higher priming effect intensity.

Vigorous Eucalyptus plantations produce 105 to 106 km ha−1 of fine roots that probably increase carbon (C) and nitrogen (N) cycling in rhizosphere soil. However, the quantitative importance of rhizosphere priming is still unknown for most ecosystems, including these plantations. Therefore, the objective of this work was to propose and evaluate a mechanistic model for the prediction of rhizosphere C and N cycling in Eucalyptus plantations. The potential importance of the priming effect was estimated for a typical Eucalyptus plantation in Brazil. The process-based model (ForPRAN – Forest Plantation Rhizosphere Available Nitrogen) predicts the change in rhizosphere C and N cycling resulting from root growth and consists of two modules: (1) fine-root growth and (2) C and N rhizosphere cycling. The model describes a series of soil biological processes: root growth, rhizodeposition, microbial uptake, enzymatic synthesis, depolymerization of soil organic matter, microbial respiration, N mineralization, N immobilization, microbial death, microbial emigration and immigration, and soil organic matter (SOM) formation. Model performance was quantitatively and qualitatively satisfactory when compared to observed data in the literature. Input variables with the most influence on rhizosphere N mineralization were (in order of decreasing importance) root diameter > rhizosphere thickness > soil temperature > clay concentration. The priming effect in a typical Eucalyptus plantation producing 42 m3 ha−1 yr−1 of shoot biomass, with assumed losses of 40 % of total N mineralized, was estimated to be 24.6 % of plantation N demand (shoot + roots + litter). The rhizosphere cycling model should be considered for adaptation to other forestry and agricultural production models where the inclusion of such processes offers the potential for improved model performance.

Changes with land use and management are the second largest cause of greenhouse gas emissions (E-GHG) into the atmosphere. So, it is important to better understand agricultural systems regarding C and N stocks, which are directly associated with soil quality. The objective of this work was to evaluate the impacts of pastoral use systems on C and N stocks and the natural abundance of 13C (δ13C) in the fractions of soil organic matter in the Brazilian Amazon biome. The pastoral systems evaluated involved two silvopastoral systems with 30% (SP30) and 60% (SP60) of shading, two full-sun pasture systems, one in use and under intensive management (IMP) and other in fallow and is considered degraded (DP), as well as a control area under forest (NV). Dominant the grass of the pasture areas was Mombasa grass (Megathyrsus maximus). The experiment design consisted of four replicates where the collection sites (trenches) were systematically distributed within the areas (SP30, SP60, NV, DP and IMP) and the layers (0–5, 5–15, 15–30, 30–60 and 60–100 cm). Silvopastoral system with 60% shading had the greatest carbon stock, even greater than the control forest at some depths. In comparison to the control condition (NV) in soil profile, SP30, DP and IMP, reduced C-total stocks by 24%, 17% and 20%, and N-total by 14%, 10% and 18%, respectively. The δ13C values were higher in the IMP and DP systems and lower in SP60 and NV, both in organic matter associated with minerals (MaOM) and particulate organic matter (POM). An isotopic enrichment occurred in all systems in the soil profile.

The connection between litter chemistry and the pathways controlling soil organic matter (SOM) formation and decay in forest ecosystems remains poorly understood, particularly in tropical soils. We addressed this question by incubating samples of a Ferralsol for 200 days with typical forest litter (leaves, twigs, bark, and roots) obtained from 13C-enriched Eucalyptus seedlings. Throughout the incubation, we monitored 13C/12C–CO2 evolved from the soil to quantify the microbial respiration of the 13C-labeled fresh plant litter and of the native SOM. Afterwards, we used density fractionation to obtain particulate organic matter (POM) with density 53 μm and mineral-associated SOM (MAOM, with particle-size

Author(s): Souza, IF; Almeida, LFJ; Jesus, GL; Pett-Ridge, J; Nico, PS; Kleber, M; Silva, IR | Abstract: Interactions with pedogenic oxides are a known mechanism of soil organic matter (SOM) protection, but little is known about how the protective power of pedogenic oxides varies with soil depth in highly weathered tropical soils. To address this issue, we followed the decomposition of a double-labeled plant litter (13C/15N) in microcosm experiments using samples collected at four depths (0-10, 10-20, 20-40, and 60-100 cm) from six Brazilian Oxisols. These soils were selected to include a range of taxonomic subtypes, spanning wide variations in mineralogy and texture. After a 12-mo incubation, we quantified the proportion of isotopically-labeled SOM (13C/15N) within the mineral fraction l53 μm (i.e., clay+silt). We found that litter-C retention increased with depth, while the opposite occurred for litter-N. Correlations between isotopicallylabeled SOM and short-range order (SRO) Al-/Fe-(hydr)oxides were insignificant in topsoil (0-10 cm), but increased with depth, reaching peak significance in the 20- to 40-cm interval (r = 0.64 and 0.58, for litter-C and -N, respectively). A similar trend was observed for crystalline Al-/Fe-(hydr)oxides, which were more strongly correlated with the retention of 13C than 15N. We posit that in subsoil, both SRO and crystalline Al-/Fe-(hydr)oxides are more readily involved in the neoformation of mineral-organic associations. Overall, litter-C is less efficiently transferred into the clay+silt fraction of C-rich topsoil relative to C-depleted subsoil horizons, which may represent a significant C sink for Oxisols.

Harvesting and reform in eucalyptus can lead to changes in soil organic matter (SOM). The objective of this work was to evaluate the changes in total organic C (TOC) and N (NT), C and N in humic acid (HA), fulvic acid (FH), light organic matter (LOM) and microbial biomass (MB), in Ultisol of eucalypt stands over one, two and four years of renovated areas in Rio Grande do Sul state. After the first year of eucalyptus reform, there were 78% of increases in TOC (0.0-1.0-m soil layer). After two years, there was an increase in TN and an average reduction of 52% in LOM-C (0.0-0.1-m soil layer). FA-C, HA-C, and HU-C presented mean reductions of 43 Mg ha–1 (0.0-1.0-m soil layer) after four years of reform. The litter contribution of previous crop and the crop residues from the harvest resulted in increments of the SOM fractions. However, there is a negative effect in later years.

Despite the benefits of grass cultivation and organic fertilization in mining areas undergoing reclamation, these practices may be associated to CO2 emissions and soil organic matter (SOM) losses by priming effect. In the present study, we evaluated the changes on SOM pools and C–CO2 emissions in a bauxite-mined area under reclamation fertilized with poultry litter (PL) (0, 10, 20, and 40 Mg ha−1) and cultivated with Brachiaria brizantha. Increases of about 3·5 times in the soil labile C were observed 1 year after experiment establishment. High C–CO2 fluxes and a significant positive priming effect were observed in the presence of B. brizantha, increasing native C mineralization by nearly 4·9 times. Nevertheless, no net soil C loss was detected, probably because of the C inputs derived from B. brizantha, which offset these losses. In fact, the grass increased total organic C by 45% when fertilized with 40 Mg PL ha−1. The data obtained suggest that the cultivation of B. brizantha fertilized with PL can be a promising option for rapid recovery in SOM in areas under reclamation. Copyright © 2016 John Wiley & Sons, Ltd.

Retaining harvest residues (HR) in the field is considered an option to recycle carbon (C) and nutrients in short-rotation plantations, but the fate of HR in the soil remains unclear. The effects of HR management and nitrogen (N) availability on soil organic matter (SOM) pools and microbial community structure of forest plantations represent a major knowledge gap. To fill this gap, we explored how management scenarios that differed in the amount and type of HR [removal of all residues (-R), residue retention without (+R/-B), or with bark (+R/+B)] and N availability [0 (-N) or 200 kg N ha-1 (+N)] influenced microbial activity and structure and SOM fractions in a recently afforested grassland. Specifically, we measured HR decomposition, carbon dioxide efflux (respired CO2), phospholipid fatty acid (PLFA), and determined changes in particulate (POM) and mineral-associated organic matter (MAOM) over 12 months using differences in δ13C natural abundance between Eucalyptus HR and grassland soils (δ13C ∼-28 ‰ and -13 ‰, respectively). Microbial respiration was stimulated by HR retention (+R). Bark retention reduced HR half-life by ∼70 days, on average, while N had little influence. Bacterial groups (Gram-positive and Gram-negative) were the primary decomposer of eucalypt HR, while Actinobacteria used more of the former soil organic carbon (SOC). +R/+B increased fungal biomarker PLFA concentration and fungal:bacterial ratio, suggesting a key role of Fungi in the fragmentation of woody HR. N influence on microbial community structure and SOM pools was dependent upon HR management. Retaining HR increased SOC concentrations, most significantly in the 0–1 cm soil depth and POM fraction, and when bark was included. +R/+B resulted in higher POM-C concentrations (up to +37%) and more depleted δ13C-POM compared with -R and +R/-B, respectively. Collectively, our results suggest it is feasible to increase POM-C and fungal abundance through HR management practices in the early stages of decomposition, which may potentially contribute to SOC stabilization in the long-term. Yet our findings remain to be tested in long-term studies, we provide quantitative evidence of the potential of a more conservative HR management to contribute to the sustainability of eucalypt plantations.

The southern Brazilian grasslands (Pampas) are a new frontier of eucalypt plantation expansion, but the impacts of this land use change (LUC) on soil organic matter (SOM) are not well documented. In this study, we evaluated changes in the particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions following the afforestation of a grassland in southern Brazil over an entire eucalypt rotation. Additionally, SOM dynamics was assessed in two soil zones [row (R) and inter-row (IR)] created by soil preparation, which consists on scraping the soil surface from the IR to the R along with subsoiling to build ridges for eucalypt plating (i.e., ridge tillage). The soil was sampled to a depth of 60 cm in the R and IR positions, and the carbon (C) and nitrogen (N) in the SOM fractions were determined. We used natural 13C abundance differences to compare the contributions of the ‘new’ C input (C3, eucalypt stands) to the ‘native’ SOM (C4, grasslands). The conversion of the grassland to eucalypt stands resulted in losses in C and N stocks in both the R and IR after 68 months. Soil management controlled the magnitude and directions of C and N changes. The C loss in the R was ∼1.3-fold higher than that in the IR (20.6 Mg ha−1; p

Changes in carbon (C) and nitrogen (N) reserves reflect the impacts of land use and management. The objective was to evaluate the changes in organic matter stocks and quality in an Oxisol under intensive vegetable cultivation in the Brazilian Cerrado region. Four areas were selected: one under Cerrado vegetation (CV) and three under vegetable production with 15 (VG15), 20 (VG20) and 30 (VG30) years of cultivation. Soil samples were collected for the determination of total C (TC) and total N (TN) in soil organic matter fractions and the biochemical composition of particulate organic matter (POM) and mineral-associated organic matter (MOM). The smallest C stocks were observed on VG30, whereas the CV area had the highest values. The TN stocks were similar across treatments. The levels of labile C decreased with cultivation time. Increases and reductions in carbohydrate and lignin ratios in POM and MOM occurred in VG15 and VG20, respectively, with POM being more sensitive to changes in land use. There was greater similarity between VG15 and VG20, but differences between CV and VG30. There was a correlation between higher labile reserves (labile C and carbohydrates) and areas with shorter land use and between Cmic and agricultural use for 30 years. There was also a negative correlation between soil nutrients and the total and MOM C and N contents as well as the MOM carbohydrate levels, and there was also a negative correlation between Cmic and TC. The intensive use of Oxisols promoted a reduction in soil organic matter stocks and altered their quality.

A competição entre plantas daninhas e culturas é um dos principais fatores responsáveis por perdas de produtividade nos campos agrícolas. Esta revisão teve como objetivo apresentar e discutir como a interação entre plantas daninhas e microrganismos pode afetar a capacidade competitiva de plantas daninhas, bem como as propriedades físico-químicas do solo. Ainda, abordamos como as mudanças na estequiometria elementar de plantas daninhas podem refletir a capacidade competitiva e adaptativa das mesmas. Embora as plantas daninhas sejam mais dependentes de associações com microrganismos do solo do que as culturas para o crescimento, há poucos trabalhos que avaliaram a contribuição da microbiota do solo para o sucesso competitivo dessas plantas em agroecossistemas. Quando em competição, as plantas podem mudar a estequiometria elementar de seus tecidos em ambientes com variada disponibilidade de nutrientes. A estequiometria elementar de plantas tem sido particularmente bem estudada em abordagens ecológicas sobre a dinâmica de populações de plantas invasoras em ecossistemas naturais e parece ser ferramenta promissora para conhecer a capacidade das plantas daninhas para adaptar a diferentes manejos agrícolas. As plantas controlam os ciclos biogeoquímicos do carbono e do nitrogênio na rizosfera por meio de um fenômeno denominado efeito priming rizosférico (EPR). Embora esta revisão tenha levantado um conjunto de informações da literatura que fornecem fortes indícios de que a convivência das culturas com plantas daninhas pode aumentar a mineralização da matéria orgânica do solo (MOS), não se tem conhecimento de estudos que investiguem os efeitos da competição entre tais plantas sobre o EPR. The competition between weeds and crops is one of the main factors responsible for productivity losses in agricultural fields. This review aimed at presenting and discussing how the interactions between weeds and microorganisms can affect the competitive capacity of weeds and soil physicochemical properties. We also discuss how changes in the elemental stoichiometry of weeds can reflect their competitive and adaptative capacity. Although weeds are more dependent on associations with soil microorganisms than crops for growth, few studies have assessed the contribution of the soil microbiota to their competitive success in agroecosystems. When in competition, plants can change the elemental stoichiometry of their tissues in environments with varied nutrient availability. Elemental stoichiometry of plants has been particularly well studied using ecological approaches on the dynamics of weed populations in natural ecosystems, being a promising tool for understanding weed capacity to adapt to different agricultural managements. Plants control the biogeochemical cycles of carbon (C) and nitrogen (N) in the rhizosphere through a phenomenon known as the rhizosphere priming effect (RPE). Although this review has found some information in the literature that provides strong indications that the coexistence of weeds and crops may increase soil organic matter mineralization, we are not aware of studies investigating the effects of competition among these plants on RPE.

Linking plant litter biochemistry, its decomposition and soil organic matter (SOM) formation is not straightforward. In this research, we evaluated the decomposition of four biochemical fractions operationally defined as i) hot-water extractable (HWE), ii) total solvent (acetone) extractable (TSE), iii) acid-base (HNO3-KOH) unhydrolyzable cellulosic fraction (CF), and iv) acid (H2SO4) unhydrolyzable (AUR) and the transfer of C from these fractions to SOM. Each biochemical fraction was Soxhlet-extracted from isotopically labeled (13C) leaves, twigs, bark and roots of eucalypt plants (120 days old). The molecular composition of each fraction was inferred from thermochemolysis with tetramethylammonium hydroxide (TMAH), followed by gas chromatography coupled to mass spectrometry (GC-MS). For the incubation, we collected soil samples from the topsoil (0–20 cm) of a sandy-clay loam, kaolinitic Haplic Ferralsol. Four plant organs and four biochemical fractions were arranged into a (4 × 4) + 1 factorial scheme, including one control treatment (soil only). The samples were incubated at 80% of their water-holding capacity and kept under controlled temperature (25 °C). The decomposition of the biochemical fractions was monitored by determining the CO2 concentration into the headspace of the vials at 1, 2, 3, 4, 7, 13, 21, 28, 38, 46, 70, 80, 92, 112, 148, 178, and 200 days after the incubation had started. After the incubation, soil samples were submitted to density followed by particle-size fractionation. HWE and CF decomposed at faster rates than TSE and AUR throughout the incubation. The soil fraction < 53 μm retained a significantly higher proportion of the initial C input of HWE (32%) and AUR (31%) than TSE (19%) or CF (15%). Light fraction of organic matter (LFOM) with density < 1.8 g cm− 3, retained a significant proportion of AUR (37%) and TSE (32%) while CF was mostly lost as CO2 (79%). Selective preservation of organic materials (e.g., long-chain lipids) within the AUR and TSE fractions appears to be a significant pathway for the formation of SOM. SOM formation through a microbial-driven pathway cannot be ruled out for any biochemical fraction evaluated, but it seems more relevant for HWE and CF. In short-term, substrate biochemistry exerts a strong influence on the conversion of eucalypt litter fractions into either CO2 or SOM. Despite inherent challenges upfront, considering such dynamics at the ecosystem level will help to improve our current understanding on C storage and CO2 emissions from soils in long-term scales.

A expansão de plantações de eucalipto no sul do Brasil na última década, especialmente em áreas com campos naturais, potencialmente altera a dinâmica do carbono orgânico total (COT ) e nitrogênio total (NT ) do solo. Objetivou-se testar o modelo Century 4.5 na simulação das mudanças de estoques de COT e NT na camada superficial (0,0-0,2 m) do solo, em povoamentos de Eucalyptus sp. com diferentes idades em três municípios no RS. Áreas adjacentes com campos naturais foram usadas para estimar os estoques originais de COT e NT desses locais. O modelo Century 4.5 apresentou bom ajuste na simulação do estoque de COT no solo, com valores similares entre os simulados e observados, podendo ser empregado na simulação da dinâmica da matéria orgânica no campo natural e nas plantações de eucalipto dos diferentes locais estudados. Para o estoque de NT do solo, mesmo com as alterações de parâmetros, o modelo Century 4.5 não conseguiu simular esse estoque do solo com eficácia e acurácia estatística aceitável. The expansion of eucalyptus plantations in southern Brazil, especially over regions with natural grasslands, which occurred in the last decade potentially changes total organic carbon and nitrogen in soil. The aim of this study was to test the Century model 4.5 in the simulation of changes in inventories of TOC and TN in the surface layer (0.0-0.2 m) soil in Eucalyptus sp. com different ages in three municipalities in Rio Grande do Sul State, Brazil. Areas adjacent to grasslands were used to estimate the original stocks of TOC and TN these sites. The Century model 4.5 showed a good adjustment in the simulation of the soil TOC stock, with similar values of simulated and observed, and can be used to simulate organic matter dynamics in pasture and in eucalyptus plantations. Even with changes of parameters, the model failed to simulate the TN stocks with acceptable statistical accuracy and efficiency.

Nitrogen (N) fertilization effects on Eucalyptus growth and soil carbon (C) stocks are still controversial. We set up an N fertilization experiment in southern Brazil to evaluate initial tree growth and changes in soil organic matter (SOM). Four N levels (24–Reference, 36, 48 and 108 kg ha−1 of N) were tested and tree growth was assessed during the first two years. Afterwards, representative trees were chosen to evaluate fine-root biomass (FRB) and its spatial distribution. Soil was sampled to a 40-cm depth and SOM was fractionated in Particulate (POM) and Mineral-Associated Organic Matter (MAOM) for C and N content, and δ13C determination. Positive N effect on tree growth was seen only for tree height. N addition resulted in higher FRB. Changes in SOM were more expressive in top-soil layers. Overall, afforestation had positive effects on soil C. Increasing reference N dose resulted in higher C and N content in both SOM fractions. C and N dynamics were tightly correlated, especially in MAOM. Eucalypt-derived C was on average three-fold higher in POM. In summary, we showed that N fertilization may have positive but limited effects on tree growth, nevertheless it enhances fine-root biomass and C and N accumulation in SOM pools.

Eucalyptus forests in southern Bahia (BA) are planted in soils with a sandy surface layer and humid tropical climate, conditions that lead to soil carbon (C) decomposition. Recent studies have shown that nitrogen (N) may be important for soil C stabilization. The aim of this study was to evaluate the contribution of Eucalyptus harvest residues and nitrogen fertilization to C stabilization in Ultisols of southern BA. The experiment was conducted in Eucalyptus clonal plantations cultivated in two regions of Eunápolis, BA, Brazil, with different clay content: southern region (140 g kg-1 of clay) and western region (310 g kg-1 of clay). Five treatments were evaluated: one control (CTR), without Eucalyptus harvest residues and N fertilization, and four treatments with harvest residues combined with four rates of N fertilization: 0, 25, 50, and 100 kg ha-1. Soil samples were collected from the 0.00-0.10, 0.10-0.20, 0.20-0.40, and 0.40-0.60 m layers at the beginning and the end of the experiment (36 months). The amount of C and N and the C and N isotopic ratio (δ13C and δ15N) of particulate organic matter (POM) and mineral-associated organic matter (MAOM) were determined. In the southern region after 36 months, the C-MAOM stocks in the 0.00-0.10 m layer of the CTR decreased by 33 %. The addition of harvest residue followed by 100 kg ha-1 N increased C-POM and N-POM stocks (0.00-0.10 m) compared to the CTR, and the final N-POM stocks and residue-C recovery in the surface soil layer were positively correlated with the increase in N fertilization rates. In the western region, residue maintenance resulted in increased C-MAOM stocks (0.00-0.10 m) compared to the CTR, but an increase in N availability reduced this increment. The increase in N fertilization rates did not alter C stocks, but reduced N stocks of POM and MAOM in the upper soil layer. At the end of the experiment, N fertilizer recovery (0.00-0.60 m) was similar among the regions evaluated. In soil with lower clay content, higher N availability led to higher C and N stocks in the particulate fraction. In soils with high clay content, physical and chemical protections are more important than N fertilization for soil C stabilization, and just maintaining harvest residues may suffice to increase C and N in the more stable SOM fraction.

RESUMOA contaminação de solos por chumbo no município de Santo Amaro da Purificação-BA se originou a partir da instalação na cidade de uma metalúrgica na década de 60 e representa um sério risco à população local. A determinação dos teores disponíveis de Pb no solo é uma ferramenta importante para monitorar os riscos desta contaminação. No entanto, diferentes métodos são utilizados para determinar os teores disponíveis de Pb no solo, o que gera valores distintos entre os extratores. O objetivo do presente trabalho foi comparar a eficiência das extrações feitas com solução Mehlich 1, DTPA e CaCl2, na avaliação da disponibilidade Pb em solo tratado com diferentes amenizantes. A amostra de solo utilizada no experimento foi coletada próximo à metalúrgica. As doses de amenizantes foram estabelecidas com base no teor de carbono dos materiais (ácido húmico de compostagem, ácido húmico comercial e carvão vegetal) e corresponderam a 0; 0,75; 1,5; 3 e 7,5 g kg-1 de C. O conteúdo de Pb disponível foi extraído por Mehlich 1, DTPA pH 7,3 e CaCl210 mmol L-1. Foi realizado um fracionamento do solo, determinando-se o metal ligado às frações trocável e matéria orgânica do solo. A adição do AHv reduziu o teor de Pb no solo em 60; 89 e 47% para extrações realizadas com Mehlich 1, CaCl2 e DTPA, respectivamente. O teor de Pb na fração matéria orgânica foi diretamente proporcional às doses dos amenizantes. Os três extratores avaliados foram eficientes na predição dos teores de Pb disponíveis. ABSTRACTSoil contamination by lead in the town of Santo Amaro, in the state of Bahia, Brazil, originated with the installation of a metal plant in the city in the 1960s, and poses a serious risk to the local population. Determination of the available Pb levels in the soil is an important tool in monitoring the risks of this contamination. However, different methods are used to determine the available Pb content of the soil, resulting in differing values for the extractants. The aim of the present work was to compare the efficiency of extraction using Mehlich 1, DTPA and CaCl2solutions in the evaluation of Pb availability in soil treated with different soil amendments. The soil sample used in the experiment was collected near the metal plant. The dosages of soil amendments were set up based on the carbon content of the materials (compost humic acid, commercial humic acid and charcoal), being: 0, 0.75, 1.5, 3 and 7.5 g C kg-1. The available Pb content was extracted by Mehlich 1, DTPA pH 7.3 and CaCl2 10 mmol L-1. Soil fractionation was carried out to determine the metal attached to the exchangeable and organic matter fractions of the soil. The addition of AHv reduced the Pb content of the soil by 60, 89 and 47% for extractions carried out with Mehlich 1, CaCl2 and DTPA respectively. The Pb content of the organic matter fraction was directly proportional to the dosages of soil amendments. The three extractants under evaluation were efficient in predicting the available Pb content.

The amount of harvest residues retained in Eucalyptus plantations strongly influences soil organic matter (SOM), but the efficiency of conversion to SOM may vary according to the type of residue. This study evaluated the recovery of C from Eucalyptus residue components - leaves, bark, branches, roots, and a mix of all residues - in different SOM fractions with or without mineral-N supplementation (200 mg kg-1 of N). Variation in natural 13C abundance was used to trace the destination of residue-derived C in the soil. The C content of the light fraction (LF) and heavy fraction (HF) of SOM increased over a 240-days decomposition period in response to incorporation of Eucalyptus residues in the soil. Bark and leaf residues showed the best results. Bark residues increased the C content of the HF by 45 % over the initial condition. Leaf residues made the largest contribution to LF-C, increasing it by 8.6 times. Leaf residues also led to the highest N contents in the LF and HF, whereas branches, roots, and the mixture of residues caused significant net transfers of N from the HF. Mineral-N supplementation had no effect on stabilization of organic C in the HF of SOM, in which the C could be maintained for longer periods due to physical/colloidal protection against microbial decomposition. These results highlight the importance of keeping Eucalyptus harvest residues in the planted area, especially the bark, which is the most abundant harvest residue component under field conditions, for maintenance of SOM.

The aim of the present study was to evaluate the effect of eucalyptus plantations on total organic C (TOC) and total N (TN) stocks as well as the C and N in the fulvic acid (FA), humic acid (HA), humin (HU), light organic matter (LOM) and microbial biomass (MB) fractions in soils with different textures in the eastern part of the state of Rio Grande do Sul, Brazil. Soil samples were collected from the 0-10-cm, 10-20-cm, 20-40-cm, 40-60-cm and 60-100-cm soil layers in a completely randomized experimental design with subdivided plots. Under the eucalyptus plantation, clay loam soil presented lower HA, HU and MB C stocks in the 0-100-cm soil layer compared to reference vegetation, whereas sandy soil presented higher and C and N in HA and LOM as well as C in MB. The observed increase in TOC in eucalyptus plantations was more pronounced in the surface soil layer (0-10 cm), i.e., approximately 150% higher than under native vegetation, which was probably due to the high contribution of the eucalyptus litter. Differences in C and N stocks in soil organic matter (SOM) fractions between eucalyptus plantations and areas with reference vegetation were more pronounced in sandy soil, showing the capacity of the clay fraction to protect SOM. RESUMO O objetivo desse trabalho foi avaliar o efeito do cultivo de eucalipto no estoque total de C orgânico (COT) e N total (NT), C e N associados às frações ácido fúlvico (AF), ácido húmico (AH), humina (HU), matéria orgânica leve (MOL) e biomassa microbiana (BM), em solos de diferentes texturas, no Leste do Rio Grande do Sul. O delineamento adotado foi inteiramente casualizado em parcelas subdivididas. As amostras foram coletadas nas camadas de 0-10, 10-20, 20-40, 40-60 e 60-100 cm. Em solo franco-argiloso, a área sob povoamento de eucalipto apresentou redução nos estoques de C associado ao AH, HU e BM em relação à vegetação de referência na camada de 0-100 cm. Por outro lado, no solo arenoso houve aumento nos estoques de COT, NT, C e N associados àAH, HU, MOLe de C-BM. O aumento no estoque de COTfoi principalmente na camada de superficial (0-10 cm), aproximadamente 150 % em relação a área de vegetação nativa, provavelmente devido a grande contribuição do resíduo do eucalipto. Ao comparar as áreas com eucalipto e áreas de vegetação de referência, no solo arenoso, nota-se alterações de maior magnitude nos estoques de C e N associados às frações da MOS, o que evidência a capacidade da fração argila em proteger a matéria orgânica do solo.

http://dx.doi.org/10.5902/1980509816607A busca de sistemas sustentáveis de manejo do solo tem levado pesquisadores a desenvolverem novas técnicas de cultivo. Dentre elas, destacam-se no Brasil estudos realizados com espécies florestais capazes de fixar N2 atmosférico e aumentar o estoque de C e N em frações lábeis e estáveis da matéria orgânica do solo (MOS). O presente estudo teve por objetivo avaliar as alterações nos estoques de C e N em frações das substâncias húmicas, da fração leve da MOS e da matéria microbiana em solos cultivados com Eucalyptus urograndis em rotação curta, eucalipto em rotação longa e povoamentos de Acacia mangium que sucederam plantios de eucalipto em rotação curta, tendo como referência o solo de floresta nativa (Mata Atlântica) adjacente. Foram quantificados os estoques de C orgânico total e N total (COT e NT), nas frações das substâncias húmicas (fração ácido fúlvico-AF, fração ácido húmico-AH e fração humina-H), C e N da fração leve da MOS (C-MOL e N-MOL) e estoque de C e N da matéria microbiana (C-BM e N-BM). Os resultados indicaram que o cultivo do eucalipto em rotação curta apresentou menores estoques de COT e NT, nas frações das substâncias húmicas e, de N na matéria microbiana em comparação ao solo sob rotação com Acacia mangium. O cultivo de Acacia mangium e o aumento do tempo da rotação do eucalipto incrementaram os estoques de C e N da maioria das frações lábeis (C-MOL, N-MOL e C-BM) e estáveis (C e N nas substâncias húmicas). Indicando tendência de recuperação dos seus estoques para valores próximos daqueles originais (mata nativa), sendo superiores aos estoques obtidos na área de eucalipto em rotação curta.

Land-use changes with natural vegetation removal may impact the quantity and quality of humic substances (HS), including their molecular nature. Nuclear magnetic resonance (NMR) experiments were used in the present study to evaluate the alterations in the molecular composition of fulvic (FA) and humic (HA) acids from soils under eucalypt plantations in three major biomes in Brazil: Atlantic Forest, Cerrado, and Grassland. The major NMR-identifiable components in these HS were aromatics including aromatic C-O, COO/NC = O groups, peptides, carbohydrates, lignin-derived moieties and nonpolar alkyls. In all biomes the dipolar dephasing technique indicated the presence of significant amounts of condensed aromatic C, possibly inherited from charred materials derived from natural and anthropogenic fires in the region. The nonpolar alkyl C to O-alkyl C ratio averaged 1.4 for HA and 1.1 for FA. Humic substances from eucalypt soils showed greater contribution of nonpolar alkyl groups and a smaller abundance of O-alkyl groups in comparison to the native vegetation soil. Sugarcane (Saccharum officinarum L.) cultivation increased HA aromatics in comparison to those from the native Atlantic Forest soil, but when sugarcane was substituted by eucalypt the aromatics decreased and O-alkyl C recovered in HA and FA. There was evidence of greater contribution of lignin-derived C for HA and FA in sites planted with Brachiaria spp. pastures. Except for the HA from one Cerrado soil (Itacambira), aromaticity of HA decreased following planting to eucalypt. These changes in HS molecular composition across biomes may have impact on soil organic matter processes and they should be taken into account in future studies.

A matéria orgânica do solo (MOS) tem importantes funções no ciclo de C e na qualidade do solo. Considerando a complexidade dos fatores que controlam a ciclagem da MOS e o tempo necessário para que as mudanças sejam nela observadas, a modelagem constitui uma ferramenta muito importante para entender a ciclagem da MOS em solos florestais. As hipóteses do estudo foram: o cultivo do eucalipto por várias rotações resultaria no aumento dos estoques de C do solo, quando comparado a pastagens degradadas; e os valores dos estoques de C do solo simulados pelo modelo Century descreveriam a realidade melhor que a média das observações. Assim, os objetivos do presente estudo foram: avaliar a dinâmica da MOS utilizando o modelo Century para simular as mudanças dos estoques de C para duas cronossequências de plantações de eucalipto no Vale do Rio Doce, Minas Gerais, Brasil; e comparar os estoques de C simulados pelo Century com os estoques de C determinados em diferentes ordens de solo em distintas regiões do Vale do Rio Doce cultivadas com eucalipto. Em Belo Oriente (BO), região de menor altitude e temperatura mais elevada, as plantações de eucalipto têm sido cultivadas por 4, 13, 22, 32 e 34 anos, ao passo que em Virginópolis (VG) - região de maior altitude e temperatura mais baixa - os tempos de cultivos com eucalipto foram de 8, 19 e 33 anos. Assim, foram determinados os estoques de C do solo na camada de 0-20 cm. Os resultados indicaram que os estoques de C simulados pelo modelo Century decresceram após 37 anos de pastagem mal manejada em áreas previamente cobertas por mata nativa nas regiões de BO e VG. A substituição da pastagem mal manejada por eucalipto nos anos 70 resultou, em geral, no aumento de 0,28 e 0,42 t ha-1 ano-1 de C em BO e VG, respectivamente. Os estoques de C determinados sob eucalipto cultivado em distintas ordens de solo e independentes regiões com variáveis condições edafoclimáticas foram ligeiramente próximos aos valores estimados pelo modelo Century (RMSE = 20,9; eficiência do modelo - EF = 0,29), apesar do oposto resultado obtido com o procedimento estatístico para testar a identidade de métodos analíticos. Apenas sob condições de menores estoques de C no solo, o modelo superestimou os estoques de C na camada de 0-20 cm. Assim, o modelo Century tem grande potencial para detectar mudanças nos estoques de C em distintas ordens do solo sob eucalipto em futuras rotações, além de indicar o impacto do manejo do resíduo da colheita na MOS. Soil organic matter (SOM) plays an important role in carbon (C) cycle and soil quality. Considering the complexity of factors that control SOM cycling and the long time it usually takes to observe changes in SOM stocks, modeling constitutes a very important tool to understand SOM cycling in forest soils. The following hypotheses were tested: (i) soil organic carbon (SOC) stocks would be higher after several rotations of eucalyptus than in low-productivity pastures; (ii) SOC values simulated by the Century model would describe the data better than the mean of observations. So, the aims of the current study were: (i) to evaluate the SOM dynamics using the Century model to simulate the changes of C stocks for two eucalyptus chronosequences in the Rio Doce Valley, Minas Gerais State, Brazil; and (ii) to compare the C stocks simulated by Century with the C stocks measured in soils of different Orders and regions of the Rio Doce Valley growing eucalyptus. In Belo Oriente (BO), short-rotation eucalyptus plantations had been cultivated for 4.0; 13.0, 22.0, 32.0 and 34.0 years, at a lower elevation and in a warmer climate, while in Virginópolis (VG), these time periods were 8.0, 19.0 and 33.0 years, at a higher elevation and in a milder climate. Soil samples were collected from the 0-20 cm layer to estimate C stocks. Results indicate that the C stocks simulated by the Century model decreased after 37 years of poorly managed pastures in areas previously covered by native forest in the regions of BO and VG. The substitution of poorly managed pastures by eucalyptus in the early 1970´s led to an average increase of C of 0.28 and 0.42 t ha-1 year-1 in BO and VG, respectively. The measured C stocks under eucalyptus in distinct soil Orders and independent regions with variable edapho-climate conditions were not far from the values estimated by the Century model (root mean square error - RMSE = 20.9; model efficiency - EF = 0.29) despite the opposite result obtained with the statistical procedure to test the identity of analytical methods. Only for lower soil C stocks, the model over-estimated the C stock in the 0-20 cm layer. Thus, the Century model is highly promising to detect changes in C stocks in distinct soil orders under eucalyptus, as well as to indicate the impact of harvest residue management on SOM in future rotations.

RESUMO O cultivo de florestas plantadas de eucalipto em áreas de pastagem tem propiciado alterações no estoque de C e N no solo, em especial pelas diferentes características de cada planta e manejo dos resíduos vegetais; o eucalipto pode localizar maior estoque de C e N originário de resíduos da parte aérea na camada superficial do solo, e aumentar a relação C/N dos resíduos vegetais em decomposição. Tais diferenças podem causar redução no estoque de C e N nas frações químicas, físicas e biológicas do solo, tanto nas frações mais lábeis, quanto nas mais recalcitrantes. Este trabalho teve como objetivo avaliar os estoques de C orgânico total (COT) e N total (NT), estoque de C e N nas substâncias húmicas (ácidos fúlvicos, húmicos e húminas), fração leve (C-MOL e N-MOL) e biomassa microbiana (C-BM e N-BM) em amostras de solos coletadas nas camadas de 0-0,10, 0,10-0,20, 0,20-0,40, 0,40-0,60 e 0,60-1,00 m de profundidade, na linha (EL) e entrelinha (EEL) do solo sob cultivo de eucalipto na quarta rotação com seis anos de idade, e em área de pastagem cultivada com brachiaria (P). O estoque de carbono orgânico total e de carbono na fração leve do solo nas primeiras camadas do solo (até 0,60 m) foi superior na entrelinha do eucalipto em comparação àquele cultivado com pastagem e na linha de cultivo do eucalipto, possivelmente favorecido pela incorporação de resíduos da colheita do ciclo anterior de eucalipto na atual entrelinha. Por meio da determinação da abundância natural de 13C na camada de 0-0,20m de solo verificou-se que 29 e 37% do carbono presente na matéria orgânica do solo (MOS) após 28 anos de cultivo derivaram do eucalipto (planta C ), respectivamente, na linha e entrelinha, o que correspondeu à taxa de ciclagem média da MOS nesse solo de 1,04% (linha) e 1,32% (entrelinha) ao ano. No solo da entrelinha, observou-se que o C-C proveniente do eucalipto incrementou o estoque de C orgânico do solo, mesmo com a substituição do C-C proveniente da pastagem, após 28 anos de cultivo do eucalipto. Para o nitrogênio nas substâncias húmicas, verificou-se estoques semelhantes entre o solo da entrelinha do eucalipto e o de pastagem, e superior ao do solo da linha do eucalipto, até 1 m de profundidade. O solo de eucalipto (EL e EEL) teve estoques de C e N na biomassa microbiana semelhante àquele cultivado com pastagem ABSTRACT The cultivation of eucalypt in areas of pasture has resulted in changes of C and N storage in soil, mainly due to the different characteristics of each plant and their biomass management; the largest stock of C and N can be found under eucalypt, originating from the biomass at the soil surface layer and increasing the C/N in decomposition of plant residues. These differences may cause a decrease in the storage of C and N fractions in the chemical, physical and biological soil, both in the more labile, and the more recalcitrant fractions. This study aimed to evaluate the stocks of total organic carbon (TOC) and total N (TN), the stock of C and N in humic substances (fulvic acids, humic and humin), the light fraction (C-MOL and N-MOL) and the microbial biomass (C-BM and N-BM) in samples of collected from the layers 0-0.10, 0.10-0.20, 0.20-0.40, 0.40-0.60 and 0.60-1.00 m in depth, line (EL) and interline (EEL) from the soil under eucalypt in the fourth rotation with six years of age, and from an area of cultivated pasture with Brachiaria (P). The total organic carbon stock and carbon in the light fraction in the first layers of soil (0.60 m) was higher in the interline of eucalypt compared with that from the cultivated pasture and the line of eucalypt, probably favored by harvest residue from the previous cycle of eucalypt buried in the current interline. By determining the natural abundance of 13C in the 0-0.20m soil, it was found that 29% and 37% of carbon present in the soil organic matter (SOM) after 28 years of cultivation in the line and interline, respectively, was derived from eucalypt (C plants), which corresponded to an average rate of cycling of SOM in this soil of 1.04% (line) and 1.32% (interline) per year. In the soil interline it was observed that the C-C from the eucalypt increased the stock of soil organic C, even with the replacement of C-C from the pasture, after 28 years of eucalypt cultivation. For nitrogen in humic substances, a similar stock was obtained for the soil of the interline of eucalypt and that of the pasture, which was higher than that of the soil line of eucalypt, up to 1 m deep. The soil of eucalypt (EL and EEL) showed a stock of C and N in microbial biomass similar to that of cultivated pasture

The objective of this study was to investigate the effect of maize rhizosphere in the dynamics of soil organic matter (SOM) fractions (particulate organic matter—POM and mineral associated organic matter—MAOM) under different soil nitrogen (N) availability. The experiment was laid out as a factorial scheme 2 × 2: (i) without (-Plant) and with maize plant (+Plant); (ii) without (-N) and with N fertilization (+N) throughout 56 d under greenhouse conditions. Soil surface CO2-C efflux was monitored by static PVC chambers, and soil- and root-derived CO2-C efflux was determined using the mass-balance approach. In the absence of plants, N addition reduced total soil CO2-C efflux over the 56-d experiment and increased MAOM-C. This was followed by an increase of 366% in microbial biomass (MB-C), indicating a high C assimilation by soil microbes. In the presence of plants, there was evidence of positive rhizosphere priming effect (RPE) at 42-d after planting when N was applied. The new C input by rhizodeposition promoted an increase of 495% and 50% in MB-C under -N and +N addition, respectively. The short-term effect of maize rhizosphere appeared to preserve native SOM and allowed higher incorporation of microbial residues-derived C into the MAOM.

Little is currently known about modifications in edaphic characteristics caused by short-rotation eucalyptus and the impacts of these alterations on the sustainability of eucalyptus wood production. This study was carried out to identify theses changes at five sites of eucalyptus plantation in the region of the Rio Doce Valley, state of Minas Gerais, Brazil. Areas with more than three previous eucalyptus cycles, adjacent to pasture land or native forest, were chosen. Soil samples were collected and soil fertility analyzed by routine methods and other fractionation methods in order to measure alterations in the K, Ca and Mg contents as a consequence of eucalyptus cultivation. In the eucalyptus areas, reductions in the exchangeable Ca2+, Mg2+ and K+ contents and pH were observed and increased Al3+ and H + Al contents. Of all nutrients, only P contents (Mehlich-1 P) increased in the eucalyptus areas. The reduction in exchangeable forms and in medium-term soil nutrient pools indicates the need for higher nutrient rates than the currently applied in order to prevent nutritional limitations and soil nutrient exhaustion. After several eucalyptus rotations there was a recovery in the SOM content in comparison to degraded pasture soils, although not to the level of the native forest soil. The positive correlation between effective CEC and medium-term non-exchangeable Ca, Mg and K with SOM emphasizes the need for adequate fertilizer and plant residue management to sustain or even increase forest productivity in future cycles.

The knowledge concerned to the horizontal and vertical distribution of organic carbon and organic matter fractions on remaining eucalyptus stumps in the field leads to a better comprehension about the changes that may occur in these features along the cultivation. This work aimed at determining the variation of total organic carbon in soil and organic matter fractions at different horizontal distances from remaining eucalyptus stump, at different ages, in the direction of the stump line. So, soil samples were collected around the eucalyptus’s stump from the previous rotation, with 31 and 54 months-aged, and at the distances of 30, 60, 90, 120 and 150 cm, at 0-10, 10-20 e 20-40 cm depth layers. The soil samples were analyzed about: total organic carbon (COT) and carbon concentration on humic substances (SH) and on the light free fraction (FLL). The results indicated variation on organic carbon concentration from several fractions of soil organic matter as long as it hold off from the remaining eucalyptus’s stump (CRE) with 31 and 54 months-aged. The C concentrations of the fulvic acids fraction, humic fraction, SH and COT get variable with CRE’s age. The major C concentrations of the several soil organic matter fractions on the surface layer reflect the deposition and maintenance of forest residues on the soil surface, mainly after forest harvesting.

Afforestation of degraded pastures can potentially enhance carbon sequestration, but little is known about the effects of eucalyptus plantations on soil organic matter (SOM) fractions. We used density and particle size SOM fractionations to evaluate changes in SOM for a chronosequence of eucalyptus plantations established on degraded pastures in two contrasting regions in southeastern Brazil. Declines in the content of soil C derived from the pasture (C4 photosynthetic pathway) and the accumulation of that derived from the eucalyptus (C3 photosynthetic pathway) were followed through 13C natural abundance of the SOM. The two study areas were in the Rio Doce River Valley, Minas Gerais State, namely: 1, Belo Oriente (BO, a region with lower elevation, higher mean annual temperature, lower forest productivities and dominated by clayey Typic Haplustoxs); 2, Virginopolis (VG, a region of higher elevation, higher forest productivities and dominated by clayey Rhodic Ustoxs). In the BO region the chronosequence included 0, 4.2, 13.2, 22.2, 32.0 and 34.2 years of eucalyptus cultivation and in the VG region soils were cultivated with eucalyptus for 0, 8.2, 19.2, 29.2 and 33.2 years. The accumulated cultivation time corresponds to about five rotations. In both regions the initial soil condition was represented by a site that was still under pasture. Soil samples collected at 0–10 and 10–20 cm depths were analyzed for: total organic carbon (TOC); C in the fulvic (FAF), humic (HAF) and humin (HF) fractions; C in the free (FLF) and occluded (OLF) light fractions; C associated with the heavy fractions, namely, the sand (SF), silt plus clay (S + CF) and clay (CF) fractions; and C in the microbial biomass (MB). Carbon stocks of virtually all SOM fractions were about twice as high in the VG region than in the BO region. Eucalyptus cultivation in the BO region increased C stocks in all SOM fractions, except for the MB fraction that was not altered and the SF fraction that declined with time of eucalyptus cultivation. Increases in TOC, FAF and HF organic C fractions were observed in the VG region. Mean annual accumulations across the entire cultivation period in the 0–10 cm soil layer showed virtually no difference averaging 0.22 Mg C ha−1 year−1 for the BO region and 0.23 Mg C ha−1 year−1 for the VG region. However, gains of TOC peaked by the end of the third rotation in the BO region and the second rotation in the VG region, so the TOC accumulation rate during the respective aggrading periods would correspond to 0.35 and 0.57 Mg C ha−1 year−1. There was no clear evidence that the more labile, fast cycling SOM fractions were more sensitive indicators of the impact of eucalyptus cultivation in the BO region, whereas in the VG region the FLF and OLF were more sensitive to the land use change than TOC.

Este estudo teve por objetivo avaliar os teores de fenóis derivados de lignina, carboidratos e aminoaçúcares, bem como o estádio de decomposição e a contribuição de compostos de origens vegetal e microbiana para a matéria orgânica do solo (MOS) em sistemas de cultivo convencional e fertirrigado de eucalipto. Foram realizadas coletas de serapilheira e solo (Argissolo) em dois sistemas de cultivo de eucalipto (convencional e fertirrigado) em dois locais de coleta (linha e entrelinha), com três repetições. Foram avaliados a produção de serapilheira, os teores de C orgânico total (COT), N total (NT), lignina e fenóis derivados da lignina e suas relações entre grupamentos ácidos e aldeídos (Ac/Al), carboidratos e suas relações hexose/pentose, aminoaçúcares e a relação glucosamina/ácido murâmico no solo. O sistema de cultivo de eucalipto fertirrigado promoveu maior produção de serapilheira, diminuindo o teor de carboidratos, e não alterou o teor de lignina na serapilheira, em comparação com o convencional. No solo, o maior acúmulo de serapilheira (22%) no sistema fertirrigado aumentou o teor de carboidratos totais, lignina e aminoaçúcares, principalmente nas entrelinhas de cultivo, em que foram amontoados e enterrados os resíduos da colheita anterior. Também houve aumento na relação hexoses/pentoses, indicando maior presença de compostos originários da comunidade microbiana na MOS. Entre os compostos de origem microbiana, observou-se maior presença de glucosamina, indicando maior contribuição fúngica para a formação da MOS do cultivo fertirrigado. The study aimed to evaluate the content of phenols derived from lignin, carbohydrates, amino sugars, the stage of decomposition of compounds and the relative contribution of plant-and microbial-derived compounds to the soil organic matter (SOM) in the conventional and fertirrigated till age of eucalyptus. Samples of litter and soil (Ultisol) were taken from two tillage systems (conventional and fertirrigated) at two collection sites (line and between-line) with three replications. It was evaluated the litter production, content of total organic C (TOC), total N (TN), lignin and phenols-derived lignin and its Ac/Alratio, carbohydrates and their hexose/pentose ratio, amino sugars and the glucosamine/muramic acid ratio in the soil. The cultivation of eucalyptus in fertirrigated system promoted higher litter fall, reduced the carbohydrate content and maintained the content of lignin in the litter in comparison to the conventional system. In soil, the greater accumulation of the litter (22%) in fertirrigated system increased the contents of carbohydrates, lignin and amino sugars, mainly in the between-lines position. The hexoses/pentoses ratio also increased, indicating a greater presence of compounds of microbial origin. Among the compounds of microbial origin, a larger abundance of glucosamine was observed, indicating a greater contribution of fungal to the SOM formation under fertirrigated cultivation.

Alterações no sistema de manejo pelo cultivo de diferentes povoamentos florestais e a utilização de rotação de culturas podem levar a alterações na qualidade da matéria orgânica do solo (MOS) e deposição de resíduos vegetais. O presente estudo avaliou o estádio de decomposição e a contribuição de compostos de origem vegetal e microbiana para a MOS por meio de biomarcadores, tais como: fenóis derivados de lignina, carboidratos e aminoaçúcares em monocultivos do Eucalyptus urograndis (clone do Eucalyptus urophylla S. T. Blake x Eucalyptus grandis W. Hill ex Spreng) de ciclo curto (sete anos), sistemas de cultivo de rotação de acácia (Acacia mangium Willd.) após monocultivo de eucalipto, monocultivo de eucalipto de ciclo longo (24 anos) e mata nativa (Mata Atlântica) como condição original de solo do litoral Norte do Espírito do Santo. Para tanto, foram estimados nas amostras de solo e serapilheira os teores de C orgânico total (COT), N total (NT), teores de fenóis derivados de lignina (VSC), carboidratos e aminoaçúcares derivados da atividade microbiana no solo e, as relações ácidos e aldeídos dos grupamentos vanilil ((Ac/Al)Vanilil) e siringil ((Ac/Al)Siringil) da lignina, relação hexoses/pentoses (H/P) dos carboidratos e, relações glucosamina/ácido murâmico (Gluc/Mur.) e glucosamina/galactosamina (Gluc/Gal) dos aminoaçúcares. Os resultados indicaram que na serapilheira do monocultivo de eucalipto de ciclo curto houve maior deposição da matéria seca, teor de lignina (VSC) e carboidratos, relação C/N e VSC/N; Semelhante proporção de resíduos grossos, resíduos finos e teor de C e, menor teor de N, comparado àquela sob rotação de acácia. No solo, o cultivo de acácia aumentou os teores de C, N e carboidratos, aumentou a relação ácido/aldeído do grupamento vanilil da lignina e a relação glucosamina/ácido murâmico dos aminoaçúcares derivados da atividade microbiana. O aumento do tempo de cultivo do eucalipto (24 anos) incrementou o teor de C e reduziu a relação VSC/N na MOS em comparação ao monocultivo de eucalipto de ciclo curto, e apresentou teor de C e N inferiores àqueles observados no solo de acácia e mata nativa. A menor relação ácidos/ aldeídos (Ac/Al) dos fenóis derivados de lignina no solo cultivado com eucalipto (de curto e longo ciclo) indica que a MOS encontra-se em estádio menos avançado de decomposição do que no solo cultivado com acácia, e naquele de mata nativa. Nos solos de acácia, seguido por aquele de eucalipto, a maior relação glucosamina/ácido murâmico sugere maior participação de fungos na comunidade microbiana, enquanto nas áreas de mata nativa e eucalipto de ciclo longo há mais abundância de compostos derivados de bactérias. Neste sentido, observou-se a recuperação na qualidade do solo cultivado com eucalipto de ciclo longo e rotacionado com acácia em relação ao monocultivo de eucalipto de ciclo curto. Changes in the management systems for the cultivation of different forest stands and the use of species rotation can lead to alterations in the quality of soil organic matter (SOM) and plant residue deposition. This study evaluated the stage of decomposition and the contribution of plant and microbial origin for SOM through biomarkers, such as lignin-derived phenols, carbohydrates and amino sugars in continuous short- rotation eucalypt (Eucalyptus urophylla x Eucalyptus grandis hybrid) (seven years) compared to a rotation system including acacia (Acacia mangium Willd.) after short-rotation eucalypt; and long-rotation eucalypt (24 years). A native vegetation (Atlantic Forest) was used as a reference for the original site condition representative for the northern coast of Espírito Santo state. To do so, we estimated the content of total organic C (TOC), total N (TN) and the contents of lignin-derived phenols (VSC), the carbohydrates and the amino sugars derived from soil microorganisms, and acids/aldehyde ratio of groups vanilil ((Ac/Al)vanilil) and syringyl ((Ac/Al)syringyl) of the lignin, hexoses/pentoses (H/P) ratio of carbohydrates, besides glucosamine/ muramic acid (Gluc/Ac. Mur) and glucosamine/galactosamine (Gluc/Gal) ratios for soil and litter samples. The results indicated that litter of the continuous short-rotation eucalypt has greater dry mass, lignin (VSC) and carbohydrates contents, C/N and VSC/N ratios; a similar proportion of coarse litter to fine litter and C content, but a lower N content in comparison to the species rotation system that includes the leguminous acacia. In the soil, acacia cultivation increases C, N and carbohydrates content, widened the acid/aldehyde ratio of vanilil groups of lignin and the glucosamine/muramic acid ratio of amino sugars derived from microbial activity. The longer rotation of eucalypt (24 years) increased C content and decreased the VSC/N ratio in SOM compared to the continuous short-rotation eucalypt, but still having C and N content lower than in soil of acacia and native forest. The smallest Ac/Al ratio of lignin-derived phenols in soils cultivated with eucalypt (in long and short-rotation) indicates that the SOM is in less advanced stage of decomposition (humification) than in the soil cultivated with acacia, and that under native forest. In soils under acacia, followed by that of short-rotation eucalypt, the higher glucosamine/muramic acid ratio suggested a greater contribution of fungi in SOM cycling, whereas in the native forest and long-rotation eucalypt there is greater abundance of bacteria-derived compounds. Overall, the results indicate that there was a recovery in the quality of the soils cultivated with eucalypt for a longer rotation and also with acacia in relation to the continuous short-rotation eucalypt.

Apesar da grande quantidade de N acumulada em plantações de eucalipto de alta produtividade, o aumento em volume do tronco em resposta à aplicação de N não tem sido expressivo nem consistente. O objetivo deste trabalho foi verificar o efeito de doses e fontes de N sobre o crescimento e o acúmulo de N em plantas de eucalipto, na serapilheira, além do impacto nas frações da matéria orgânica do solo (MOS). O experimento foi instalado em campo, no município de Itamarandiba-MG, em blocos ao acaso com três repetições, consistindo da aplicação em cobertura de doses (0, 60, 120 e 240 kg ha^-1) e fontes de N distintas (sulfato de amônio e nitrato de amônio) em clone de eucalipto (AEC1528®). O efeito dos tratamentos sobre o crescimento e acúmulo de N nas plantas foi avaliado aos 30 meses de idade, abatendo-se árvores com DAP médio e separando-as em lenho, casca, galhos e folhas, para determinação da produção de matéria seca e dos teores e conteúdos de nutrientes das plantas. Amostras de solo e de serapilheira foram coletadas para análises de nutrientes. Os teores de C e N total da matéria orgânica particulada (MOP) e da matéria orgânica associada à fração mineral (MOAM) foram determinados por espectrometria de massa de razão isotópica, após separação física da MOS. As análises estatísticas consistiram de análise de variância e de regressão. A aplicação de adubos nitrogenados promoveu aumento no crescimento volumétrico do tronco e na matéria seca da parte aérea. A dose de N como sulfato de amônio para obter 90 % da produção máxima foi de 74 kg ha^-1, a qual resultou em incremento de 42,3 % no volume de tronco em relação à testemunha sem adubação nitrogenada. Na dose de 120 kg ha^-1 de N, não houve diferença de resposta à aplicação de sulfato de amônio e nitrato de amônio. Não foram detectadas alterações nos estoques de C e N da MOS com a adubação nitrogenada. No entanto, houve aumento da absorção de Ca, Mg e S. A taxa de recuperação aparente de N no campo foi maior na dose de 120 kg ha^-1 de N, atingindo 34,4 %. In spite of the high amount of N accumulated in high-yielding, short-rotation eucalyptus plantations, no significant or consistent increases in trunk volume have been observed in response to N fertilization. This experiment evaluated the effect of N fertilization on the growth and chemical composition of eucalyptus as well as its role in soil organic matter (SOM) mineralization. The field experiment was carried out in a randomized block design with three replications, in Itamarandiba- MG. Two N sources (ammonium sulphate and ammonium nitrate) of increasing N rates (0, 60, 120, 240 kg ha^-1) were tested on the eucalyptus clones AEC1528®. The effect of the treatments on tree growth and N accumulation were evaluated in 30 month-old trees with average DBH, which were harvested and separated in stem, bark, branches, and leaves to determine dry matter production and nutrient contents. Soil and forest litter samples were collected for chemical analyses. Total C and N content of particulate organic matter (POM) and organic matter associated to the mineral fraction (OMAM) were quantified by mass spectrometry (IRMS) after physical separation of SOM. The statistical analyses consisted of variance analysis and regression. Nitrogen fertilization increased trunk volume and aerial dry matter. The estimated N rate to obtain 90 % of the maximum trunk volume was 74 kg ha^-1 in the form of ammonium sulfate, and increased the yield by 42.3 % in comparison to the control plot without N fertilization. In response to N rates of 120 kg ha^-1, no difference was observed between ammonium sulfate and ammonium nitrate. No alterations in SOM C and N stocks were detected due to N application. However, there was an increase in Ca, Mg and S absorption. The apparent N recovery rate in the field was highest at the N rate of 120 kg ha^-1, reaching 34.4%.

The use of machinery in agricultural and forest management activities frequently increases soil compaction, resulting in greater soil density and microporosity, which in turn reduces hydraulic conductivity and O2 and CO2 diffusion rates, among other negative effects. Thus, soil compaction has the potential to affect soil microbial activity and the processes involved in organic matter decomposition and nutrient cycling. This study was carried out under controlled conditions to evaluate the effect of soil compaction on microbial activity and carbon (C) and nitrogen (N) mineralization. Two Oxisols with different mineralogy were utilized: a clayey oxidic-gibbsitic Typic Acrustox and a clayey kaolinitic Xantic Haplustox (Latossolo Vermelho-Amarelo ácrico - LVA, and Latossolo Amarelo distrófico - LA, respectively, in the Brazil Soil Classification System). Eight treatments (compaction levels) were assessed for each soil type in a complete block design, with six repetitions. The experimental unit consisted of PVC rings (height 6 cm, internal diameter 4.55 cm, volume 97.6 cm³). The PVC rings were filled with enough soil mass to reach a final density of 1.05 and 1.10 kg dm-3, respectively, in the LVA and LA. Then the soil samples were wetted (0.20 kg kg-1 = 80 % of field capacity) and compacted by a hydraulic press at pressures of 0, 60, 120, 240, 360, 540, 720 and 900 kPa. After soil compression the new bulk density was calculated according to the new volume occupied by the soil. Subsequently each PVC ring was placed within a 1 L plastic pot which was then tightly closed. The soils were incubated under aerobic conditions for 35 days and the basal respiration rate (CO2-C production) was estimated in the last two weeks. After the incubation period, the following soil chemical and microbiological properties were detremined: soil microbial biomass C (C MIC), total soil organic C (TOC), total N, and mineral N (NH4+-N and NO3--N). After that, mineral N, organic N and the rate of net N mineralization was calculated. Soil compaction increased NH4+-N and net N mineralization in both, LVA and LA, and NO3--N in the LVA; diminished the rate of TOC loss in both soils and the concentration of NO3--N in the LA and CO2-C in the LVA. It also decreased the C MIC at higher compaction levels in the LA. Thus, soil compaction decreases the TOC turnover probably due to increased physical protection of soil organic matter and lower aerobic microbial activity. Therefore, it is possible to conclude that under controlled conditions, the oxidic-gibbsitic Oxisol (LVA) was more susceptible to the effects of high compaction than the kaolinitic (LA) as far as organic matter cycling is concerned; and compaction pressures above 540 kPa reduced the total and organic nitrogen in the kaolinitic soil (LA), which was attributed to gaseous N losses. A mecanização em atividades agrícolas e florestais tem incrementado a compactação do solo, o que resulta no aumento da densidade e microporosidade, bem como na redução da condutividade hidráulica e das taxas de difusão de O2 e CO2, entre outros efeitos negativos. Assim, a compactação do solo tem o potencial de alterar negativamente a atividade dos seus microrganismos e os processos envolvidos na decomposição da matéria orgânica e ciclagem de nutrientes. Este trabalho foi realizado sob condições controladas para avaliar os efeitos da compactação sobre a atividade microbiana e mineralização do carbono (C) e nitrogênio (N) do solo. Foram utilizados dois Latossolos com distintas mineralogias: um oxídico-gibbsítico (Latossolo Vermelho-Amarelo ácrico - LVA) e um caulinítico (Latossolo Amarelo distrófico - LA). Oito tratamentos (níveis de compactação) foram avaliados para cada tipo de solo em um delineamento em blocos casualizados, com seis repetições. A unidade experimental foi composta por anel de PVC com 6 cm de altura e 4,55 cm de diâmetro interno, com volume útil de 97,6 cm³. Os anéis de PVC foram preenchidos com uma massa de solo suficiente para alcançar a densidade final de 1,05 e 1,10 kg dm-3, no LVA e LA, respectivamente. A seguir, as amostras de solo foram umedecidas (0,20 kg kg-1 = 80 % da capacidade de campo) e compactadas em uma prensa hidráulica. As pressões de compactação foram de 0, 60, 120, 240, 360, 540, 720 e 900 kPa. Após a compactação dos solos, determinou-se a densidade resultante, por meio do novo volume ocupado pelo solo. Em seguida, cada vaso foi individualmente disposto no interior de um pote plástico de 1 L, que foi hermeticamente fechado. Procedeu-se a uma incubação aeróbia dos solos por um período de 35 dias, e a respiração basal (C-CO2 mineralizado) foi estimada nas duas últimas semanas. Depois do período de incubação, foram realizadas análises químicas e microbiológicas em amostras dos solos: C da biomassa microbiana (C MIC), C orgânico total (COT), N total e N mineral (N-NH4+ e N-NO3-). Depois disso, foram calculados o N mineral, o N orgânico e a taxa de mineralização líquida de N. A compactação ocasionou aumento do N-NH4+ e da taxa de mineralização líquida de N nos dois solos e do N-NO3- no LVA; e decréscimo da taxa de perda de COT nos dois solos, de N-NO3- no LA, de C-CO2 no LVA e de C MIC em maiores níveis de compactação no LA. Assim, a compactação reduziu a ciclagem de COT provavelmente devido ao aumento da proteção física da matéria orgânica do solo e à menor atividade microbiana. Portanto, é possível concluir que, sob condições controladas, o solo oxídico (LVA) foi mais suscetível aos efeitos de maiores níveis de compactação do que o caulinítico (LA), considerando a ciclagem de matéria orgânica; e que as pressões de compactação superiores a 540 kPa reduziram o nitrogênio total e orgânico do solo caulinítico (LA), o que foi atribuído às perdas gasosas de N.

Nas últimas décadas, a utilização de isótopos estáveis em várias áreas de pesquisa vem se destacando, como na análise de fluxos e rotas metabólicas, análise de efeitos de estresses em plantas e, em grande escala, no estudo da matéria orgânica do solo (MOS). Estudos de alterações e dinâmica da MOS usando a variação da abundância natural do ^13C requerem mudanças na razão isotópica do C. Quando não existe essa possibilidade, uma das alternativas é enriquecer o material vegetal (planta) com ^13C, via fixação de ^13CO2, de modo que a razão isotópica seja distinta daquela da MOS original. O objetivo deste trabalho foi investigar a magnitude e a homogeneidade do enriquecimento em ^13C em diferentes componentes da planta de eucalipto. No processo de marcação, três plantas de eucalipto, com 4 meses de idade, cultivadas em solução nutritiva foram expostas a uma atmosfera enriquecida com ^13CO2, em uma câmara de vidro (448 dm^3), com temperatura em torno de 24 ºC. A concentração de CO2 e a razão ^13C/^12C foram monitoradas por um espectrômetro de massa de razão isotópica (IRMS) em amostras de ar retiradas ao longo do processo (126 dias com três pulsos de ^13CO2 semanais). Após o período de marcação, as plantas foram separadas em folha (folha-fonte e folha-dreno), galho, casca, lenho e raiz e analisadas em IRMS. O resultado foi expresso em partes por mil (‰) em relação ao padrão internacional de C denominado Pee-Dee Belemnite (PDB), obtendo-se a δ^13CPDB delas: folha-fonte (828,07 ‰), folha-dreno (645,72 ‰), galho (672,49 ‰), casca (691,86 ‰), lenho (632,02 ‰) e raiz (536,55 ‰). O padrão de alocação e enriquecimento de ^13C entre os componentes das plantas foi homogêneo, embora com diferenças numéricas da ordem de 291 ‰ na δ^13CPDB. As plantas de eucalipto mantiveram alta taxa de absorção de CO2 e, consequentemente, alta taxa fotossintética em concentrações de CO2 muito acima (180,4 mmol L^-1 - 7.934 ppmv) da encontrada na atmosfera (8,64 mmol L^-1 - 380 ppmv). O ^13C fixado durante o dia foi liberado em menor escala na respiração noturna, em comparação com o ^12C. O grau de enriquecimento com ^13C obtido indica que a técnica empregada permite o enriquecimento suficiente do material para traçar o C em estudos de decomposição e estabilização de litter de eucalipto em frações da MOS. In the last decades the use of stable isotopes has gained importance in several research areas, e.g., for metabolic flux and pathway analysis and studies on effects of biotic and abiotic plant stresses and on soil organic matter (SOM). Studies on the alterations and dynamics of SOM based on the variation in natural ^13C abundance require variations in the C isotopic composition, which are not always observed. One alternative is to label the plant material with ^13C by ^13CO2 fixation, so that the C isotope ratio is different from that of native SOM. The objective of this study was to investigate the magnitude and homogeneity of ^13C labeling in the different plant parts of eucalypts. Three 4-month-old eucalypt plants grown in nutrient solution were labeled, using a pulse technique. Plants were exposed to a ^13CO2 enriched atmosphere in a 448 dm^3 glass chamber, at around 24 ºC. The CO2 concentration and the ^13C/^12C ratio were monitored by gas sample analysis in an isotope ratio mass spectrometer (IRMS) during the pulse-labeling period (126 days, three ^13CO2 pulses per week). After the labeling period the plants were separated in leaves (leaf-source and leaf-sink), branches, bark, stem, and roots and analyzed in an IRMS. The results were expressed in parts per one thousand (‰) relative to the Pee-Dee Belemnite standard (δ^13CPDB). The δ^13CPDB were: 828.07 ‰ for young leaves, 645.7 ‰ for mature leaves, 672.5 ‰ for branches, 691.9 ‰ for bark, 632.02 ‰ for stem, and 536.6 ‰ for roots. The ^13C allocation and enrichment pattern among plant parts was homogeneous (statistically equal), although variations in δ^13CPDB of 291 ‰ between plant parts were observed. The eucalypt plants maintained a high CO2 absorption, and consequently photosynthetic rates well above the normal atmospheric CO2 concentration (8.64 mmol L-^1 - 380 ppmv), even at high CO2 concentrations (180.4 mmol L^-1 - 7.934 ppmv). The ^13C fixed during the day was proportionally less respired at night in comparison to ^12C. The degree of ^13C enrichment obtained indicates that the technique allows a sufficient plant enrichment to trace C in studies of litter decomposition and eucalypt litter stabilization in SOM fractions.

O cultivo de espécies florestais como o eucalipto em solos ocupados por pastagens pode levar a alterações no processo de decomposição de resíduos vegetais e na formação da matéria orgânica do solo (MOS), principalmente por alterar a contribuição de diferentes compostos orgânicos e modificar a atividade e a diversidade microbiana. Neste sentido, o presente estudo teve o objetivo de avaliar alterações causadas pelo cultivo do eucalipto em compostos orgânicos, tais como: fenóis derivados de lignina, carboidratos e aminoaçúcares, visando identificar o estádio de decomposição e a contribuição de compostos de origem microbiana para a MOS em solos do litoral Norte do Espírito Santo em comparação àqueles de pastagem. Os resultados indicaram menor aporte de carboidratos e teor de lignina similar na serapilheira de eucalipto em comparação àqueles da pastagem. Os fenóis derivados de lignina, que se caracterizaram pela presença de compostos do grupo cinamil (ácidos ferúlico e p-coumárico) em maior teor, refletiram em maior acúmulo destes compostos e carboidratos totais no solo de pastagem, embora os teores totais de lignina, COT, NT e aminoaçúcares tenham sido semelhantes entre os cultivos. Nos solos cultivados com eucalipto obteve-se maior contribuição de componentes de origem vegetal (menor relação H/P) para a MOS que aqueles de origem microbiana em comparação aos solos sob pastagem. O teor de determinados aminoaçúcares, como a glucosamina, nos solos indicou que os fungos são mais abundantes que bactérias e actinomicetos, especialmente no solo de pastagem. Forest species cultivation like eucalyptus pasture soils can lead to changes in the decomposition process of plant residues and in the soil organic matter formation (SOM), mainly by altering the different organic compounds contribution and modifying the activity and microbial diversity. Thus, this study was to evaluate the changes caused by the eucalyptus cultivation in organic compounds such as phenols from lignin, carbohydrates, and amino sugars to identify the decomposition stage and the compounds contribution from microbial origin for the SOM in the North Coast's soils from Espírito Santo, Brazil, in comparison with those ones of pasture. The results indicated a lower carbohydrates' contribution and lignin content in the Eucalyptus leaf litter similar to those ones of pasture. The phenols from lignin, which are characterized by the compounds presence from cinnamyl group (ferulic and p-coumaric acids) in higher levels, reflected an increased of these compounds accumulation and of the total carbohydrates in the pasture soil, although the total lignin content, TOC (total organic carbono), TN (total nitrogen), and amino sugars were similar among crops. In cultivated soils with eucalyptus, a higher components contribution from plant origin (lower H/P relation) for the SOM was got than those from microbial origin, compared with the soils under pasture. Certain amino sugars content such as glucosamine in the soils indicated that fungi are more abundant than bacteria and actinomycetes, especially in the pasture soil.

O eucalipto é a principal espécie florestal plantada no Brasil. Todavia, são escassas informações acerca do C estocado no solo e na biomassa desses plantios. Este trabalho teve como objetivos avaliar o estoque de C no solo (ECS) em plantações de eucalipto e determinar quais características edafoclimáticas determinam esse estoque. O estudo foi conduzido em eucalipto cultivado na região centro-leste do Estado de Minas Gerais, abrangendo cinco regiões: Cocais (CO), Rio Doce (RD), Sabinópolis (SA), Santa Bárbara (SB) e Virginópolis (VI). Foi calculado o estoque de C no solo até 100 cm de profundidade em plantações de eucalipto em áreas com predomínio de seis classes de solo: Cambissolo Háplico (CX), Latossolo Amarelo (LA), Latossolo Vermelho (LV), Latossolo Vermelho-Amarelo (LVA), Neossolo Flúvico (RU) e Plintossolo Pétrico (FF). Os estoques de C no solo variaram entre regiões e classes de solo. O maior ECS ocorreu no LV, com 183,07 t ha-1 de C, seguido pelas classes de CX, LVA, LA, FF e RU, com 135,65, 130,95, 121,58, 112,01 e 95,08 t ha-1 de C, respectivamente. Em relação ao estoque médio de C no solo por região, considerando todas as classes de solo, o maior ECS foi de 141,22 t ha-1; determinado na região de VI, na profundidade até 100 cm de profundidade, seguida pelas regiões SA, CO, SB e RD, com 135,54, 127,26, 112,89 e 80,79 t ha-1 de C, respectivamente. Ao se considerar o estoque de C total no sistema solo-planta, aos 84 meses de idade, a região de SA foi a que apresentou maior estoque, com 251,61 t ha-1; e a região de RD, o menor estoque, com 186,84 t ha-1 de C. O ECS pode ser estimado por equações compostas por características edofoclimáticas, sendo a variação em ECS explicada por características como teor de argila e de Al3+; pela altitude e pelo déficit hídrico da região. Eucalypt is the main commercial forest species in Brazil, but very little information is available in the literature on the amount of carbon stored in the soil and in the biomass of these forest stands. The main objective of this study was to estimate the amount of soil-stored carbon (SSC) of eucalypt plantations and determine soil and climate characteristics that influence SSC. The study was carried out in the Central-Eastern region of Minas Gerais State, Brazil, in five micro-regions (CO, RD, SB, SA, and VI) with varying soil and climatic conditions. Soil carbon was determined to a depth of 100 cm. Carbon in the forest floor was estimated by allometric equations. The carbon stored in the soil-plant system differed among micro-regions and soil classes. SSC ranged from 183.1 t ha-1 in Red Latosol to 95.1 t ha-1 in Inceptisol, and was negatively correlated to soil K, Ca2+; and Mg2+ content and density in the top soil layer. SSC was highest in the micro-region VI (141.2 t ha-1; average value for all soil types) and lowest in RD (80.8 t ha-1). Considering the soil-plant ecosystem and the usual rotation age (84 months), the absolute SSC value was greatest in the micro-region SA (251.6 t ha-1) and lowest in RD (186.8 t ha-1). Regression equations showed that the clay and aluminum content and altitude and water stress explained most of the SSC variation.

Morphologically differentiated Spodosols usually occur in the Coastal Plain of the South of Bahia and North of Espírito Santo. They are found in profiles known as "muçungas", i.e. sandy soils that accumulate water. In these areas, two kinds of Spodosols, different from those in the Restinga area, can be found: Spodosols with E albic horizon (white muçunungas) and without this horizon (black muçunungas). Eight soil profiles with spodic characteristics were collected and described in order to evaluate differences in the formation process of Barreiras and Restinga Spodosols in the South of Bahia. The soil profiles were also characterized chemically, physically and mineralogically. Additionally, texture and chemical analysis, Fe and Al extraction by sodium dithionite-citrate-bicarbonate (DBC), acid ammonium oxalate and sodium pyrophosphate, ammonium oxalate extract optic density (DOox), sulphuric acid attack, and X ray difractometry of the clay fraction were performed. In the Spodosols of the Barreiras area, fragipan was found the spodic layers. Cemented B spodic horizon were observed in the white muçunungas, and granular structure and dark color from the surface in the black muçunungas. There was no fragipan or hard spodic horizon in the Restinga Spodosol. This soil is acid, dystrophic and alic, with sandy texture and high clay percentages in the spodic horizons. The CEC, based on H + Al, is predominantly represented by the organic matter. The most representative components of the mineral phase of the clay fraction are kaolinite and possibly vermiculite traces with interlayered hydroxy. Chemical, physical, morphological and mineralogical differences were observed between the Barreiras and Restinga environments. The black and white muçunungas differ in morphologic and chemical properties only. Nos domínios dos Tabuleiros Costeiros do sul da Bahia e norte do Espírito Santo é comum a ocorrência de Espodossolos morfologicamente diferenciados, que ocorrem em depressões popularmente conhecidas como muçunungas - terras arenosas que acumulam água. Nessas áreas, observam-se Espodossolos com horizonte E álbico (muçunungas brancas) e sem este horizonte (muçunungas pretas), que apresentam características diferenciadas entre si e em relação àqueles encontrados em áreas de Restinga. Com o objetivo de avaliar as possíveis diferenças nos processos de formação dos Espodossolos da Formação Barreiras e da Restinga no sul da Bahia e caracterizá-los química, física e mineralogicamente, foram descritos e coletados oito perfis de solos com materiais espódicos e realizadas análises textural, químicas de rotina, extração de Fe e Al pelo ditionito-citrato-bicarbonato de sódio (DCB), oxalato ácido de amônio e pirofosfato de sódio, densidade ótica do extrato do oxalato de amônio (DOox), ataque sulfúrico e difratometria de raios-x das frações areia, silte e argila. No ambiente Barreiras, os Espodossolos apresentam fragipã abaixo dos horizontes espódicos. As muçunungas brancas apresentam horizonte B espódico cimentado, enquanto as pretas possuem estrutura pequena granular e coloração escura desde a superfície. No Espodossolo de restinga não se observou horizonte espódico endurecido nem fragipã. Os solos apresentam textura arenosa e aumento dos teores de argila nos horizontes espódicos. São solos ácidos, distróficos e álicos. A CTC, dominada por H + Al, é quase exclusivamente representada pela matéria orgânica. Os principais componentes da fase mineral da fração argila dos horizontes espódicos são caulinita e, possivelmente, traços de vermiculita com hidroxi entre camadas (VHE). Foram constatadas diferenças químicas, físicas, morfológicas e mineralógicas entre os Espodossolos da Formação Barreiras e os da Restinga. As muçunungas pretas e brancas apresentaram apenas diferenças morfológicas e químicas entre si.

O presente estudo teve o objetivo de avaliar os estoques de C e N nas frações lábeis e estáveis da MOS e da biomassa microbiana em resposta ao cultivo florestal com o uso de fertirrigação quando comparado ao cultivo convencional. Para tanto, duas áreas cultivadas com eucalipto num mesmo solo (Argissolo Amarelo), mas com manejos fertirrigado e convencional foram amostradas nas camadas de 0-10, 10-20, 20-40, 40-60 e 60-100cm de profundidade na linha e entrelinha de cultivo. A amostragem da entrelinha foi justificada pelo enleiramento e enterro de resíduos vegetais provenientes da colheita do cultivo anterior na entrelinha do cultivo atual. No final do ciclo de cultivo do eucalipto (sete anos), a deposição de serapilheira e os estoques de C e N na maioria das frações lábeis e estáveis da MOS foram maiores no sistema fertirrigado, com incremento de 50,34t ha -1 de C total (COT) e 5,62t ha -1 de N total (NT), na camada de 0- 100cm de profundidade, em comparação àqueles do sistema convencional. O solo da entrelinha de plantio, principalmente no sistema fertirrigado, apresentou maior estoque de C e N total e C nas frações das substâncias húmicas e na fração leve da MOS que no solo da linha de plantio, possivelmente em virtude da maior deposição e incorporação de resíduos vegetais do ciclo anterior na entrelinha do cultivo atual. A intensificação do manejo de florestas comerciais de eucalipto com o uso de fertirrigação possibilita aumentar a deposição de resíduos vegetais e sequestrar mais C e N, tanto em frações estáveis, quanto em frações mais lábeis da MOS, especialmente nas camadas de solo de até 40cm de profundidade. The objective of this study was to evaluate the C and N stocks in fractions of labile and more stable SOM fractions in response to intensification of forest cultivation through fertirrigation when compared to conventional stands. To that end, two areas planted with eucalyptus in the same (Ultisol), but with different managements (fertilized and irrigated and conventional system) were sampled in the 0-10, 10-20, 20-40, 40-60 and 60-100cm deep soil layers in line and interrow of the cultivation. The sampling interrow soil was justified by the ridged and burial of plant residues from previous crop cultivation in-between the substrate. At seven years old, eucalypt under fertirrigation increased the litterfall and the C and N stocks in the most labile and stable SOM fractions, leading to greater stocks of C and N in the 0-100 cm deep soil layer. The increments due to fertirrigation observed for total C was 50.34 and for the labile fraction was 5.62t ha -1 . The eucalypt interrow soil, especially in the fertirrigated treatment, presented larger stocks of total C and N and also larger C stocks in humic substances and light fraction of SOM possibly due to greater deposition and incorporation of vegetable residues from the previous cycle in the interrow current cultivation. The intensification of water and nutrients usage in short-rotation eucalypt stands is an alternative to increase the deposition of vegetable residues and C sequestration in more stable and labile SOM fractions, especially in the 0-40cm soil layer.

Sistemas agrossilvopastoris (SASPs) representam práticas de manejo agroflorestal que têm como principal objetivo permitir maior diversidade e sustentabilidade do sistema. A matéria orgânica do solo (MOS) é uma das principais propriedades indicadoras da sustentabilidade de um sistema de cultivo, principalmente em solos sob condições tropicais. Os objetivos deste trabalho foram estudar o impacto dos SASPs de cultivo do eucalipto em diferentes frações de C e N da MOS ao longo do tempo. Para isso, foi amostrada uma cronossequência composta por seis tempos de implantação dos SASPs. O tempo zero foi representado pela área de referência (Cerrado), e os demais, com dois, três, quatro, sete e 10 anos de implantação. Foram coletadas amostras de solo das camadas de 0-10, 10-20, 20-40, 40-60 e 60-100 cm, na linha e na entrelinha de cultivo do eucalipto. Foi feita a caracterização física e química desses solos, assim como as determinações de C orgânico total (COT), N total (NT), C na matéria orgânica leve (MOL), N na MOL e C oxidável por permanganato de potássio, como uma medida de labilidade do C (CL). A partir desses dados foram calculados o IMC CL, que é um Índice de Manejo do C, e o IMC CMOL, que é uma modificação do índice original. O estoque total de C orgânico no solo (COS) (0-100 cm) na linha de plantio do eucalipto foi mantido ao longo do tempo, enquanto na entrelinha houve tendência de perda inicial de C, sem que esses estoques retornassem aos valores originais (Cerrado) no final do décimo e último ano estudado. O estoque total de N mostrou comportamento semelhante na linha e na entrelinha de cultivo do eucalipto. Após perda inicial acentuada de N, houve tendência de acúmulo desse elemento ao longo do tempo de implantação dos SASPs. Contudo, esse acúmulo não foi sustentável e, ao final do período estudado, esses valores eram menores que os originais. A fração da MOL foi a que apresentou as maiores alterações, confirmando sua sensibilidade a alterações de manejo. As perdas de C e N da MOL também foram maiores para a entrelinha de plantio de eucalipto do que para a linha. O C lábil (CL), assim como o índice calculado a partir dele (IMC CL), não possibilitou ajustes dos seus resultados como variável do tempo de implantação dos SASPs. Entretanto, o IMC modificado, calculado a partir do C da MOL (IMC CMOL), apresentou bons ajustes para todas as camadas estudadas; além disso, apresentou correlação mais elevada com o COT, mostrando-se mais indicado para a comparação dos sistemas de manejo estudados. Pôde-se perceber a importância de estudar camadas mais profundas do solo, já que elas podem apresentar perdas de C significativas e de difícil recuperação. A implantação dos SASPs acarretou diminuição dos estoques de COT das camadas superficiais, e o tempo necessário para a recuperação dos estoques originais ficou além do período máximo estudado. A fração mais sensível ao manejo adotado foi a MOL livre. Agrosilvipastoral systems represent agro-forestry practices that allow greater diversity and sustainability of a cropping system. Soil organic matter (SOM) is one of the main indicators of the sustainability of a cropping system, especially in tropical soils. The objectives of this study were to investigate the impact of eucalypt cultivation on different C and N of SOM fractions on agrosilvipastoral systems over time. To this end, a chronosequence consisting of six implantation times of an agrosilvipastoral system was sampled. Time zero was represented by the adjacent native vegetation (Cerrado), followed by 2, 3, 4, 7 and 10 year periods since the agrosilvipastoral system implantation in previously Cerrado land. The layers 0-10, 10-20, 20-40, 40-60, and 60-100 cm were sampled in each area, within and in-between eucalypt rows. The samples were chemically and physically analyzed to determine: total organic carbon (TOC), total nitrogen (TN), C in the light organic matter (LOM), N in the LOM, and C oxidizable by potassium permanganate, the labile carbon (LC). From these data, the CMI proposed by Blair et al. (1995) was calculated (CMI LC), as well as the modified CMI, which is based on C in the LOM (CMI LOM). In the planting row, the TOC stocks in the 0-100 cm layer remained stable over time, while in-between the rows the TOC stocks initially decreased and then recover over time, but have yet to return to the level found in the native vegetation soil. The trend for the TN stock (0-100 cm) was similar for both in and between the eucalypt rows. After a sharp initial decrease, the TN stocks recovered over the years. However, this increase was not sustained and at the end of the cycle the TN stocks were lower than the original values found for the soil under native vegetation. Major changes were observed in the free LOM fractions, confirming its great sensitivity to management changes. More free LOM C and N were lost in-between than in the rows. Labile C, as well as the CMI, were found not sensitive to describe SOM changes along the implantation time, whereas the CMI CLOM adjusted well for all soil layers. In addition, it was best correlated with TOC and is therefore more adequate to compare the studied cropping system. These results show the importance of studying deeper soil layers; since the loss of C at greater soil depths can be relatively high and difficult to recover. The agrosilvipastoral systems induce a decrease in soil organic C in the upper layers and the time to recover the original stocks was beyond the study period. The fraction of free light organic matter was most sensitive to the management.

A determinação de C orgânico do solo tem sido realizada, principalmente, por métodos baseados na oxidação química. Contudo, recentemente, o C tem sido determinado por métodos de combustão seca, como o CHNS/O. Este trabalho teve como objetivo comparar três métodos para determinação do C de solos cultivados com eucalipto em regiões com diferentes características edafoclimáticas. Foram comparados os métodos: Walkley-Black, Yeomans & Bremner e combustão seca (CHNS/O), utilizando amostras de diferentes classes de solos obtidas nas seguintes profundidades: 0-20, 20-80 e 80-130 cm nos Cambissolos; 0-25, 25-70 e 70-110 cm nos Latossolos; e 0-18, 18-50 e 50-110 cm nos Neossolos. Amostras de TFSA foram trituradas em almofariz, passadas em peneira de malha de 0,2 mm e submetidas aos três métodos de determinação de C. Os teores de C obtidos correlacionaram-se positiva e significativamente entre si. Os métodos Walkley-Black e Yeomans & Bremner tenderam a subestimar os teores de C em relação ao método de referência, CHNS/O, tanto no que se refere às camadas superficiais quanto àquelas mais profundas, com menores teores de C. Equações de regressão linear com elevados valores de R² permitiram transformar os teores de C obtidos por Walkley-Black e Yeomans & Bremner em C por combustão seca. The soil C content has been determined most frequently by wet combustion methods. More recently, however, the dry combustion method has been used as the reference method for soil C determination. The main purpose of this study was to compare three C determination methods for soils under eucalypt plantations in southeastern Brazil. Samples of different soil types and located under distinct climate conditions were collected. Soil C contents were determined by two wet (Walkley-Black and Yeomans & Bremner) and one dry combustion method (CHNS/O Analyzer) in samples from six soil classes and different horizons: 0-20, 20-80 and 80-130 cm in Inceptsols; 0-25, 25-70 and 70-110 cm in Oxisols; and 0-18, 18-50 and 50-110 cm in Entisols. The soil samples were ground and sieved (0.2 mm) and the C concentration was determined by the three methods. The soil C contents determined by the three methods were positively correlated. However, the C content tended to be underestimated by the Walkley-Black and Yeomans & Bremner methods as compared to the reference dry combustion method, for surface as well as for the deeper soil layers with lower C contents. Linear regression equations with high R² values were adequate to transform C contents determined by the wet combustion methods into C contents equivalent to that determined by the dry combustion method.

A matéria orgânica leve tem se mostrado um indicador precoce das mudanças na matéria orgânica do solo (MOS) acarretadas por diferentes usos e sistemas de manejo. Várias soluções são empregadas na sua separação, mas pouco se sabe sobre o efeito delas na quantidade e qualidade da fração extraída, tampouco se conhecem as possíveis interferências que a solução de separação pode ocasionar na posterior quantificação do carbono (C) orgânico. Assim, foram selecionados 20 solos com diferentes teores de argila e que se encontravam sob diferentes usos, onde foram coletadas amostras, em três repetições, na profundidade de 0–20 cm. Depois de secas ao ar, destorroadas, passadas em peneira com malha de 2 mm e homogeneizadas, retiraram-se subamostras para determinação do teor de argila e para realização do fracionamento densimétrico da MOS. Do fracionamento densimétrico com água ou NaI (1,8 kg L-1), foram obtidas a fração leve (FL) e a fração pesada (FP) da MOS. O teor de C da FL foi determinado por combustão via seca, em analisador elementar (CHN), e o teor de C da FP foi determinado por oxidação via úmida com aquecimento externo, segundo Yeomans & Bremner (1988) (Y & B), e em analisador elementar (CHN). Adicionalmente, foi determinado o teor equivalente em C orgânico por via úmida em soluções-padrão com NaI em diferentes concentrações. Foi observado que a quantidade da FL extraída com NaI foi, em média, 4,86 vezes maior em relação à extraída com água destilada. A relação C:N da fração extraída com NaI foi, em média, 1,85 vez superior à apresentada pela extração com água destilada, mas com grande variação entre solos. Com relação à FP da MOS, o teor de C determinado por Y & B foi similar quando se procedeu à separação densimétrica com água, mas foi cerca de 1,5 vez superior ao determinado pelo CHN quando esta fração foi separada com NaI, indicando que houve superestimação do teor de C orgânico devido à interferência do NaI. Esse fato foi confirmado pelo aumento linear no consumo de Cr2O7-2 quando da presença de concentrações crescentes de NaI em soluções isentas de C. Dessa forma, a extração com NaI recupera maior proporção da FL, mas que difere em qualidade daquela extraída com água. Além disso, algum possível resíduo de NaI usado na separação pode interferir na quantificação do C orgânico quando este for determinado por dicromatometria. Resultados de estudo adicional sugerem que cada mol de I- consome aproximadamente 1 mol de Cr2O7-2. The light organic matter fraction is known as an early indicator of changes in soil organic matter (SOM) as a result of different soil use and management. Several solutions are used in its separation, but little is known about the effect of these solutions on the quantity and quality of the extracted fraction, as well as the possible interferences of the separation solution on the quantification of organic carbon (C) content. Twenty soils with variable clay contents and under different uses were selected. Soil samples were collected in the 0–20 cm layer, with three replications. After air-drying, they were ground manually, passed through a 2 mm sieve and homogenized; sub-samples were taken for clay content determination and densimetric SOM fractionation. Based on the fractionation scheme using water or NaI (1.8 kg L-1) the SOM light fraction (LF) and the heavy fraction (HF) were obtained. The C concentration of LF was determined by dry combustion in an elemental analyzer (CHN) and the C content of HF by wet oxidation, according to Yeomans & Bremner (1988) (Y & B) and by dry combustion. Additionally, we determined the equivalent organic C concentration by wet oxidation of standard solutions containing increasing NaI concentration. It was found that the amount of NaI-extracted LF averaged across soils was 4.86 times greater than when extracted with distilled water. The C:N ratio of this fraction was, on average, 1.85 times greater than that extracted with water, and the variation among soils was high. With regard to HF, the C concentration determined by wet oxidation was similar to that separated with water, but 1.5 times larger than when determined by dry combustion if separated with NaI. These results indicate that the C content was overestimated, possibly due to the interference of NaI. This finding was confirmed by the linear increase in the consumption of Cr2O7-2 during organic C determination in the presence of increasing NaI concentration of organic-C free solutions. Therefore, the NaI extraction of LF recovers a larger amount of LF, which differs however in quality from that extracted with water. Additionally, any potential NaI remaining after the separation, especially in the HF, may interfere with the organic C quantification if wet oxidation with dichromate as oxidant agent is the chosen method. Results of a complementary study indicate that each mol of I- consumes approximately 1 mol of Cr2O7-2.