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Relatively few studies have documented the impacts of afforestation, particularly production forestry, on belowground carbon dioxide (CO2) effluxes to the atmosphere. We evaluated the changes in the soil CO2 efflux—a proxy for soil respiration (Rs)—for three years following a native grassland conversion to eucalypt plantations in southern Brazil where minimum tillage during site preparation created two distinct soil zones, within planting row (W) and between-row (B). We used root-exclusion and carbon (C)- isotopic approaches to distinguish Rs components (heterotrophic-Rh and autotrophic-Ra respirations), and a CO2 profile tube (1-m deep) to determine the concentration ([CO2]) and isotopic C signature of soil CO2 (δ13[CO2]). The soil CO2 efflux in the afforested site averaged 0.37 g CO2 m−2 h−1, which was 56% lower than the soil CO2 efflux in the grassland. The δ13CO2 in the afforested site ranged from − 14.1‰ to − 29.4‰, indicating a greater contribution of eucalypt-derived respiration (both Rh and Ra) over time. Higher soil CO2 efflux and lower [CO2] were observed in W than B, indicating that soil preparation creates two distinct soil functional zones with respect to C cycling. The [CO2] and δ13[CO2] decreased in both zonal positions with eucalypt stand development. Although the equilibrium in C fluxes and pools across multiple rotations is needed to fully account for the feedback of eucalypt planted forests to climate change, we provide quantitative information on soil CO2 dynamics after afforestation and show how soil preparation can leverage the feedback of planted forests to climate change.

Concerns about the negative effects of agriculture on soil physical quality and soil organic carbon (SOC) pools have spurred on the adoption of conservation tillage systems in tropical regions. This study aimed to evaluate the long-term impacts (16th year) of conventional (CT), minimum (MT) and no-tillage (NT) practices and different cover crops (sunn hemp and a bean/millet sequence) on soil physical properties and SOC content of a corn cultivated Ultisol in the northeast of Brazil. Soil bulk density (Bd), soil penetration resistance (SPR), soil aggregation, and total aggregate-associated carbon (C) (4-2 mm) were evaluated. Tillage practices exerted strong control on soil physical properties and total aggregate-C content but were influenced by cover crop species. Minimum tillage presented the lowest Bd, irrespective of cover crop, while NT with bean/millet sequence resulted in the lowest SPR. However, as regards soil aggregation and total aggregate-C, the results indicated that there were no differences between MT and NT, with both systems presenting greater mean weight diameter (MWD) and total aggregate-C content than CT. Total aggregate-C content in the 0.00-0.05 m soil layer in conservation tillage was increased by the adoption of a bean/millet sequence. Increased mechanical disturbance through CT practices was harmful to Bd, soil aggregation and SOC accrual. Overall, more conservative tillage practices and the proper choice of cover crops might yield greater soil quality. Low intensity of soil disturbances due to the adoption of MT favors soil aggregation and the accrual of SOC in weakly structured soils through increases in contact between organic and mineral particles while not causing significant destruction of soil aggregates.

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

In Brazil, studies addressing soil carbon (C) stocks under native vegetation are scarce and often limited to land use change comparisons at the plot level, while studies across larger areas are based on legacy data that address multiple land uses, use wet-oxidations methods to determine C concentrations (Cc) and lack proper soil bulk density (ρb) determinations. Together, these factors may result in either underestimations or biased soil C stocks, thereby hampering the development of adequate land use policy. In this study, we aimed to (i) determine the soil C stocks at different depths (0–10, 10–20, 20–40, 40–60 and 60–100 cm) in Cerrado woodland remnants in Minas Gerais (MG), southeastern Brazil, (ii) study the spatial distribution of soil C stocks using regression-kriging (RK), and (iii) provide a description of the soil C stock distribution and its relationship with variables that drive the soil C stock in the MG Cerrado. We designed a large soil inventory, in which soil samples were collected for soil Cc and ρb determination, to compute soil C stocks in the Cerrado biome of MG. Afterwards, we used RK to model the soil C stocks across the MG Cerrado to a depth of 1 m. Our results show that using RK to map C stocks across different soil layers in the MG Cerrado resulted in accurate estimates. The soil C stocks in the MG Cerrado tend to increase in the west-east direction, are positively correlated with altitude and silt + clay content, and are negatively correlated with the mean annual temperature and total annual precipitation. On average, the C stored at the 0–40 cm layer accounted for 56% of the total C stocked in the top 1 m of soil. Approximately 1.03 Pg of C is stored in the MG Cerrado soils at a depth of 1 m. The average soil C stock in the MG Cerrado is approximately 53% larger than the average indicated for the Brazilian Cerrado. Our results highlight the need for more detailed regional scale soil C inventories in the country to refine national C accounting. We believe our study can aid the development of adequate land use policy and could be used as a baseline for land use change studies in the MG Cerrado.

Near-infrared (NIR) spectroscopy and chemometric methods were used to predict the chemical properties of decomposing eucalyptus harvest residues to better understand the decomposition process of these materials. Leaves, twigs, branches, and bark from a decomposition experimental set up in commercial plantations were sampled for one year. The contents of carbon (C), nitrogen (N), extractives (EX), acid-soluble lignin (SL), Klason insoluble lignin (KL) and holocellulose (HC) were determined by the reference method in the collected samples. Principal component analysis (PCA) was employed to distinguish the types of harvest residues throughout the decomposition period. Multi-residue regression models were built from the NIR spectra using partial least squares regression (PLS). Two feature selection methods, i.e., ordered predictors selection (OPS) and genetic algorithm (GA), were applied and compared. The OPS and GA did not differ statistically; however, compared with the GA, OPS was more computationally efficient and selected fewer variables. Using the PLS-OPS models, the root mean square errors of prediction (RMSEP) for C, N, EX, SL, KL and HC were 19.70, 0.08, 0.74, 0.39, 28.13 and 33.99, respectively, and the prediction correlations (Rp) for these properties were 0.94, 0.99, 0.99, 0.99, 0.96 and 0.98, respectively. PLS-discriminant analysis (PLS-DA) was used to classify the samples over the decomposition time and provided a good separation. Some mismatches obtained in the modeled classes were explained by the differences in the decomposition rate and changes in the chemical composition of the different harvest residue components that were evaluated. The results showed the feasibility of NIR spectroscopy and chemometric methods to evaluate the chemistry of decomposing eucalyptus harvest residues, indicating that these methods can be used as rapid and inexpensive alternatives to conventional methods to help understand the decomposition process.

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.

The potential export of nutrients by eucalypt plantations harvest raises concern about environmental and wood production sustainability. Keeping non-commercial components in the field can minimize it. However, little is known about changes in chemistry of decomposing eucalypt harvest residues. Simulated harvest residue management practices fully replicated across 11 Brazilian representative sites were undertaken to evaluate the dynamics of macronutrient release. We studied the influence of de-barking in the field, the use of an external source of nitrogen and the placement of harvest residues (remained on the surface or incorporated into the soil). After the experiment set up, residues were sampled five times and contents of Ca, Mg, N, P, K and S were determined. We used the remaining mass and nutrient concentration to fit the single exponential decay model (X = X0e−kt) and calculate half-life (hl = ln(2)/k) of each nutrient. At the end, we calculate element ratios to see differences in residue chemistry. The presence of bark and residue incorporation into soil enhanced decomposition and decreased nutrient half-life across all site. External N had little or no effect on nutrient dynamics. Site had significant effect on nutrient half-life, but only low correlations with climate or edaphic properties could be found. C:N, C:S and C:Ca ratios were much narrower at the end. On the other hand, N:P, N:K, Ca:S and Ca:Mg were wider. C:P varied among sites, but averaged almost the same as in the beginning. Ca was the nutrient that presented the greatest immobilization, particularly in the absence of bark, indicating a critical role of this element on decomposition under the conditions tested. Our results suggest that nutrient release is more controlled by management, chemical properties of the residues and specific decomposer community needs than climate or soil properties. Microbial community seems to change their carbon use efficiency and immobilize nutrients in limiting conditions.