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Soil organic carbon (SOC) plays a key role in improving soil quality and optimizing crop yield. Yet little is known about the fate of macroaggregates (>0.25 mm) under long-term fertilization and their relative importance in SOC sequestration in reclaimed calcareous soil. Therefore, the effects of mineral fertilizers and organic manure on the mechanisms of organic carbon (OC) stabilization in macroaggregates were investigated in this study. Four treatments were used: unfertilized control (CK), mineral fertilizer (NPK), compost chicken manure alone (M), and mineral fertilizers plus manure (MNPK). Samples from the 0–20 cm layer of soil receiving 11-year-long fertilization were separated into four fractions based on the macroaggregates present (unprotected coarse and fine particulate organic matter, cPOM and fPOM; physically protected intra-microaggregate POM, iPOM; and biochemically protected mineral associated OM, MOM) by the physical fractionation method. Compared with the control, the long-term application of NPK had little effect on SOC content, total nitrogen (TN) content, and OC and TN contents of macroaggregate fractions. In contrast, incorporation of organic manure (MNPK) significantly increased SOC (45.7%) and TN (24.3%) contents. Application of MNPK increased OC contents within macroaggregate-extracted fractions of cPOM (292.2%), fPOM (136.0%) and iPOM (124.0%), and TN contents within cPOM (607.1%), fPOM (242.5%) and iPOM (127.6%), but not the mineral associated organic carbon (MOM-C) and nitrogen (MOM-N) contents. Unprotected C fractions were more strongly and positively correlated with SOC increase than protected C fractions, especially for cPOM-C, indicating that SOC sequestration mainly occurred via cPOM-C in the studied calcareous soil. In conclusion, MNPK increased the quantity and stability of SOC by increasing the contents of cPOM-C and cPOM-N, suggesting that this management practice (MNPK) is an effective strategy to develop sustainable agriculture.

The Fe(II)-catalyzed transformation of ferrihydrite into highly crystalline forms may represent an important pathway for soil organic matter (SOM) destabilization under moderately reducing conditions. However, the link between redox-driven changes in soil Fe mineral composition and crystallinity and SOM chemical properties in the field remains elusive. We evaluated abiotic Fe(II)-catalyzed mineralogical transformation of Fe (oxyhydr)oxides in bulk soils and two particle-size SOM fractions, namely the fine silt plus clay (

The persistence of soil organic matter (SOM) constituents as a function of their recalcitrance has been recently questioned, with several papers showing the influence of the combined action of physical protection and chemical stabilization mechanisms as the main factors affecting SOM mean residence time. Using a physical fractionation method, SOM located between aggregates (FR), occluded within macro- (MA) and micro-aggregates (MI), and associated with the mineral fractions (MIN) were isolated from an agricultural soil differently amended (using compost, sewage sludge and biochar), and the occurrence of (total, bacterial and plant) DNA in these SOM pools was investigated. Following physical fractionation, total DNA (tDNA) was recovered from all SOM pools and from all treatments. Independently from the amendment, most tDNA accumulates in the FR fraction (30–70%), followed by the MIN pool (25–55%). The positive correlation between tDNA contents and C/N ratios (both tending to decrease following the order FR, MA and MI) observed for the light SOM fractions, together with the opposite pattern characterizing the heavy, MIN fraction (characterized by relatively high tDNA contents and low C/N ratios), suggests a different origin of the latter SOM pool, i.e., new molecules resulting from microbial transformations rather than highly degraded litter inputs. Therefore, tDNA may represent a promising proxy of organic matter dynamics in mineral soils. Finally, the MI fraction shows the highest number of microbial taxa and diversity, and seems to constitute a separate microbial niche in which different bacterial communities carry out their activity.

Accrual of carbon (C) and nitrogen (N) in soil is a significant and realizable management option to mitigate climate change; thus, a clear understanding of the mechanisms controlling the persistence of C and N in soil organic matter (SOM) across different ecosystems has never been more needed. Here, we investigated SOM distribution between physically and chemically stabilized fractions in soils from a variety of ecosystems (i.e., coniferous and broadleaved forest soils, grassland soils, technosols, and agricultural soils). Using elemental and thermal analyses, we examined changes in the quantity and quality of physically fractionated SOM pools characterized by different mechanisms of protection from decomposition. Independently of the ecosystem type, most of the organic C and total N were found in the mineral-associated SOM pool, known to be protected mainly by chemical mechanisms. Indexes of thermal stability and C/N ratio of this heavy SOM fraction were lower (especially in agricultural soils) compared to light SOM fractions found free or occluded in aggregates, and suggested a marked presence of inherently labile compounds. Our results confirm that the association of labile organic molecules with soil minerals is a major stabilization mechanism of SOM, and demonstrate that this is a generalizable finding occurring across different mineral soils and ecosystems.

The use of organic residues and compost is a common practice to improve soil quality and content of organic matter. In this study, the labile and stable fractions of soil organic matter were evaluated after application of layers of fresh (non-composted) or composted organic residues in a 6-year-old citrus orchard. The experiment was set up as a randomized block design, with 6 treatments: control without NPK, control with NPK, non-composted organic residue (NCOR, with and without NPK), and composted organic residue (humus, with and without NPK), with three replicates. The treatments were applied under the plant canopy. Soil samples were collected from the 0-0.05, 0.05-0.10, and 0.10-0.15 m layers. There were increases of 10.3, 22.4, 16.3, and 37.1 % in the organic carbon contents of the surface soil for the treatments using NCOR without NPK, NCOR with NPK, humus with NPK, and humus without NPK, respectively. The organic carbon contents of the labile fraction varied from 1.0 to 12.8 g kg-1, representing between 8 and 62 % of the total carbon. The carbon concentrations in the stable fraction varied from 3.1 to 9.7 g kg-1, representing between 38 and 92 % of the total carbon, and this was the dominant fraction for most of the treatments. O uso de resíduos orgânicos e composto é uma prática comum para aumentar os teores de matéria orgânica e a qualidade do solo. Neste estudo, foram aplicados resíduos orgânicos compostados (húmus) e frescos, em compostagem laminar, em um pomar de citros com 6 anos com o objetivo de avaliar as frações lábeis e estáveis da matéria orgânica. O experimento foi disposto em blocos casualizados, com seis tratamentos (Controle–NPK, controle + NPK, resíduo fresco com e sem NPK, resíduo compostado, com e sem NPK), com três repetições, aplicados na projeção da copa. As amostras de solo foram coletadas nas camadas de 0-0.05, 0.05-0.10 e 0.10-0.15 m. Houve aumento de 10,3; 22,4; 16,3 e 37,1 % no teor de carbono nos tratamentos com resíduo não compostado, com e sem NPK, e com resíduo compostado, com e sem NPK, respectivamente. O teor de carbono na fração lábil variou de 1 a 12,8 g kg-1, representando de 8 a 62 % do carbono total, e de 3,1 a 9,7 g kg-1, representando 38 a 92 % do carbono total, sendo esta a fração dominante na maioria dos tratamentos.

The state of Rio Grande do Sul has about 20% of the total area as lowland soils, suitable for flooded rice (Oryza sativa). In order to mitigate damage caused by rice monoculture, new crops such as sorghum (Sorghum bicolor) and soybean (Glycine max) have been cultivated in these areas. With different qualities of crop residues, it is expected a change in soil organic matter (SOM) dynamics and consequently, nitrogen (N) availability. The objective of this study was to evaluate the influence of rice, soybean and sorghum crop residues on the N present in physical fractions of SOM of two lowland soils, using labeled 15N technique, under incubation for 180 days in aerobic condition and more 180 days in anaerobic condition. At 30, 180 and 360 days of incubation the remaining N of the plant residues and N destination from the residues in both soils were quantified in the physical fractions of SOM >250 μm, 250-53 μm and 250 μm, 250-53 μm e

Evidence is accumulating that belowground crop residues contribute more efficiently to the build-up and maintenance of native soil organic carbon (SOC) than aboveground plant parts. We corroborated previous testing of the hypothesis that root-C would preferentially accumulate in soil microaggregates, where it is physically protected against microbially mediated decomposition. In three European field trials with C-3 to C-4 crop transitions, we compared the content of maize-C (Zea mays L.) in soil from rotations with grain maize (MG) or silage maize (MS) (i.e. with incorporation of roots and shoots or roots only). After decades of maize cultivation, SOC content did not differ within three out of four MG-MS pairs, although obviously larger amounts of shoot biomass were added to soil in the case of MG. We found that relative contribution of roots was on average 3.5 times more than shoots to the build-up of SOC per equivalent mass of residue C added to soils. Preferential occlusion of root-C as silt-sized intra-microaggregate particulate organic matter (iPOM) was not observed. There were much larger effects from shoot incorporation on maize-C in the >53-mu m fraction and free silt and clay. Storage of root-C as sand-sized iPOM was not quantified here, but first estimates suggested that physical entrapment at this level could only partly explain the longevity of root-C in soil. We reconfirm the relative stability of root-C in soil, but do not conclude that this stems from preferential physical entrapment over shoot-C. Future work should investigate the cause of preferential root-C association with the clay-sized fraction and if this occurs before or after microbial processing. Highlights The hypothesized preferential physical stabilization of root-C was assessed. Aboveground biomass had a minor effect on SOC storage in European long-term trials. Contribution of maize root-C to maintenance of SOC was three-fold that of shoot-C inputs. There was no preferential occlusion of root-C in silt-sized microaggregates.

The capacity of soils to sequestrate carbon (C) is mainly related to the formation of organo-mineral complexes. In this study, we investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate. Samples from a Humic Hapludox and a Rhodic Hapludox, clayey soils cultivated for approximately 30 years under no-tillage (NT) and conventional tillage (CT) were collected from six layers distributed within 100-cm soil depth from each site and from an adjacent native forest. After the removal of particulate organic matter (POM), the suspension (

Revegetation is essential to improve the quality of constructed soils. This study investigated effects of perennial grasses on total organic carbon (TOC) stock, organic matter (OM) fractions and on quality of a recently constructed soil, after coal mining. Soil samples were collected from the 0.00-0.03 m layer two years after the beginning of the experiment. The treatments consisted of Cynodon dactylon cv vaquero (T1); Urochloa brizantha (T2); Panicum maximun (T3); Urochloa humidicola (T4); Hemarthria altissima (T5); Cynodon dactylon cv tifton (T6); bare constructed soil (T8); and natural soil (T9). The treatments with grass species increased the TOC stock by 57 % and increased the OM lability in comparison with T8. Higher C accumulation in the coarse and free light fractions and a higher C management index were observed in T2 and T3, indicating greater suitability of these species for the initial recovery of OM of the constructed soil.

O uso e manejo do solo podem afetar as frações lábeis e humificadas da matéria orgânica (MO), mas a magnitude destas alterações é pouco conhecida em ambientes subtropicais. Este estudo avaliou os efeitos de quatro sistemas de uso e manejo do solo (mata, campo nativo, preparo convencional e plantio direto na sucessão trigo/soja) sobre (i) o estoque de carbono orgânico total (COT) (0-250mm), e nos (ii) estoques de carbono (C) em frações lábeis (grosseira, leve) e humificadas (associada aos minerais, substâncias húmicas) da MO na camada superficial (0-25mm) de um Latossolo bruno, no Sul do Brasil. Comparado à mata, o solo sob preparo convencional apresentou 36% (46,2Mg ha-1) menos COT na camada de 0-250mm, bem como um decréscimo generalizado no estoque de C em todas frações da MO na camada de 0-25mm. As frações grosseira (>53 m) e leve (53 mm) and light (

7 páginas.-- 4 tablas.-- 40 referencias, Integrated crop-livestock (ICL) is a promising land use system for the Brazilian Cerrado, but little is known about what this system might change in chemical composition of soil organic matter. In three long-term experiments (9–11 years old), located on Cerrado Ferralsols in Dourados, Maracaju and Campo Grande (Mato Grosso do Sul State, Brazil), we assessed the impact of continuous cropland (CC), ICL, and permanent pasture of Brachiaria decumbens (PP) on the C concentration and composition of the free light fraction (FLF), occluded light fraction (OLF) and heavy fraction (HF) of soil in the 0–5 cm layer. CPMAS13C NMR spectroscopy was used to determine the percentage of alkyl, O-alkyl, aromatic and carboxyl C types. In Dourados and Maracaju, PP had the highest concentrations of organic C in whole soil and physical fractions, while ICL was intermediate and CC lowest. In Campo Grande, soil organic C concentration was similar among management systems. Distribution of organic C across physical fractions was not affected by management nor by experimental site, and on average the FLF, OLF and HF contained 7%, 26% and 67% of the total storage, respectively. Signal peaks of the four main C types appeared in all CPMAS 13C NMR spectra, but at different intensities. O-alkyl was the major C type (about 50%), carboxyl was the minor representative (generally less than 10%) and alkyl and aromatic C were intermediates. From FLF to OLF, the alkyl and aromatic C concentrations increased, possibly due to selective preservation of waxes, resins, cutin, suberin and lignin. The HF had greater O-alkyl and lower aromatic C concentrations than OLF, which might have been related to the accumulation of microbial carbohydrates on mineral surfaces of the HF. Along the sequence CC–ICL–PP, the most evident changes were greater of O-alkyl and lower alkyl C types, practically in all fractions and sites. In FLF and OLF, these changes were attributed to greater biomass input and less seed drill-induced disturbance of soil surface (lower decomposition of residues) in the PP and ICL. Additionally, in OLF, greater O-alkyl concentration in PP and ICL was attributed to physical protection of particulate organic matter derived from grass roots occluded inside soil aggregates. Our results suggest that PP and ICL systems increased or maintained soil organic C concentrations compared to CC, associated with a qualitative increase of the chemically labile O-alkyl C type which was possibly related to greater biomass addition and less soil disturbance.

A cana-de-açúcar é a principal cultura utilizada na produção de etanol biocombustível no Brasil e sua colheita pode ser feita com ou sem queima das folhas, aumentando ou diminuindo a emissão de gases do efeito estufa e a deposição de C no solo. Por meio deste trabalho, avaliou-se o efeito de sistemas de colheita de cana-de-açúcar (com e sem queima da palha, com um, três e seis anos após a última reforma do canavial) sobre os teores e estoques de C no solo, a qualidade física da matéria orgânica e a imobilização de C na biomassa microbiana do solo. As áreas de colheita sem queima apresentaram maior teor de C na camada superficial e maiores estoques de C, independentemente do tempo após a última reforma. Diferenças na qualidade física da matéria orgânica ocorreram principalmente na fração com tamanho entre 53 e 75 µm, na qual a proporção de C foi maior nas áreas sem queima. Na camada de 0-10 cm, o sistema sem queima apresentou maior teor de C microbiano. A colheita sem queima da palha é eficiente em acumular C em formas que possuem alto tempo de residência no solo. Sugarcane is the main crop used in ethanol biofuel production in Brazil and it may be harvested with or without burning, increasing or decreasing greenhouse gases emissions and soil carbon deposition. In this study, we evaluated the effect of the sugarcane harvest system (with and without burning, at one, three, and six years after the last replanting) on C contents and C stocks in the soil, the physical quality of soil organic matter, and C immobilization in soil microbial biomass. The areas harvested without burning showed higher C content in the surface layer and higher C stocks, regardless of the time after the last replanting. Differences in the physical quality of organic matter mainly occurred in the 53 to 75 µm particle size fraction, in which the C ratio was higher in the unburned areas. In the 0-10 cm layer, harvesting without burning was associated with higher microbial C. Harvest without straw burning is effective in accumulating C in forms with long residence time in the soil.

4 páginas, 4 referencias.-- Functions of Natural Organic Matter in Changing Environment presents contributions from the 16th Meeting of the International Humic Substances Society (IHSS 16) held in Hangzhou, China on September 9-14, 2012.-- cmartin@irnase.csic.es, The analysis of the humic material in the particle-size fractions of fire-affected (FA) and fire-unaffected (FU) Cambisols and Leptosols from the Sierra de Aznalcóllar confirms that already shortly after a fire, degraded charred organic matter can interact strongly with the mineral phase. Applying solid-state 13C NMR spectroscopy, we identified pyrogenic organic matter in all particle-size separates, although the clearest evidence for its presence was obtained for the sand fraction. Difference spectra indicated that in the sand fractions of the FA soils, the O-alkyl C content decreased relative to that of the FU soil. This observation may be explained by the fact that in general, this fraction contains a considerable amount of particulate organic material with low humification degree and high accessibility for microorganisms. After the fire event, such material entering the soil as litter was quickly degraded, whereas the more resistant pyrogenic organic matter accumulated. Some of the latter must have suffered further disintegration due to weathering and partly degradation by microorganisms, leading to the production of smaller particles which can interact with the minerals of the small fractions. This interaction is likely to contribute to their stabilization and may explain the observation that in several soils with ancient fire history, charcoal residues were mainly found in the silt and clay fractions.

O fracionamento físico densimétrico é uma importante ferramenta no estudo da proteção física da matéria orgânica (MO) em agregados de solo, porém seus resultados podem ser alterados pela malha da peneira utilizada no preparo da amostra. A hipótese do presente estudo é de que a desagregação excessiva do solo pelo uso de peneira de 2 mm libera MO originalmente protegida dentro de agregados e, portanto, subestima o teor de C na fração leve-oclusa (C-FLO) e a magnitude da proteção física da MO intra-agregados, em comparação à peneira de 9,5 mm. Amostras (0–5 cm) de cinco solos foram coletadas em experimentos de longa duração (9–18 anos) nas regiões Centro-Oeste (quatro solos) e Sul do Brasil (um solo), sob sistemas de manejo convencional e conservacionista. Tomando as amostras ≤ 9,5 mm como referência, verificou-se subestimação de 2,9 a 15,7 % no C-FLO quando utilizadas amostras ≤ 2,0 mm, a qual foi mais expressiva nos sistemas conservacionistas de manejo de solo e positivamente correlacionada com os teores de C no solo (r = 0,92, p < 0,04), massa de macroagregados > 2 mm (r = 0,90, p < 0,02) e índice de estabilidade de agregados (r = 0,94, p < 0,01). Em três dos cinco solos estudados, o uso de amostras ≤ 2,0 mm levou à conclusão errônea de que os sistemas conservacionistas de manejo do solo não são efetivos na proteção física da MO do solo. Em estudos da proteção física da MO intra-agregados indica-se o uso da peneira de malha de 9,5 mm no preparo da amostra ou, alternativamente, da mesma malha da peneira empregada na preparação de amostras para avaliação da estabilidade de agregados, usualmente 8 mm ou maior. Density fractionation is an important tool to determine the physical protection of organic matter within soil aggregates, but results may be affected by the mesh size used to sieve soil samples. The hypothesis of this study is that in comparison to 9.5 mm sieve, sieving through 2 mm mesh results in excessive soil disaggregation by releasing organic matter originally protected within aggregates and consequently leads to underestimations of the carbon content in the occluded light fraction (OLF-C) and of the magnitude of physical protection of organic matter within soil aggregates. Soil samples (0-5 cm) of five Brazilian soils under long-term experiments (9-18 yrs) were collected in the regions Center West (4 soils) and South (1 soil), under conventional and conservation soil management systems. Based on ≤ 9.5 mm soil samples as reference, an underestimation of 2.9–15.7% of the OLF-C pool was observed in comparison to ≤ 2.0 soil samples mm. This effect was more pronounced for conservation soil management systems and positively correlated with soil organic C content (r = 0.92, p < 0.04), macroaggregate mass > 2 mm (r = 0.90, p < 0.02) and aggregate stability index (r = 0.94, p < 0.01). In three of the evaluated soils, the use of ≤ 2.0 mm soil samples led to the erroneous conclusion that conservation soil management systems are not efficient to stabilize organic matter within soil aggregates. Therefore, a mesh size of 9.5 mm is indicated to evaluate the physical protection of organic matter in aggregates, or alternatively, the sieves used to prepare soil samples for aggregate stability analysis, i.e., usually greater than 8 mm.

Nutrientes preservados em microagregados podem constituir reserva importante em solos altamente intemperizados. Com o objetivo de melhor compreender a distribuição de nutrientes em microagregados contidos em frações granulométricas de 200-50 μm e 50-20 μm, de um Latossolo Vermelho distrófico típico, utilizou-se um procedimento de fracionamento físico, adaptado para o objetivo deste estudo, em conjunto com análises químicas e mineralógicas. Parte destas frações foi caracterizada como microagregados constituídos por minerais da fração argila e silte, com altos teores de macro e micronutrientes. P, Cu, Mn e Zn foram mais eficientemente acumulados nos microagregados e menos susceptíveis à remoção, pela ação do intemperismo e lixiviação, em comparação ao K e Mg. Maiores teores de C foram encontrados nos microagregados de 50-20 μm do horizonte superficial, devido à proteção física mais eficiente contra a mineralização. Os estoques de macro e micronutrientes (teores totais), nos microagregados (200-50 μm e 50-20 μm), corresponderam a 5-19% do total de nutrientes no solo, e 24-26% do total de carbono do solo esteve sequestrado nestes microagregados. Apesar da baixa capacidade de reserva destes microagregados, eles tornam-se um compartimento de alta estabilidade, com maior eficiência na função de preservar os nutrientes, frente à degradação física e ao intemperismo e lixiviação. Nutrients preserved in microaggregates are an important fraction of the pool of nutrients in highly weathered soils. With the objective of better understanding the nutrients distribution in microaggregates of fine-sand (200-50 μm) and coarse-silt (50-20 μm), in a Rhodic Haplustox, the present study utilized an adapted physical fractionation procedure with mineralogical and chemical characterization. Part of the soil fractions was characterized as microaggregates formed by clay and silt minerals, with high macro and micronutrients contents. P, Cu, Mn, and Zn were more efficiently accumulated in the microaggregates and less susceptive to removal by the weathering and leaching actions, when compared to K and Mg. Higher C contents were found in the superficial horizon microaggregates (50-20 μm), showing evidences of an efficient physical protection against mineralization. Macro and micronutrients pools (total contents), in the 200-50 μm and 50-20 μm microaggregates, corresponded to 5-19% of the total soil nutrients content, and 24-26% of the total soil carbon was seized by those microaggregates. Although these microaggregates present a low capacity to store nutrients, they become an important and highly stable compartment to preserve nutrients, as far as physical degradation and weathering and leaching are concerned.

Mechanisms leading to high mean residence times of organic matter in subsoil horizons are poorly understood. In lower parts of the soil profile root material contributes greatly to soil organic matter (SOM). The objective of this study was to elucidate the decomposition dynamics of root-derived C and N in different soil depths during a 3 year field experiment and to examine the importance of different protection mechanisms as well as abiotic factors for the decomposition dynamics. Additionally, we assessed the effect of root litter addition on native SOM. Our conceptual approach included the exposure of litterbags with (13)C and (15)N labeled wheat root material mixed to loamy agricultural soil at three different soil depths (30, 60 and 90 cm). During the incubation period, we monitored soil temperature, humidity and the incorporation of root derived C and N into the soil microbial biomass and physical SOM fractions. Our results showed that abiotic decay conditions were better in subsurface horizons compared to the topsoil. Root litter addition significantly increased the size of microbial biomass in all three soil horizons. In the topsoil, root-derived C decomposition was significantly higher in the first 6 months of incubation compared to subsoil horizons. In 60 and 90 cm depths, a lag phase with development of soil microbial biomass seemed to be prevailing before decomposition was activated. For root-derived N, similar decomposition kinetics could be observed in top- and subsoil horizons. Despite of higher SOM contents, better soil structure and higher microbial activity in the topsoil horizon compared to subsoil horizons, the amounts of root-derived C and N remaining after 3 years were similar for all three depths. Most of the root-derived C and N was present as organo-mineral complexes or occluded in soil aggregates (oPOM), illustrating similar importance of these two protection mechanisms in all three soil depths. Addition of fresh root litter caused small losses of native soil C whereas native N was retained. We conclude that despite of similar SOM protection mechanisms, there are distinct differences in decomposition dynamics of root litter between top- and subsoil horizons. In the long run, the better abiotic decay conditions prevailing in subsoil horizons may compensate for their poorer physico-chemical characteristics.

Sistemas de manejo conservacionistas aumentam o conteúdo de carbono (C) e nitrogênio (N) orgânico do solo. Todavia, os efeitos sobre as frações leves da matéria orgânica do solo (MOS) ainda são pouco conhecidos, especialmente em ambiente tropical. O objetivo deste trabalho foi avaliar o efeito dos sistemas de preparo do solo, cultivado com oleráceas, na distribuição das frações leves (livre e intra-agregado) da matéria orgânica em Latossolo Vermelho em Paty do Alferes (RJ). Os tratamentos estudados foram: preparo convencional (PC), preparo em nível (PN) e cultivo mínimo (CM). Como referência, foram utilizadas uma área com cobertura vegetal de gramíneas (GR) e uma parcela sem cobertura vegetal (SC). As amostras de terra foram coletadas no verão 1998/99, nas profundidades de 0-5 e 5-10 cm. A distribuição do peso das frações leves diferiu significativamente entre os tratamentos. Em relação ao tratamento PC, houve aumento de 70 e 58 % na quantidade das frações leves livre e intra-agregado, respectivamente, no sistema CM. O conteúdo de C orgânico e N total das frações leves foi maior na parcela GR. Considerando os atributos avaliados, os tratamentos PN e cm são recomendados para as condições de solo e ambiente para o cultivo de oleráceas em Paty do Alferes. Conservational management systems increase the content of soil organic C and N. However, their effects on the light fractions of the soil organic matter (SOM) are not very well known, particularly under tropical climate conditions. The objective of this study was to evaluate the effect of soil tillage systems on the distribution of light fraction (free and intra-aggregate) organic matter in a Red Latosol (Rhodic Haplustox) cultivated with vegetable crops. The study site was located in Paty do Alferes (Rio de Janeiro State, Brazil). The studied treatments were: conventional tillage (PC), contour tillage (PN), and minimum tillage (CM). Additionally, soil samples from two reference plots - one with grass coverage (GR) and the other under bare soil (SC) were collected. Soil samples were collected from the 0-5 and 5-10 cm deep layers in the 1998/99 summer. The light fraction distribution on a weight basis differed statistically among the soil tillage systems. The CM system showed higher content of light (70 %) and intra-aggregate (58 %) fractions when compared to the PC system. The total organic C and total N content of the light fractions were higher in the GR plot. Considering the evaluated soil attributes, PN and CM systems are recommended for vegetable crop cultivation under soil and environmental conditions in Paty do Alferes.