Influence of summer crop residues on 15N present in organic matter fractions under two lowland soils / Influência de resíduos de culturas de verão sobre o 15 N em frações da matéria orgânica de dois solos de terras baixas

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 <53 μm. The soil with higher amount of clay+silt received a larger quantity of 15N from residues, while flooding of the soil after 180 days caused a loss of N added to the soil by the soybean and sorghum residues. In general, larger amounts of 15N were reported in the fraction <53 μm, associated with clay minerals, throughout the incubation period. These N losses should be considered in N fertilization for the following crops in rotation with flooded rice.(AU)

O Rio Grande do Sul possui cerca de 20% da área total de solos de terras baixas, propícias para o cultivo do arroz (Oryza sativa) irrigado por inundação. Buscando mitigar danos ocasionados com o monocultivo de arroz, novas culturas, como o sorgo (Sorghum bicolor) e a soja (Glycine max), têm sido cultivadas nestas áreas. Com diferentes qualidades de resíduos culturais, espera-se uma alteração na dinâmica da matéria orgânica do solo (MOS) e, consequentemente, do nitrogênio (N). O objetivo deste estudo foi avaliar a influência de resíduos culturais de arroz, soja e sorgo na quantidade de N em frações físicas da MOS de dois solos de terras baixas, utilizando a técnica de marcação isotópica de 15N das culturas e uma incubação por um ciclo de 180 dias em condição aeróbica e mais 180 dias em condição anaeróbica. Aos 30, 180 e 360 dias de incubação foi quantificado o N remanescente dos resíduos vegetais e destino do N proveniente dos resíduos nos dois solos em frações físicas da MOS >250 μm, 250-53 μm e <53 μm. O solo com maior quantidade de argila+silte recebeu maior quantidade de 15N dos resíduos, enquanto que o alagamento do solo após 180 dias de incubação ocasionou uma perda do N adicionado ao solo pelos resíduos de soja e sorgo. De maneira geral, foram encontradas maiores quantidades de 15N na fração <53 μm, associado aos argilominerais. As perdas de N devem ser consideradas na adubação nitrogenada para as culturas sequentes nos sistemas de rotação de culturas com arroz irrigado.(AU)

Influence of summer crop residues on 15N present in organic matter fractions under two lowland soils / Influência de resíduos de culturas de verão sobre o 15 N em frações da matéria orgânica de dois solos de terras baixas

ABSTRACT: 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 <53 μm. The soil with higher amount of clay+silt received a larger quantity of 15N from residues, while flooding of the soil after 180 days caused a loss of N added to the soil by the soybean and sorghum residues. In general, larger amounts of 15N were reported in the fraction <53 μm, associated with clay minerals, throughout the incubation period. These N losses should be considered in N fertilization for the following crops in rotation with flooded rice.

RESUMO: O Rio Grande do Sul possui cerca de 20% da área total de solos de terras baixas, propícias para o cultivo do arroz (Oryza sativa) irrigado por inundação. Buscando mitigar danos ocasionados com o monocultivo de arroz, novas culturas, como o sorgo (Sorghum bicolor) e a soja (Glycine max), têm sido cultivadas nestas áreas. Com diferentes qualidades de resíduos culturais, espera-se uma alteração na dinâmica da matéria orgânica do solo (MOS) e, consequentemente, do nitrogênio (N). O objetivo deste estudo foi avaliar a influência de resíduos culturais de arroz, soja e sorgo na quantidade de N em frações físicas da MOS de dois solos de terras baixas, utilizando a técnica de marcação isotópica de 15N das culturas e uma incubação por um ciclo de 180 dias em condição aeróbica e mais 180 dias em condição anaeróbica. Aos 30, 180 e 360 dias de incubação foi quantificado o N remanescente dos resíduos vegetais e destino do N proveniente dos resíduos nos dois solos em frações físicas da MOS >250 μm, 250-53 μm e <53 μm. O solo com maior quantidade de argila+silte recebeu maior quantidade de 15N dos resíduos, enquanto que o alagamento do solo após 180 dias de incubação ocasionou uma perda do N adicionado ao solo pelos resíduos de soja e sorgo. De maneira geral, foram encontradas maiores quantidades de 15N na fração <53 μm, associado aos argilominerais. As perdas de N devem ser consideradas na adubação nitrogenada para as culturas sequentes nos sistemas de rotação de culturas com arroz irrigado.

Vinasse application and cessation of burning in sugarcane management can have positive impact on soil carbon stocks.

Bioenergy crops, such as sugarcane, have the potential to mitigate greenhouse gas emissions through fossil fuel substitution. However, increased sugarcane propagation and recent management changes have raised concerns that these practices may deplete soil carbon (C) stocks, thereby limiting the net greenhouse gas benefit. In this study, we use both a measured and modelled approach to evaluate the impacts of two common sugarcane management practices on soil C sequestration potential in Brazil. We explore how transitions from conventional (mineral fertiliser/burning) to improved (vinasse application/unburned) practices influence soil C stocks in total and in physically fractionated soil down to one metre. Results suggest that vinasse application leads to an accumulation of soil C of 0.55 Mg ha-1yr-1 at 0-30 cm depth and applying unburned management led to gains of ∼0.7 Mg ha-1yr-1 at 30-60 cm depth. Soil C concentration in the Silt+Clay fraction of topsoil (0-20 cm) showed higher C content in unburned management but it did not differ under vinasse application. The CENTURY model was used to simulate the consequences of management changes beyond the temporal extent of the measurements. Simulations indicated that vinasse was not the key factor driving increases in soil C stocks but its application may be the most readily available practice to prevent the soil C losses under burned management. Furthermore, cessation of burning may increase topsoil C by 40% after ∼50 years. These are the first data comparing different sugarcane management transitions within a single area. Our findings indicate that both vinasse application and the cessation of burning can play an important role in reducing the time required for sugarcane ethanol production to reach a net C benefit (payback time).

Factor analysis of organic soils for site discrimination in a forensic setting.

Organic soils are generally located in fluvial settings such as river floodplains that are commonly used for the disposal of bodies. Therefore, the aim of this study was to provide a protocol for the analysis of small amounts of organic soils for forensic purposes. The protocol was applied in five representative sites within the Curitiba metropolitan region (Brazil), with each site supplying four composite samples separated from one another by 3m. The soil samples were collected at a depth of 0 to 5cm. One gram of soil sample was used to determine the total elemental content and perform physical fractionation of the soil (>53µm and <53µm). For both soil size fractions, total C and N contents were determined, and the elements adsorbed to organic matter was determined only for the <53µm size fraction (Na-pyrophosphate extraction). Chemometric multivariate analyses were conducted for the total data set, where more than 77% of the variation was explained by the first three factors. It was determined that Ca, Ba, and Mg adsorbed to organic matter, and total Ba, Ca, K, Mg, Mo, and C contents were most important in sample groupings. As expected in forensic science, the five sites were efficiently distinguishable from each other and the four replicates collected at the same individual site were clearly grouped. This protocol for sampling, chemical analysis, and data treatment of organic soils can be used in real crime situations.

Quantity and quality of soil organic matter as a sustainability index under different land uses in Eastern Amazon

Soil organic matter (SOM), which influences chemical, physical and biological soil attributes, is the main form of C found in the soil which can also be used as a soil sustainability index. The aim of this study was to use the quantity and quality of SOM as an indicator to determine the sustainability of different land uses (native vegetation, secondary vegetation, fruit orchards, horticultural areas, degraded pasture, improved pasture, and fields with annual crops) in the eastern Amazon. Improved pasture had higher soil C stock than the other land uses and was similar to the native vegetation, and also presented the highest quantity of C in a stable form in the soil (fraction < 53 μm). According to the C management index, improved pasture is the most similar in use to native vegetation. Changes in land use reduced the soil microbial C content, although the more conservationist systems (fruit orchards, secondary vegetation, and improved pasture) had contents similar to those of the native vegetation. The highest soil microbial quotients were found in fruit orchards and horticultural areas. Well-managed pastures were effective in accumulating C as stable forms in the soil, which demonstrates the sustainability of this land use in the region studied.

Quantity and quality of soil organic matter as a sustainability index under different land uses in Eastern Amazon

Soil organic matter (SOM), which influences chemical, physical and biological soil attributes, is the main form of C found in the soil which can also be used as a soil sustainability index. The aim of this study was to use the quantity and quality of SOM as an indicator to determine the sustainability of different land uses (native vegetation, secondary vegetation, fruit orchards, horticultural areas, degraded pasture, improved pasture, and fields with annual crops) in the eastern Amazon. Improved pasture had higher soil C stock than the other land uses and was similar to the native vegetation, and also presented the highest quantity of C in a stable form in the soil (fraction < 53 μm). According to the C management index, improved pasture is the most similar in use to native vegetation. Changes in land use reduced the soil microbial C content, although the more conservationist systems (fruit orchards, secondary vegetation, and improved pasture) had contents similar to those of the native vegetation. The highest soil microbial quotients were found in fruit orchards and horticultural areas. Well-managed pastures were effective in accumulating C as stable forms in the soil, which demonstrates the sustainability of this land use in the region studied.(AU)

Compartments of organic matter in an Oxisol under different types of no-tillage systems / Compartimentos da matéria orgânica de um Latossolo sob diferentes tipos de semeadura direta

The objective of this study was to evaluate the changes in soil carbon (C) as well as its fractions ofphysical (particulate organic carbon associated with minerals), chemical (fulvic and humic acids andhumins), and oxidizable (F1, F2, F3, and F4) forms of an Oxisol under no-tillage systems (SPD) underdifferent successions and crop rotations in the Cerrado area. In order to do this, the areas of consolidatedSPD (10 to 20 years) under the same soil and climatic conditions were selected in Montividiu (GO),with soybean-corn succession (SSM), soybean-millet succession (SSMt), soybean-millet-bean-cottonrotation (RSMFA), crop-livestock integration (ILP), and a native cerrado (CE) area used as a controltreatment In each area, samples were collected at depths of 0.0-0.05, 0.05-0.10, 0.10-0.20, and 0.20-0.40 m, respectively. The no-tillage system with crop rotation, followed by no-tillage system withcrop-livestock integration, were the ones that presented the greatest potential to increase total carboncontent in soil, presenting higher C contents of the granulometric, humic, and oxidizable fractions ofthe soil organic matter (SOM), compared with the other areas of no-tillage systems with succession ofthe respective evaluated crops. The no-tillage areas with corn and soybean/millet succession systemsshowed a similar pattern in soil C accumulation, as well as in the compartments of SOM evaluated.(AU)

O objetivo deste estudo foi avaliar as alterações no carbono do solo, bem como, suas frações físicas(carbono orgânico particulado e associado aos minerais), químicas (ácido fúlvico, húmico e humina),e oxidáveis (F1, F2, F3 e F4) de um Latossolo Vermelho em áreas de sistema de plantio direto (SPD)sob diferentes sucessões e rotações de culturas no cerrado goiano. Para isso, em Montividiu (GO)foram selecionadas áreas de SPD consolidado (10 a 20 anos) sob mesmas condições edafoclimáticascom: sucessão soja-milho (SSM), sucessão soja-milheto (SSMt), rotação soja-milho-milheto-feijãoalgodão(RSMFA), integração lavoura-pecuária (ILP) e uma área de cerrado nativo (CE) utilizada comoreferência. Em cada uma das áreas, foram coletadas amostras nas profundidades de 0,0-0,05; 0,05-0,10; 0,10-0,20; 0,20-0,40 m. O sistema de plantio direto com rotação de culturas seguido do sistemade plantio direto com integração lavoura-pecuária foram aqueles que apresentaram maior potencialde aumento do carbono total do solo, apresentando maiores teores de C das frações granulométricas,húmicas e oxidáveis da MOS (matéria orgânica do solo) em relação as demais áreas de sistema deplantio direto com sucessão de culturas avaliadas. As áreas de sistema de plantio direto com sucessãosoja/milho e soja/milheto exibiram padrão semelhante no acúmulo de C do solo, bem como, nos compartimentos da MOS avaliados.(AU)

Compartments of organic matter in an Oxisol under different types of no-tillage systems / Compartimentos da matéria orgânica de um Latossolo sob diferentes tipos de semeadura direta

The objective of this study was to evaluate the changes in soil carbon (C) as well as its fractions ofphysical (particulate organic carbon associated with minerals), chemical (fulvic and humic acids andhumins), and oxidizable (F1, F2, F3, and F4) forms of an Oxisol under no-tillage systems (SPD) underdifferent successions and crop rotations in the Cerrado area. In order to do this, the areas of consolidatedSPD (10 to 20 years) under the same soil and climatic conditions were selected in Montividiu (GO),with soybean-corn succession (SSM), soybean-millet succession (SSMt), soybean-millet-bean-cottonrotation (RSMFA), crop-livestock integration (ILP), and a native cerrado (CE) area used as a controltreatment In each area, samples were collected at depths of 0.0-0.05, 0.05-0.10, 0.10-0.20, and 0.20-0.40 m, respectively. The no-tillage system with crop rotation, followed by no-tillage system withcrop-livestock integration, were the ones that presented the greatest potential to increase total carboncontent in soil, presenting higher C contents of the granulometric, humic, and oxidizable fractions ofthe soil organic matter (SOM), compared with the other areas of no-tillage systems with succession ofthe respective evaluated crops. The no-tillage areas with corn and soybean/millet succession systemsshowed a similar pattern in soil C accumulation, as well as in the compartments of SOM evaluated.

O objetivo deste estudo foi avaliar as alterações no carbono do solo, bem como, suas frações físicas(carbono orgânico particulado e associado aos minerais), químicas (ácido fúlvico, húmico e humina),e oxidáveis (F1, F2, F3 e F4) de um Latossolo Vermelho em áreas de sistema de plantio direto (SPD)sob diferentes sucessões e rotações de culturas no cerrado goiano. Para isso, em Montividiu (GO)foram selecionadas áreas de SPD consolidado (10 a 20 anos) sob mesmas condições edafoclimáticascom: sucessão soja-milho (SSM), sucessão soja-milheto (SSMt), rotação soja-milho-milheto-feijãoalgodão(RSMFA), integração lavoura-pecuária (ILP) e uma área de cerrado nativo (CE) utilizada comoreferência. Em cada uma das áreas, foram coletadas amostras nas profundidades de 0,0-0,05; 0,05-0,10; 0,10-0,20; 0,20-0,40 m. O sistema de plantio direto com rotação de culturas seguido do sistemade plantio direto com integração lavoura-pecuária foram aqueles que apresentaram maior potencialde aumento do carbono total do solo, apresentando maiores teores de C das frações granulométricas,húmicas e oxidáveis da MOS (matéria orgânica do solo) em relação as demais áreas de sistema deplantio direto com sucessão de culturas avaliadas. As áreas de sistema de plantio direto com sucessãosoja/milho e soja/milheto exibiram padrão semelhante no acúmulo de C do solo, bem como, nos compartimentos da MOS avaliados.

Effect of Soil Water Content on the Distribution of Diuron into Organomineral Aggregates of Highly Weathered Tropical Soils.

We evaluated the effects of soil water content on the retention of diuron and its residual distribution into organomineral aggregates in four Brazilian oxisols. (14)C-Diuron was incubated for days at 25, 50, and 75% of maximum water-holding capacity for each soil. After 42 days, the physical fractionation method was used to obtain >150, 53-150, 20-53, 2-20, and <2 µm aggregate sizes. Diuron retention increased with increasing soil water content for all soils. At lower soil water content, diuron's retention was higher in the sandier soil. It was mostly retained in the fine (<20 µm) aggregates of sandier soil, and for clayed soils, retention was higher in the coarse aggregates (>53 µm). The sorption coefficients (Kd and Koc) generated by batch studies should be carefully used because they do not provide information about aggregation and diffusion effects on pesticides soil sorption.

Organomineral Interactions and Herbicide Sorption in Brazilian Tropical and Subtropical Oxisols under No-Tillage.

We evaluated the effects of the soil organic matter (SOM) composition, distribution between soil aggregates size, and their interactions with the mineral phase on herbicide sorption (alachlor, bentazon, and imazethapyr) in tropical and subtropical Oxisols under no-till systems (NT). Using soil physical fractionation approach, sorption experiments were performed on whole soils and their aggregates. SOM chemistry was assessed by CP/MAS (13)C NMR. The lower sorption observed in tropical soils was attributed to the greater blockage of SOM sorption sites than in subtropical soils. When these sites were exposed upon physical fractionation, sorption of the three herbicides in tropical soils increased, especially for imazethapyr. High amounts of poorly crystallized sesquioxides in these soils may have contributed to masking of sorption sites, indicating that organomineral interactions may lead to blockage of sorption sites on SOM in tropical soils.

Carbon sequestration in clay and silt fractions of Brazilian soils under conventional and no-tillage systems

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 ( 53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.

Carbon sequestration in clay and silt fractions of Brazilian soils under conventional and no-tillage systems

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 ( 53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.(AU)

Carbon stocks in organic matter fractions as affected by land use and soil management, with emphasis on no-tillage effect

Land use and soil management may affect both labile and humified soil organic matter (SOM) fractions, but the magnitude of these changes is poorly known in subtropical environments. This study investigated effects of four land use and soil management systems (forest, native pasture, and conventional tillage and no-tillage in a wheat/soybean succession) on (i) total soil organic carbon (SOC) stocks (0 to 250mm depth) and on (ii) carbon (C) stocks in labile (coarse, light) and humified (mineral-associated, humic substances) SOM fractions (0 to 25mm depth), in a Hapludox soil from southern Brazil. In comparison to the adjacent forest site, conventionally tilled soil presented 36% (46.2Mg ha-1) less SOC in the 0 to 250mm depth and a widespread decrease in C stocks in all SOM fractions in the 0 to 25mm depth. The coarse (>53 mum) and light ( 1kg dm-3) SOM fractions were the most affected under no-tillage, showing 393% (1.22Mg C ha-1) and 289% (0.55Mg C ha-1) increases, respectively, in relation to conventional tillage. Similar results were observed for mineral-associated SOM and humic substance C pools (34% and 38% increases, respectively) under no-tillage. Compared with labile SOM fraction results, the percentual increments on C stocks in humified fractions were smaller; but in absolute terms this C pool yielded the highest increases (3.06 and 2.95Mg C ha-1, respectively). These results showed that both labile and humified organic matter are better protected under the no-tillage system, and consequently less vulnerable to mineralization. Humified SOM stabilization process involving interactions with variable charge minerals is probably important in maintaining and restoring soil and environmental quality in tropical and subtropical regions.

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 mim) e leve ( 1kg dm-3) da MO foram as mais afetadas pelo sistema plantio direto, com incrementos no estoque de C de 393% (1,22Mg ha-1) e 289% (0,55Mg ha-1), respectivamente, em relação ao preparo convencional. Os estoques de C na MO associada aos minerais e nas substâncias húmicas aumentaram de forma semelhante (incrementos de 34% e 38%, respectivamente) no sistema plantio direto. Apesar dos aumentos percentualmente menores no estoque de C nas frações humificadas do que nas lábeis, em termos absolutos, os maiores incrementos ocorreram na matéria orgânica associada aos minerais e nas substâncias húmicas (3,06 e 2.95Mg C ha-1, respectivamente). O sistema plantio direto resultou num ambiente biologicamente menos oxidativo, favorável à preservação das frações lábeis e humificadas da MO. O processo de estabilização da MO pela sua interação com minerais de carga variável é provavelmente um fator fundamental na manutenção e recuperação da qualidade do solo e do ambiente em regiões tropicais e subtropicais.

Carbon stocks in organic matter fractions as affected by land use and soil management, with emphasis on no-tillage effect

Land use and soil management may affect both labile and humified soil organic matter (SOM) fractions, but the magnitude of these changes is poorly known in subtropical environments. This study investigated effects of four land use and soil management systems (forest, native pasture, and conventional tillage and no-tillage in a wheat/soybean succession) on (i) total soil organic carbon (SOC) stocks (0 to 250mm depth) and on (ii) carbon (C) stocks in labile (coarse, light) and humified (mineral-associated, humic substances) SOM fractions (0 to 25mm depth), in a Hapludox soil from southern Brazil. In comparison to the adjacent forest site, conventionally tilled soil presented 36% (46.2Mg ha-1) less SOC in the 0 to 250mm depth and a widespread decrease in C stocks in all SOM fractions in the 0 to 25mm depth. The coarse (>53 mum) and light ( 1kg dm-3) SOM fractions were the most affected under no-tillage, showing 393% (1.22Mg C ha-1) and 289% (0.55Mg C ha-1) increases, respectively, in relation to conventional tillage. Similar results were observed for mineral-associated SOM and humic substance C pools (34% and 38% increases, respectively) under no-tillage. Compared with labile SOM fraction results, the percentual increments on C stocks in humified fractions were smaller; but in absolute terms this C pool yielded the highest increases (3.06 and 2.95Mg C ha-1, respectively). These results showed that both labile and humified organic matter are better protected under the no-tillage system, and consequently less vulnerable to mineralization. Humified SOM stabilization process involving interactions with variable charge minerals is probably important in maintaining and restoring soil and environmental quality in tropical and subtropical regions.

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 mim) e leve ( 1kg dm-3) da MO foram as mais afetadas pelo sistema plantio direto, com incrementos no estoque de C de 393% (1,22Mg ha-1) e 289% (0,55Mg ha-1), respectivamente, em relação ao preparo convencional. Os estoques de C na MO associada aos minerais e nas substâncias húmicas aumentaram de forma semelhante (incrementos de 34% e 38%, respectivamente) no sistema plantio direto. Apesar dos aumentos percentualmente menores no estoque de C nas frações humificadas do que nas lábeis, em termos absolutos, os maiores incrementos ocorreram na matéria orgânica associada aos minerais e nas substâncias húmicas (3,06 e 2.95Mg C ha-1, respectivamente). O sistema plantio direto resultou num ambiente biologicamente menos oxidativo, favorável à preservação das frações lábeis e humificadas da MO. O processo de estabilização da MO pela sua interação com minerais de carga variável é provavelmente um fator fundamental na manutenção e recuperação da qualidade do solo e do ambiente em regiões tropicais e subtropicais.