The area under no-till (NT) in Brazil reached 22 million ha in 2004-2005, of which approximately 45% was located in the southern states. From the 1970s to the mid-1980s, this region was a source of carbon dioxide to the atmosphere due to decrease of soil carbon (C) stocks and high consumption of fuel by intensive tillage. Since then, NT has partially restored the soil C lost and reduced the consumption of fossil fuels. To assess the potential of C accumulation in NT soils, four long-term experiments (7-19 yr) in subtropical soils (Paleudult, Paleudalf, and Hapludox) varying in soil texture (87-760 g kg(-1) of clay) in agroecologic southern Brazil zones (central region, northwest basaltic plateau in Rio Grande Sul, and west basaltic plateau in Santa Catarina) and with different cropping systems (soybean and maize) were investigated. The lability of soil organic matter (SOM) was calculated as the ratio of total organic carbon (TOC) to particulate organic carbon (POC), and the role of physical protection on stability of SOM was evaluated. In general, TOC and POC stocks in native grass correlated closely with clay content. Conversely, there was no clear effect of soil texture on C accumulation rates in NT soils, which ranged from 0.12 to 0.59 Mg ha(-1) yr(-1). The C accumulation was higher in NT than in conventional-till (CT) soils. The legume cover crops pigeon pea [Cajanus cajan (L.) Millsp] and velvet beans (Stizolobium cinereum Piper & Tracy) in NT maize cropping systems had the highest C accumulation rates (0.38-0.59 Mg ha(-1) yr(-1)). The intensive cropping systems also were effective in increasing the C accumulation rates in NT soils (0.25-0.34 Mg ha(-1) yr(-1)) when compared to the double-crop system used by farmers. These results stress the role of N fixation in improving the tropical and subtropical cropping systems. The physical protection of SOM within soil aggregates was an important mechanism of C accumulation in the sandy clay loam Paleudult under NT. The cropping system and NT effects on C stocks were attributed to an increase in the lability of SOM, as evidenced by the higher POC to TOC ratio, which is very important to C and energy flux through the soil.

Soil organic matter (SOM) plays a crucial role in soil quality and can act as an atmospheric C-CO 2 sink under conservationist management systems. This study aimed to evaluate the long-term effects (19 years) of tillage (CT-conventional tillage and NT-no tillage) and crop rotations (R0-monoculture system, R1-winter crop rotation, and R2-intensive crop rotation) on total, particulate and mineralassociated organic carbon (C) stocks of an originally degraded Red Oxisol in Cruz Alta, RS, Southern Brazil. The climate is humid subtropical Cfa 2a (Köppen classification), the mean annual precipitation 1,774 mm and mean annual temperature 19.2 o C. The plots were divided into four segments, of which each was sampled in the layers 0-0.05, 0.05-0.10, 0.10-0.20, and 0.20-0.30 m. Sampling was performed manually by opening small trenches. The SOM pools were determined by physical fractionation. Soil C stocks had a linear relationship with annual crop C inputs, regardless of the tillage systems. Thus, soil disturbance had a minor effect on SOM turnover. In the 0-0.30 m layer, soil C sequestration ranged from 0 to 0.51 Mg ha -1 yr -1 , using the CT R0 treatment as base-line; crop rotation systems had more influence on soil stock C than tillage systems. The mean C sequestration rate of the cropping systems was 0.13 Mg ha -1 yr -1 higher in NT than CT. This result was associated to the higher C input by crops due to the improvement in soil quality under long-term no-tillage. The particulate C fraction Brazil. was a sensitive indicator of soil management quality, while mineral-associated organic C was the main pool of atmospheric C fixed in this clayey Oxisol. The C retention in this stable SOM fraction accounts for 81 and 89 % of total C sequestration in the treatments NT R1 and NT R2, respectively, in relation to the same cropping systems under CT. The highest C management index was observed in NT R2, confirming the capacity of this soil management practice to improve the soil C stock qualitatively in relation to CT R0. The results highlighted the diversification of crop rotation with cover crops as a crucial strategy for atmospheric C-CO 2 sequestration and SOM quality improvement in highly weathered subtropical Oxisols.Index terms: carbon sequestration, no-tillage, conventional tillage, soil management. 0-0,05, 0,05-0,10, 0,10-0,20 e 0,20-0,30 RESUMO: ESTOQUE E COMPARTIMENTOS DE CARBONO EM UM LATOSSOLO VERMELHO SUBTROPICAL SOB DIFERENTES SISTEMAS DE LONGO PRAZO DE PREPARO E ROTAÇÃO DE CULTURAS A matéria orgânica do solo (MOS) desempenha papel relevante na qualidade do solo e pode atuar como dreno de C-CO 2 atmosférico em solos sob sistemas de manejo conservacionista. Este estudo foi realizado com o objetivo de investigar o efeito de longo prazo (19 anos) de sistemas de preparo do solo (PC-preparo convencional e PD-plantio direto) e de rotação de culturas (R0-sucessão de monoculturas, R1-rotação de culturas de inverno e R2-rotação intensiva de culturas) no estoque de carbono (C) orgânico total, particulado e associad...

RESUMOO impacto de plantas de inverno sobre a estabilidade estrutural do solo, antecedendo a cultura do milho sob plantio direto, foi avaliado em Podzólico Vermelho-Amarelo. O experimento foi realizado no campus da Universidade Federal de Santa Maria, de maio de 1991 a maio de 1992. Os tratamentos utilizados constaram de: chícharo (Lathyrus sativus L.), tremoço azul (Lupinus angustifolius L.), ervilhaca (Vicia sativa L.), aveia preta (Avena strigosa Schieb) e pousio invernal. Em cada parcela, foram coletadas mensalmente, desde a implantação das plantas de inverno até a colheita do milho, totalizando treze coletas, amostras de solo parcialmente deformadas e indeformadas para as determinações. Foram usadas duas subamostras na profundidade de 0-5 cm, com pá de corte, compondo uma amostra para análise de agregados e carbono orgânico e cinco subamostras, na mesma profundidade, com uso de cilindro volumétrico, compondo amostra para análise da atividade microbiana e umidade do solo. As plantas de cobertura induziram variação temporal da estabilidade dos agregados no período de estudo. A aveia preta atingiu maiores valores de estabilidade estrutural durante o ciclo das culturas de inverno, enquanto o tremoço azul maiores valores durante o ciclo do milho. Isso pode ser atribuído ao sistema radicular da gramínea e à taxa de decomposição da leguminosa, criando ambiente favorável à agregação, pela ação de raízes, cobertura do solo, fornecimento de material orgânico e conservação da umidade favoráveis à ação de microrganismos. Tais fatores, provavelmente, favoreceram a formação e conservação dos agregados do solo. Termos de indexação: variabilidade temporal da agregação, Lathyrus sativus,Lupinus angustifolius, Vicia sativa, Avena strigosa, milho, plantio direto.

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SUMMARYAgricultural soils can act as a source or sink of atmospheric C, according to the soil management. This long-term experiment (22 years) was evaluated during 30 days in autumn, to quantify the effect of tillage systems (conventional tillage-CT and no-till-NT) on the soil CO 2 -C flux in a Rhodic Hapludox in Rio Grande do Sul State, Southern Brazil. A closed-dynamic system (Flux Chamber 6400-09, Licor) and a static system (alkali absorption) were used to measure soil CO 2 -C flux immediately after soybean harvest. Soil temperature and soil moisture were measured simultaneously with CO 2 -C flux, by Licor-6400 soil temperature probe and manual TDR, respectively. During the entire month, a CO 2 -C emission of less than 30 % of the C input through soybean crop residues was estimated. In the mean of a 30 day period, the CO 2 -C flux in NT soil was similar to CT, independent of the chamber type used for measurements. Differences in tillage systems with dynamic chamber were verified only in short term (daily evaluation), where NT had higher CO 2 -C flux than CT at the beginning of the evaluation period and lower flux at the end. The dynamic chamber was more efficient than the static chamber in capturing variations in CO 2 -C flux as a function of abiotic factors. In this chamber, the soil temperature and the water-filled pore space (WFPS), in the NT soil, explained 83 and 62 % of CO 2 -C flux, respectively. The Q 10 factor, which evaluates CO 2 -C flux dependence on soil temperature, was estimated as 3.93, suggesting a high sensitivity of the biological activity to changes in soil temperature during fall season. The CO 2 -C flux measured in a closed dynamic chamber was correlated with the static alkali adsorption chamber only in the NT system, although the values were underestimated in comparison to the other, particularly in the case of high flux values. At low soil temperature and WFPS conditions, soil tillage caused a limited increase in soil CO 2 -C flux.Index terms: no-till, greenhouse gases, soil temperature, soil moisture. RESUMO: EMISSÃO DE DIÓXIDO DE CARBONO EM LATOSSOLO VERMELHO ALTERADA POR SISTEMAS DE PREPARO NO SUL DO BRASILOs solos agrícolas podem atuar como dreno ou fonte de C atmosférico, dependendo do sistema de manejo adotado. Este estudo foi desenvolvido em experimento de longa duração (22 anos), durante o período de 30 dias do outono, com o objetivo de avaliar o impacto de sistemas de preparo de solo (preparo convencional-PC e plantio direto-PD) nas emissões de C-CO 2 de um Latossolo Vermelho distrófico, em Cruz Alta, RS. As emissões de C-CO 2 do solo foram avaliadas com câmaras dinâmica (Flux Chamber 6400-09, Licor) e estática (com captação em solução alcalina), imediatamente após a colheita da soja. A temperatura e a umidade do solo foram registradas, concomitantemente com as emissões de C-CO 2 , por meio de sensor de temperatura e TDR manual, respectivamente, integrantes do Licor-6400. Estimouse que, em 30 dias, uma quantidade equivalente a menos de 30 % do C aportado pelos resíduos de soj...

Com o objetivo de avaliar o inoculante "Graminante", foram realizados, na safra 1996/97, dois experimentos com a cultura de milho, em área experimental localizada na FUNDACEP FECOTRIGO, Cruz Alta, RS, conduzidos no sistema plantio direto. O produto comercial foi testado individualmente e associado à adubação nitrogenada. Os tratamentos foram: l) testemunha sem inoculação e sem fertilizante nitrogenado (N); 2) com "Graminante"; 3) com N na semeadura e em cobertura: 4) com "Graminante" + N na semeadura e em cobertura; 5) com "Graminante" + N na semeadura e, 6) com "Graminante" + N em cobertura. Foram avaliados o número de plantas e de espigas, a estatura de plantas e o rendimento de grãos. O produto comercial "Graminante " não apresentou resposta na cultura de milho.

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