RESUMOO nitrogênio, na maioria das situações, é o nutriente que mais influencia o rendimento do milho. O manejo da adubação nitrogenada deve satisfazer o requerimento da cultura com o mínimo de risco ambiental. Para tanto, é necessário que a recomendação da dose de adubo nitrogenado seja a mais exata possível. A generalização do uso do sistema de plantio direto e culturas de cobertura, no Sul do Brasil, criou a necessidade de ser a recomendação da adubação nitrogenada adaptada a este novo cenário agrícola. O presente trabalho, além de considerar o teor de MO e a expectativa do rendimento de grãos de milho na recomendação da adubação nitrogenada conforme preconiza a CFS-RS/SC (1995), propõe a introdução de um terceiro parâmetro que é a contribuição em N das culturas de cobertura antecedente. O efeito das culturas de cobertura foi considerado em três situações: leguminosas em cultivo solteiro, gramíneas em cultivo solteiro e consorciações. No caso de leguminosas e gramíneas em cultivo solteiro, a influência na disponibilidade de N foi considerada com base na produção de matéria seca, enquanto, nas consorciações, a proporção da leguminosa foi o principal fator considerado. A recomendação de adubação apresentada neste trabalho não dispensa acompanhamento de campo, visando a ajustes que se fizerem necessários, especialmente porque sistemas de produção, baseados em culturas de cobertura, dependem de processos biológicos influenciados por condições de clima, manejo e solo, que devem ser acompanhados localmente.Termos de indexação: fertilidade do solo, adubos verdes, decomposição de resíduos, Zea mays.

Conservation agriculture can provide a low-cost competitive option to mitigate global warming with reduction or elimination of soil tillage and increase soil organic carbon (SOC). Most studies have evaluated the impact of zero till (ZT) only on surface soil layers (down to 30 cm), and few studies have been performed on the potential for C accumulation in deeper layers (0-100 cm) of tropical and subtropical soils. In order to determine whether the change from conventional tillage (CT) to ZT has induced a net gain in SOC, three long-term experiments (15-26 years) on free-draining Ferralsols in the subtropical region of South Brazil were sampled and the SOC stocks to 30 and 100 cm calculated on an equivalent soil mass basis. In rotations containing intercropped or cover-crop legumes, there were significant accumulations of SOC in ZT soils varying from 5 to 8 Mg ha À1 in comparison with CT management, equivalent to annual soil C accumulation rates of between 0.04 and 0.88 Mg ha À1 . However, the potential for soil C accumulation was considerably increased (varying from 0.48 to 1.53 Mg ha À1 yr À1 ) when considering the soil profile down to 100 cm depth. On average the estimate of soil C accumulation to 100 cm depth was 59% greater than that for soil C accumulated to 30 cm. These findings suggest that increasing sampling depth from 30 cm (as presently recommended by the IPCC) to 100 cm, may increase substantially the estimates of potential CO 2 mitigation induced by the change from CT to ZT on the free-draining Ferralsols of the tropics and subtropics. It was evident that that legumes which contributed a net input of biologically fixed N played an important role in promoting soil C accumulation in these soils under ZT, perhaps due to a slow-release of N from decaying surface residues/roots which favored maize root growth.

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( 15, 29, 43, 59, 71, 82, 112, and 182 days. Remaining dry matter amounts (DM)

Avaliou-se a produção de matéria seca (MS), bem como as concentrações de nitrogênio, fósforo, potássio e carbono do tecido vegetal. O cultivo consorciado de plantas de cobertura proporcionou produção de matéria seca estatisticamente semelhante àquela da aveia e do nabo em culturas isoladas e superior à da ervilhaca. O acúmulo de N na fitomassa dos tratamentos envolvendo consórcio de aveia + ervilhaca não diferiu daquele da ervilhaca isolada e, na média dos três anos, foi superior ao da aveia isolada em 32 kg ha -1 de N. Consorciando aveia + ervilhaca, houve um aumento médio de 67 % na relação C/N da fitomassa, em relação à ervilhaca. As plantas de cobertura proporcionaram maior produção de MS e foram mais eficientes no acúmulo de N, P e K do que a vegetação espontânea do pousio invernal. Os resultados indicaram que o consórcio de aveia + ervilhaca e de aveia + nabo no outono/inverno proporcionou maior

When incorporated in soil, plant residues and their decomposition products are in close contact with mineral particles with which they can be bound to form aggregates. We measured the incorporation of carbon (C) and nitrogen (N) derived from crop residues in water-stable aggregate fractions of a silty soil in a field experiment in Northern France using 13C15N-labelled wheat straw (Triticum aestivurn L.). Soil samples were taken seven times for 18 months and separated into slaking-resistant aggregate size fractions which were analysed for total C and N contents, and I3C and I5N enrichments. During the early stages of decomposition (approximately 200 days), the enrichment of I3C increased rapidly in the macroaggregates (> 250 pm) but decreased thereafter.The macroaggregates represented only < 20% of the soil mass and at any one time, they accounted for <25% of the residual I3C in the soil. The proportion of I3C recovered in the <50-pm and 50-250-pm fractions increased during decomposition of the residues; at day 574, the 50-250-pm fraction accounted for close to 50% of the residual I3C. A greater proportion of "N than I3C was recovered in the <50-pm fraction. The results indicate that during decomposition in soil, C and N from crop residues become rapidly associated with stable aggregates. In this silty soil the 50-250-pm stable aggregates appear to be involved in the storage and stabilization of C from residues.

RESUMO

SummaryIncorporating straw into the ploughed layer of soil affects the dynamics of carbon and nitrogen. A precise quantification of its short-term effect in agricultural fields is difficult because biological and physical processes interact and take place simultaneously. As an alternative to experimentation, investigations have turned to simulations using mechanistic models, and we have taken this approach. The goal of our study was to test a mechanistic and one-dimensional model of transport and biotransformation (PASTIS) against a data set obtained in a field experiment in northern France. We tested carbon and nitrogen dynamics by measuring C mineralization rates, the rates of gross immobilization and mineralization of N (using 15 N tracing), and inorganic pools of N in the soil profile during 1 year in a bare soil with or without addition of wheat straw. Most of the model parameters were determined in independent experiments. We estimated the biological parameters from incubation experiments in the laboratory. The simulated results were in good agreement with experimental data, particularly for gross N rates. Hypotheses concerning the pathway of microbial assimilation and the dependence of decomposition on the size of the biomass were tested. The simulated net N immobilization due to addition of straw (8000 kg dry matter ha À1) reached a maximum of 64 kg N ha À1 after 2 months, whereas the observed value was 66 kg N ha À1. The model indicated that after 13 months the incorporation of straw had reduced the net amount of nitrogen mineralized by 13% and the amount of leached nitrate by 27%. The sensitivity analysis to the depth of straw incorporation indicated that the deeper was the incorporation the less was the leaching and the mineralization of nitrogen.