Variações nos estoques de carbono e emissões de gases de efeito estufa em solos das regiões tropicais e subtropicais do Brasil: uma análise crítica.

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Estimativa da difusividade de N2O no solo pela Lei de Fick utilizando os dados de fluxo e perfil de concentração de CO2 no solo.

Fluxo a partir da câmara estática fechada e amostragem de gases do perfil do solo; Estimativa da difusividade; Cálculo do fluxo e produção de N2O pela técnica de concentração no perfil do solo; Análises estatísticas. 0 objetivo do presente trabalho foi testar um novo método para avaliar a difusividade de gases do solo para estudos com N2O, baseado em medidas dos fluxos de CO2 para a atmosfera e de concentray80 de CO2 no perfil do solo. A hip6tese foi a de que 0 fluxo de N2O, calculado com as concentrações de N20 no solo e difusividade, não deveria ser diferente do fluxo de N20 estimado por uma câmara estática fechada. 0 estudo foi feito em condições controladas, em um cilindro de 1 m de diâmetro x 0,5 m de altura, preenchido ate 0,3 m com solo seco ao ar e peneirado, com peneira de 2 mm. Um sistema para amostragem de gases foi montado nas profundidades de 5 e 15 cm. Uma câmara estática foi montada sobre 0 mesmo ponto para avaliação de fluxo de gás. As amostras de gases nas diferentes profundidades do solo foram retiradas durante um período de 41 dias, sempre no mesmo horário em que se fazia a medida do fluxo de gases para a atmosfera. Cada amostra era analisada para as concentrações de CO2 e N20 por cromatografia gasosa. Pela proposta do trabalho, a difusividade foi calculada utilizando a Lei de Fick, sendo as variáveis, gradiente de concentray80 na camada de solo e fluxo do gás, determinadas com base nas analises de CO2. Também se estimou a difusividade pelo modelo de Marshall, baseado na tortuosidade, estabelecida pela umidade e porosidade do solo em cada camada. Os resultados mostraram que os fluxos de N2O, calculados usando a difusividade obtida pelo método do CO2, tendo como referencia a camada de solo de 0-5 cm, ficaram semelhantes aos medidos com a câmara estática. Considerando a camada de 0-15 cm, os fluxos calculados foram 40% superiores ao da câmara estática. Os fluxos de N20 calculados usando a difusividade pelo modelo de Marshall foram 5 a 9 vezes maiores que 0 medido com a câmara estática. Conclui-se que 0 método do CO2 permite estimativas mais reais da difusividade do N20 no solo, sendo uma opção mais simples e rápida do que aquelas baseadas no radioisótopo, como 0 Rn222. The present work aimed to evaluate a new method to estimate the soil diffusivity of gases to be used in studies of N2O, based on measurements of soil CO2 efflux and the soil CO2 profile. The hypothesis was the soil N20 efflux, estimated from the soil N20 concentrations and diffusivity would not be different from the N20 efflux estimated by using a closed static chamber. The study was carried out under controlled conditions in a cylinder of 1 m diameter by 0.5 m height, filled in with air dried 2 mm sieved soil up to 0.3 m. A soil air sampling system was built at 5 and 15 cm depth. A closed static chamber was in the soil surface at the same place for gas efflux evaluation. Gas sampling from the different soil depths were taken during 41 days, always at the same time of gas efflux measurement. Each gas sample was analyzed for CO2 and N20 concentration by gas chromatography. According to the proposal, gas diffusivity was calculated using the Fick's Law, and the variables gas concentration gradient of each layer and gas efflux determined from the analysis of CO2. The diffusivity was also estimated by the Marshall's model, taking into account the soil tortuosity, calculated using soil moisture and porosity of each layer. The results have shown that the N20 fluxes calculated from the diffusivity obtained from CO2 data from the 0-5cm soil layer were similar to the fluxes measured using the static chamber. Considering the 0-15 cm soil layer the fluxes were 40% greater than that obtained with the static chamber. Fluxes of N20 calculated using diffusivity from Marshall's model were almost five to nine times greater than the fluxes measured with static chamber. It can be concluded that the method using CO2 as a reference gas allows more reliable estimates of N20 diffusivity in soil, being a simfller and faster option than the ones based on radioisotopes such as Rn 222bitstream/CNPAB-2010/34892/1/bot025.pd

Changes in soil organic carbon during 22 years of pastures, cropping or integrated crop/livestock systems in the Brazilian Cerrado.

Abstract: In Brazil?s central savanna region, government policy is to encourage the conversion of conventional plough tillage (PT) agriculture to no-till (NT) and raise the productivity of under-utilized pastures, including their conversion to integrated crop-livestock (ICL) systems, with the objective of increasing soil organic carbon (SOC) at the expense of atmospheric carbon dioxide. An experiment was established in 1991 by liming and fertilizing at two levels an area of native vegetation (NV). The treatments, replicated in randomized plots, included pastures, continuous cropping and ICL systems under PT or NT. The aim of this study was to quantify the SOC accumulation to 100 cm depth under these treatments over time. The high C:N ratios suggested that there was a high proportion of charcoal present in the soil. Increasing fertilizer inputs had no overall significant effect on SOC stocks. Stocks of SOC changed little under pastures. Analyses of ¹³C abundance showed that higher fertilizer inputs increased the decomposition rate of C derived from NV under pure grass pastures. Continuous cropping under NT preserved SOC and under PT there were significant losses. The highest SOC stocks were found under ILP treatments, but not all ILP treatments accumulated SOC even under NT. These results indicate that government initiatives to substitute PT with NT and to intensify beef cattle production will have only modest short-term gains in SOC accumulation