Impacto da adubação verde no acúmulo de carbono e nitrogênio no solo em rotações de culturas em sistema plantio direto e convencional

Abstract

The productivity of crops has been provided by the use of substantial amounts of industrial chemical fertilizers, especially nitrogen. In systems of crop rotations managed under no-tillage it has been shown that the inclusion of a leguminous green manure may replace partially, or sometimes entirely, nitrogen fertilizer subsequently used for maize. It is likely that the presence of a legume results in an increase in soil C stocks. The main objectives of this study were to evaluate the nitrogen balance in different crop rotations with and without the inclusion of a legume used as green manure in no-tillage and the consequences for storage of carbon and nitrogen in soil in long-term experiments. The first experiment was carried out at Embrapa Soja, located in Londrina, State of Paraná. The soil was classified as a loamy red Oxisol. The treatments consisted of two production systems managed for twelve years under conventional and no tillage, and with three crop rotations. Crop rotations were composed of soybean, corn, wheat, oat and white lupine, but differed in the frequency that each crop was present in the rotation. In this study, it was evaluated the summer and winter harvests and calculated the N balance for each rotation (N input and output). There were evaluated the stocks of carbon and nitrogen in the soil up to 80 cm depth, in the years of 1997, 2000, 2003 and 2009. The losses of soil C and N were recorded in the rotations that had a negative balance of N in the system. The second experiment was conducted in the experimental area of Embrapa Trigo, located in Coxilha, Rio Grande do Sul State. The soil was classified as a loamy red Oxisol. The treatments consisted of three production systems, managed for thirteen years: system I (wheat / soybean and vetch / corn), system II (wheat / soybean and oat / corn) and system III (wheat / soybean and pea / corn). This experiment was sampled at the time of the winter harvest, in 2010. The carbon and nitrogen stocks in the soil at different depths, and the calculation of the N balance for each rotation (N input and output) were evaluated. The three treatments of crop rotation could contribute over the years with enough N to sustain agricultural productivity and maintain a positive balance between N inputs and output. All rotations maintained the total soil carbon and nitrogen stocks (0-100 cm). In both experiments the positive N balance in the system maintained stocks of C and N in the soil. The contribution of N2 fixation in legumes was essential to maintain a positive N balance to the system at some rotations. It was concluded that there is a close relation among the maintenance of soil C and N stocks with a positive nitrogen balance in the system.