Decomposição de resíduos culturais de plantas de cobertura, emissão de gases do efeito estufa e produtividade do arroz e soja em terras baixas

Abstract

Growing grasses and legumes as cover crops in autumn / winter is an uncommon practice in lowlands, especially regarding the use mixture between this species. The objective of this study was to evaluate whether the mixture of ryegrass (Lolium multiflorum) and persian clover (Trifolium resupinatum) modifies the rate of decomposition and release of N from crop residues in relation to single crops and alters methane (CH4) and nitrous oxide (N2O) emissions and the yields of rice and soybean in the lowlands. The study was carried out in the agricultural year 2019/20 in the didactic-experimental area of the lowlands of the Department of Plant Science at the Federal University of Santa Maria (UFSM). Two experiments were conducted, one with soybean and the other with rice. The treatments evaluated in each experiment were: persian clover, ryegrass, mixture of persian clover + ryegrass and fallow (without plants). The design used was a randomized block with four replications. The implantation of treatments was carried out in June 2019 in a managed area shortly after the rice harvest. After the management of the species in October 2019, rice and soybean were sown with the cover crop residues on the soil surface. The evaluations carried out were: biomass and N accumulation by the cover crops, cover crops residue decomposition and N release, inorganic N (ammonium and nitrate) in the 0-10 cm soil layer, CH4 and N2O emissions using the static chamber method, partial global warming potential (pGWP)and rice and soybean yields. The mixture of ryegrass and clover modified the chemical composition of the residues, but this change was not sufficient to reduce the rate of decomposition and N release of the residues in relation to the clover in the initial stage of decomposition. Growing cover crops in the off-season did not affect the yields of rice and soybeans in lowlands. CH4 was the main gas emitted in the rice system and N2O in the soybean system. In both systems, CH4 and N2O emissions were increased in the presence of cover crops residues. While the emission of N2O was 2 times as high with clover than ryegrass, CH4 emissions after legume in irrigated rice was 1,3 times lower than with ryegrass and intercropping. These results associated with the pGWP indicate that in lowlands with the use of cover crops, the persian clover should be cultivated preferentially preceding rice and ryegrass and the consortium preceding soybean.