Emissões de metano e óxido nitroso em sistemas de rotação de culturas em um Planossolo do Rio Grande do Sul.

Abstract

Global efforts seek to reduce greenhouse gases (GHG) emissions, such as nitrous oxide (N2O) and methane (CH4) from soil, which are favored by the addition of nitrogen fertilizers and soil flooding, respectively. Among the cropping systems adopted in Rio Grande do Sul for continuous flooding rice are the conventional tillage and no-tillage, which sometimes are established with crops rotation. Thus, the present study sought to evaluate CH4 and N2O emissions and their respective partial global warming potential (pGWP) under different crop rotation systems and soil preparation from a soil classified as Planossolo Háplico. The treatments included combinations of cropping systems factors [conventional tillage (CT) and no-tillage (NT)] and rotation of summer crops (rice-rice-rice-rice; soybean-soybean-soybean-rice; sorghum-sorghumsorghum-rice and soybean-sorghum-soybean-rice). The system adopted to collect greenhouse gases was the closed static chamber type. Methane and nitrous oxides fluxes, total emission, pGWP were evaluated during the period of nongrowing season 2016 and growing season 2016/17, as well as yield-scaled pGWP. The total emissions of N2O and CH4 were low during the nongrowing season, not differing between CT and NT, but it was higher in the rice-rice-ricerice treatment. During the growing season, N2O emissions were higher in continuous flooding rice under CT than in NT systems; while the reverse occurred with sorghum and soybean crops. The CH4 emission in the growing season was higher in irrigated rice under CT in relation to NT; and for CH4 emission in rainfed crops there was no statistical difference between cropping systems. The pGWP was higher in rice-rice-rice-rice system in both CT and NT during nongrowing season and growing season when compared to other crops rotation. During the nongrowing season, rice-rice-rice-rice system presented higher pGWP per kilogram of dry matter produced in both cropping systems; the same occurred during the growing season period. The results allowed the following conclusions: The inclusion of rainfed crops in rotation with irrigated rice in a Planossolo presents potential to mitigate CH4 emissions during nongrowing season and growing season, as well as pGWP. In no-tillage system, sorghum cultivation promotes higher N2O emissions when compared to irrigated rice. In conventional tillage system, emissions of N2O from irrigated rice exceed emissions from soybean and sorghum, grown in succession of soybean. During nongrowing season period, GHG emissions from cropping systems are similar in conventional tillage and no-tillage. On the other hand, during growing season, only emissions from irrigated rice were lower under notillage system, while the opposite occurred with rainfed crops. Growing season period of spring/summer is responsible for most of GHG emission and pGWP from cropping rotation systems in a Planossolo, corresponding to 93% and 82% of the annual total for irrigated rice and rainfed crops, respectively.