Emissões de Gases de Efeito Estufa e Estoques de Carbono em Planossolo Cultivado com Arroz Irrigado: Efeito dos Sistemas de Preparo do Solo e de Rotação de Culturas

Abstract

The interest on crop diversification including rainfed crops in rotation to irrigated rice has grown in the Lowlands of Rio Grande do Sul, favoring the adoption of no-tillage systems, replacing conventional tillage. Irrigated rice rotation with rainfed crops present potential to mitigate greenhouse gases (GHG) emissions, especially CH4, as it reduces the period the soil is kept flooded. This study aimed to evaluate the influence of crop rotation and soil tillage systems on CH4 and N2O fluxes and cumulative emissions, parcial Global Warming Potential (pGWP), Yield-sclaed GHG (study 1), and soil carbon stocks and its fractions (study 2) in a Planossolo Háplico (typic Albaqualf) cultivated with irrigated rice. In study 1, treatments included combination of two soil tillage systems (conventional – CT and no-tillage NT) and four crop rotation systems (rice-rice-rice-rice; soybeansoybean-soybean-rice; sorghum-sorghum-maize-rice and soybean-sorghumsoybean-rice). Air was sampled during the drained non-rice season 2018 and during the flooded rice-growing season 2018/2019, using the static closed chamber method. During the rice-growing season, rice-rice-rice-rice system under CT emitted more N2O in relation to NT, while the reverse occurred for soybean-soybean-soybean-rice rotation. During the rice-growing season, higher CH4 emissions occurred for rice-rice-rice-rice and soybean-soybean-soybeanrice rotations under CT, in comparison to NT. The rice-growing season pGWP accounted for most of annual GHG emissions from all treatments. The pGWP/grain yield index of rice/rice/rice/rice/CT was higher than the other rotations and tillage systems. In conclusion, introducing rainfed crops in rotation to irrigated rice in Lowlands has potential to mitigate CH4 emissions from both periods, but enhances N2O emission and pGWP from the non-rice season; NT reduces pGWP from a Planossolo cultivated with irrigated rice. In study 2, treatments included combinations of conventional tillage (CT) and no-tillage (NT) systems; and rice-rice-rice-rice and sorghum-sorghum-maize-rice crop rotation. Physical fractionation of organic matter was performed to determine total and C contend in fraction stocks. The evaluations were carried out in pre-sowing and post-harvest of rice, in the fourth phase of two crop rotation systems. Rice-ricerice-rice system provided higher total C stock, compared to sorghum-sorghummaize-rice, regardless of the evaluation period. In pre-sowing, NT system presented greater total C and non-labile C. However, in the post-harvest period, NT system reduced total C stocks from rice-rice-rice-rice system whereas increased for sorghum-sorghum-maize-rice rotation.