Soil N2O emissions from long-term agroecosystems: Interactive effects of rainfall seasonality and crop rotation in the Brazilian Cerrado

In its natural state, the Cerrado biome is a mitigator of soil emissions of nitrous oxide (N2O). However, the integration of this biome in agricultural activities induced changes in nitrogen (N) dynamics, consequently increasing N2O emissions to the atmosphere. For one year, N2O emissions were evaluated under interactive effects of rainfall seasonality and crop rotation in 19-year-old agricultural ecosystems in the Cerrado. The agricultural systems included: (I) no-tillage soybean in the main and sorghum in the late growing season (NTR1); (II) no-tillage maize in the main and pigeon pea in the late growing season (NTR2); (III) soybean in the main and fallow in the late growing season under conventional tillage (CT); (IV) and native Cerrado (NC), as a reference environment. Measurements in a closed static chamber were carried out from October 2013 to September 2014 to determine the fluxes by gas chromatography. The N2O fluxes were related to the following soil and climate variables: nitrate (NO3−), ammonium (NH4+), soil temperature (Soil temp.), and water-filled pore space (WFPS). The annual N2O average fluxes of the agroecosystems ranged from zero to 266 μg m−2 h−1. Fluxes were lowest in the native Cerrado, and in certain periods of the year, especially in the dry season, inflows were observed. The total annual cumulative fluxes from CT, NTR1 and NTR2 were: 1.36; 1.00 and 0.70 kg N2O ha−1, respectively. In NC, the annual cumulative total was 0.27 kg N2O ha−1. Under CT, N2O peaks were highest in the dry period, especially after soybean harvest, from fallow soil. Of the total cumulative emissions in CT, 50% were accumulated during the dry season and 75% during the fallow period, indicating that for the Cerrado with rainfall seasonality, monoculture soybean followed by fallow soil is not an appropriate crop rotation sequence. Among the different tillage systems, NTR2 had the lowest cumulative N2O emissions. This crop rotation is therefore indicated as the most efficient to mitigate N2O, with emission peaks not exceeding 100 μg m−2 h−1, while in NTR1, emissions in the rainy season reached almost 270 μg m−2 h−1.