Nitrous oxide and methane emissions are affected by soil tillage systems and crop diversity in the Brazilian Savanna

Population growth associated with the increasing demand for resources around the globe has increased greenhouse gas (GHG) emissions, leading to the climate change process. In this study, we assessed the impact of agrosystems intensification on N₂O and CH₄ emissions in the Brazilian Savanna. We selected two long-term experiments located in Costa Rica – MS and Itiquira – MT, which consisted of soil tillage systems (conventional (CT) and no tillage (NT) systems) and levels of crop diversification with cover crops (e.g., Brachiaria sp., Crotalaria sp., Pennisetum sp.) in soybean-cotton-based cropping systems. Gas emissions were monitored for 8 months (February 2022 to October 2022). N₂O and CH₄ fluxes were calculated following the linear changes in the concentration of each gas, and cumulative emissions were estimated after extrapolation from hourly to daily fluxes. Data were subjected to analysis of variance, and means were compared by Tukey's test (p<0.05). In Costa Rica - MS, CH₄ fluxes varied between -30 and 50 µg m^-2 h^-1 of CH₄-C and were greatly affected by rainfall seasonality. Cropping systems affected cumulative emissions, with soybean + cotton under CT and soybean + cotton + maize presenting emissions of 0.13 and 0.27 kg ha^-1 of CH₄-C, respectively. Soybean + cotton under NT and the treatments with cover crops (soybean + cotton + Brachiaria sp., and soybean + cotton + rattlebox) promoted CH₄ uptake, sequestering up to 0.6 kg ha^-1 of CH₄-C. N₂O fluxes were mostly affected by N fertilization. Peaks up to 270 µg m^-2 h^-1 of N₂O-N were observed, most associated with soybean + cotton + maize treatment, and soybean + cotton under CT. The highest cumulative N₂O emissions were observed at soybean + cotton + maize and soybean + cotton + Brachiaria sp. treatment (0.88 kg ha^-1 of N₂O-N on average). Soybean + cotton under CT and NT systems, and soybean + cotton + rattlebox showed an average cumulative emission of 0.43 kg ha^-1 of N₂O-N. At Itiquira – MT, CH₄ fluxes also were affected by rainfall seasonality, with values ranging between -40 and 90 µg m^-2 h^-1 of CH₄-C. Cumulative CH₄ emissions were significantly affected by the treatments, with the highest emissions at soybean + maize succession (i.e., 0.36 kg ha^-1 of CH₄-C). The succession between soybean + Brachiaria sp. and the other crop rotations involving rattlebox and millet did not differ from each other, presenting an average uptake of 0.13 kg ha^-1 of CH₄-C. For N₂O-N, the fluxes were highly responsive to N fertilization in maize, presenting peaks up to 75 µg m^-2 h^-1 of N₂O-N. Cumulative N₂O emissions, however, were reduced (0.18 kg ha^-1 ofN₂O-N) only at soybean + rattlebox + maize + Brachiaria sp., which was 2.7 times lower than the cumulative emissions found at soybean + maize, soybean + Brachiaria sp., and soybean + rattlebox + maize + Brachiaria sp. for one year period. Overall, our results suggest that cropping intensification in the Brazilian Savanna is an efficient strategy to reduce CH₄ and N₂O emissions, contributing to tackling the climate change process.