Soil Direct N2O Emissions Due to Bovine Excreta Deposition in Native Grassland at Southern Brazil: Effect of Different Year's Seasons.

Livestock has great impact on global nitrous oxide (N₂O) emissions for providing sources of N by grazing animal’s excreta. In Brazil, due to the large areas occupied by extensive grazing systems, excreta deposition contributes to 57% of total N₂O emitted in agricultural soils. In contrast, information on emission factors (EF) of N₂O are scarce. Aiming to determine EF of N₂O for subtropical soils and evaluate if the generated values are comparable to IPCC EF (2%), this study was performed in a native grassland in Southern Brazil. We conducted field trials to evaluate the emission factor (EF) of N₂O from cattle urine and dung in different year’s seasons. The proportional of one bovine event for each excreta type, were applied in the beginning of each season of 2016 year and N₂O emissions were measured over this year. Soil N₂O fluxes for urine treatment varied from -2.3 to 250.4 g N₂O-N ha^-1 day^-1 among year’s seasons, with observed emission peaks until 20 days after urine application. Soil N₂O fluxes due to dung deposition were low in all year’s seasons. Cumulative N₂O emissions was affected by year season and type of excreta. The higher N₂O cumulative emission were observed in spring season for urine treatment (5.85 kg N₂O-N ha^-1). The EF for bovine dung varied from 0.008 and 0.029% between year’s seasons and from 0.34 and 2.2% for urine. The dissimilarity of the N₂O emission factor for urine and dung found in this study, indicates the need for distinct EF to be considered for each type of excreta. In addition, our results showed that for subtropical region of Brazil the EF’s values vary among year’s seasons due to the influence of climate variables (temperature and rainfall), and, across all seasons, the mean EF-N₂O is lower than indexes recommended by the IPCC.