Emissions of N2O and NH3 from cattle excreta in grass pastures fertilized with N or mixed with a forage legume

Pasture is the major livestock production system in Brazil, and nitrogen (N) excretion by cattle has been identified as an important source of nitrous oxide (N2O) and ammonia (NH3). This study assessed long-term N2O and NH3 emissions from cattle urine and dung deposited on three pasture systems: (1) palisadegrass (Brachiaria brizantha) monoculture without N-fertilizer (GRASS); (2) palisadegrass monoculture fertilized with 150 kg N ha−1 (GRASS + N); (3) palisadegrass mixed with Arachis pintoi without N-fertilizer application (GRASS + LEGUME). Two trials were carried out, one beginning in the dry season and the other in the rainy season. Urine N2O emission factors (EFN2O) were the lowest for the GRASS pasture (0.41% vs. 0.56 and 0.62% for the GRASS + N and GRASS + LEGUME pastures, respectively). There was a tendency of lower EFN2O in for dung from cattle grazing the GRASS + LEGUME pasture (0.11% vs. 0.20% for the GRASS and GRASS + N pastures; P < 0.10). Excreta-N lost by NH3 volatilization (EFNH3) was highest for urine under GRASS + N pasture (12.6% vs. 5.9 and 4.7% for the GRASS + LEGUME and GRASS pastures, respectively). These figures are within the ranges reported in the IPCC 2019 guidelines but consistently in the lower range, probably due to the tropical climate. The lowest urine EFN2O estimated for the GRASS pasture showed that N fertilization increased N2O emissions from urine patches. The lower EFN2O for dung from GRASS + LEGUME pastures and the emissions avoided associated with N fertilizer use indicate that the mixed grass/legume pasture is a strategy to mitigate greenhouse gas emissions when pasture intensification requires N addition.