Soil–plant retention dominated nitrogen fate from cattle excreta under low N input in a tropical pasture system

This study quantified ammonia (NH 3 ) volatilization, nitrous oxide (N 2 O) emissions, and urinary nitrogen (N) mass balance across different pasture types in southeastern Brazil. Beef cattle excreta from Mixed pastures ( Urochloa brizantha cv. Marandu + Macrotyloma axillare cv. Java) or Grass monoculture pasture (Marandu palisade grass) were applied in a randomized block design with six replications, during the wet and dry seasons. Although Grass pasture resulted in a tendency for excreta with higher N concentration, reflecting greater levels of mineral N, forage production, and N recovery in forage mass and litter, no significant differences were observed between pasture types in gaseous losses (NH 3 , N 2 O). Seasonality influenced N fate. During the wet season, a higher concentration of urinary N was observed in the 10–20 cm soil layer, and greater recovery by Marandu palisade grass at the chamber border, together with lower total 15 N recovery, suggested greater hydrological N transport (~23% of non-recovered N). During the dry season, there was a difference in losses due to NH 3 volatilization, mainly from urine (~3.7%). Emission factors were lower than the IPCC’s averages. The soil represented the main destination of urinary N (~54%), and the soil–plant system recovered 96.2% of the N during the dry season and 75.5% during the wet season. Under the conditions of this study, NH 3 and N 2 O losses were low, and the 15 N mass balance indicated limited amounts of non-recovered N under most conditions, with stronger evidence of hydrological N transport during the wetter period.