Nitrous Oxide Emissions and Ammonia Volatilization from Brachiaria brizantha cv. Piatã Under Different Nitrogen Rates in the Brazilian Savanna

Nitrogen (N) fertilization plays a key role in pasture productivity but also contributes to environmental losses, especially under tropical conditions. This study evaluated the effects of four N rates (0, 50, 75, and 100 kg N ha−1) as urea on soil N dynamics, ammonia (NH3) volatilization, nitrous oxide (N2O) emissions, and biomass accumulation in Brachiaria brizantha cv. Piatã, cultivated in a clayey Oxisol in the Brazilian Savanna. The experiment was conducted over two pasture growth cycles during the late summer and early fall. NH3 volatilization increased with the N rate and showed significant differences in the initial samplings of both cycles. N2O emissions were low, strongly influenced by rainfall, and resulted in emission factors ≤ 0.3%. Soil NH4+ and NO3− concentrations did not differ statistically among treatments. Biomass production increased over time on Cycle 2 but plateaued at greater doses, with no significant differences between treatments. The limited biomass response suggests physiological saturation or environmental constraints. Findings indicate that N losses and use efficiency are shaped by rainfall and plant demand.