Effects of Nitrogen Rate and Fertilizer Type on Gaseous Nitrogen Losses and Soil Nitrogen Storage in Alkaline Maize Fields of the Hetao Irrigation District

Gaseous nitrogen losses and residual soil nitrogen accumulation are primary drivers of low nitrogen use efficiency in alkaline irrigated cropping systems. A two-year field experiment (2019–2020) in the Hetao Irrigation District under alkaline flood-irrigated maize evaluated the effects of nitrogen rate, fertilizer formulation, and enhanced-efficiency fertilizers—urea with urease inhibitor NBPT and ammonium sulfate with nitrification inhibitor DMPP—on NH3 volatilization, N2O emissions, post-harvest soil mineral nitrogen, and grain yield. A soil pH manipulation sub-experiment (±0.5 units, ambient pH ~8.8) was conducted to quantify the direct effect of alkalinity on volatilization. NH3 volatilization was insensitive to fertilizer formulation and inhibitor inclusion but strongly responsive to soil pH; a 0.5-unit increase in soil pH elevated volatilization efficiency by up to 25% relative to ambient conditions. N2O emissions were around 18% higher under ammonium sulfate than under urea and were reduced by 21–32% with inhibitor treatments, without increasing NH3 volatilization. Inhibitor-assisted optimized management (urea + NBPT and ammonium sulfate + DMPP) achieved higher yields and lower emission intensity than urea alone. These results confirm that NH3 and N2O losses are governed by distinct controls, and that concurrent mitigation of both pathways requires interventions that independently target each loss driver, beyond rate optimization and inhibitor application alone.