How do soil moisture and simulated rainfall drive ammonia emissions after applying inhibited urea fertilizers? – An incubation study

Ammonia (NH₃) emissions in Germany originate mainly from agricultural activities, leading to numerous negative impacts on ecosystems, human health and environment. To reduce NH₃ emissions, the addition of urease inhibitors (UI) to synthetic urea fertilizer is proposed and implemented in practical agriculture. Nevertheless, there are still uncertainties about the reduction potential of UIs under varying environmental conditions. In this incubation study, we examined the influence of soil water content (low: 10%, medium: 17.5%, high: 25% w/w) and rainfall (1, 5, 10 mm) on NH₃ emissions after the application of urea fertilizer. Ammonia was trapped by sulfuric acid in an incubation setup for three weeks after fertilizer application. Daily cumulative emissions were fitted to a logistic growth function. Cumulative emissions of the fertilized treatments, ranged between 1.6 and 24.9% of the applied N and were highest at the medium soil water content. Low soil moisture at application hindered granule dissolution, resulting in emissions of 8.4% across all fertilizers. High soil water content led to low emissions (7.2%) due to a faster diffusion of urea into the soil. The results indicate, that UI always reduced NH₃ emissions with a mean reduction of 73.3% over all soil moisture and rain treatments. Under conditions favoring high NH₃ emissions, the emission of a double inhibited urea fertilizer (UI and nitrification inhibitor) was higher when compared to fertilizer only with UI, but still lower than from untreated urea. Rainfall linearly decreased cumulative emissions, independent of fertilizer choice by 4.7% per mm of rain.