Enhanced nitrification in higher pH soils moderates ammonia emissions in global croplands

Ammonia (NH3) emissions from agricultural lands represent a major environmental concern, with current emission models heavily relying on inherent soil pH as a key determinant based on chemical equilibrium principles. Here, analyzing 1792 field measurements across global croplands, we demonstrate that soils with inherently higher pH generally have lower NH3 emissions. We identify enhanced nitrification as the key mechanism, where higher pH promotes rapid NH3 oxidation, thereby reducing substrate availability for NH3 volatilization. This biological process can reduce NH3 emissions by 11% per unit increase in inherent soil pH. In paddy rice, nitrification is inhibited due to the flooded environment, leading to higher emission factors (EF) than upland crops, with EF positively correlating with pH above approximately 7.5. Our findings highlight that current emission inventories may substantially misestimate NH3 emissions by oversimplifying pH effects and overlooking biological processes. Higher soil pH promotes rapid ammonia oxidation through enhanced nitrification, thereby reducing substrate availability for ammonia volatilization that finally leads to reduced ammonia emission, according to an analysis of 1792 field measurements across global croplands.