Soil pH Determines Nitrogen Effects on Methane Emissions From Rice Paddies

ABSTRACTRice paddies account for approximately 9% of human-induced methane (CH4) emissions. Nitrogen (N) fertilization affects CH4 emissions from paddy soils through several mechanisms, leading to conflicting results in field experiments. The primary drivers of these N-related effects remain unclear and the contribution of N fertilization to CH4 emissions from the rice paddies has not yet been quantified for global area. This uncertainty contributes to significant challenges in projecting global CH4 emissions and hinders the development of effective local mitigation strategies. Here, we show through a meta-analysis and experiments that the impact of N fertilization on CH4 emissions from rice paddies can be largely predicted by soil pH. Specifically, N fertilization stimulates CH4 emissions most strongly in acidic soils by accelerating organic matter decomposition and increasing the activities of methanogens. Accounting for the interactions between soil pH and N fertilization, we estimate that N fertilization has raised current area-scaled and yield-scaled CH4 emissions across the total global paddy area by 52% and 8.2%, respectively. Our results emphasize the importance of alleviating soil acidification and sound N management practices to mitigate global warming.