Suppression of Nitrogen Deposition on Global Forest Soil CH4 Uptake Depends on Nitrogen Status.

Methane (CH4) is the second most important atmospheric greenhouse gas (GHG) and forest soils are a significant sink for atmospheric CH4. Uptake of CH4 by global forest soils is affected by nitrogen (N) deposition; clarifying the effect of N deposition helps to reduce uncertainties of the global CH4 budget. However, it remains an unsolved puzzle why N input stimulates soil CH4 uptake in some forests while suppressing it in others. Combining previous findings and data from N addition experiments conducted in global forests, we proposed and tested a "stimulating‐suppressing‐weakened effect" ("three stages") hypothesis on the changing responses of soil CH4 flux (RCH4) to N input. Specifically, we calculated the response factors (f) of RCH4 to N input for N‐limited and N‐saturated forests across biomes; the phased changes in f values supported our hypothesis. We also estimated the global forest soil CH4 uptake budget to be approximately 11.2 Tg yr−1. CH4 uptake hotspots were predominantly located in temperate forests. Furthermore, we quantified that the current level of N deposition reduced global forest soil CH4 uptake by ∼3%. This suppression effect was more pronounced in temperate forests than in tropical or boreal forests, likely due to differences in N status. The proposed "three stages" hypothesis in this study generalizes the diverse effects of N input on RCH4, which could help improve experimental design. Additionally, our findings imply that by regulating N pollution and reducing N deposition, soil CH4 uptake can be significantly increased in the N‐saturated forests in tropical and temperate biomes. Plain Language Summary: Methane is an important greenhouse gas. Forest soils can absorb methane from the atmosphere and mitigate its warming effect. Meanwhile, forests suffer from high atmospheric nitrogen deposition, yet the effect of nitrogen on the methane uptake by forest soils remains unclear. Using data from global nitrogen addition experiments, we validated a "stimulating‐suppressing‐weakened effect" ("three stages") hypothesis, which could explain the diverse responses of soil methane flux to nitrogen input observed in different forests. On this basis, we quantified that nitrogen deposition decreased global forest soil methane uptake by approximately 3%. Our findings also imply that by regulating nitrogen pollution, soil methane uptake can be significantly increased in the nitrogen‐saturated forests in tropical and temperate biomes, potentially mitigate global warming. Key Points: A "three stage hypothesis" was developed to generalize the diverse responses of forest soil CH4 flux to N inputCH4 uptake by global forest soils was estimated to be 11.2 Tg yr−1, with N deposition suppressing ∼3% of this uptakeEffective regulation to reduce N deposition would promote CH4 uptake by N‐saturated forests and mitigate global warming [ABSTRACT FROM AUTHOR]