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 (R_CH4) to N input. Specifically, we calculated the response factors (f) of R_CH4 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 R_CH4, 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.