Impacts of global change drivers on terrestrial methane emission and uptake

 Methane (CH₄) is the second largest greenhouse gas and affects global climate change. In turn, global changes strongly affect CH₄ fluxes from the terrestrial biosphere to the atmosphere. However, it is unclear how CH₄ fluxes are affected by warming (W), precipitation patterns (P), elevated carbon dioxide (eCO₂), and nitrogen (N) addition. Here, we synthesized terrestrial CH₄ fluxes data from 1,165 observations performed under changes in W, P, eCO2, and N across different vegetation types over the globe. Results showed N addition significantly reduced CH₄ emission and uptake across upland ecosystems (-31% and -14%, P<0.05), but stimulated CH₄ emission in rice paddies (3%, P>0.05) and wetlands (24%, P<0.05). CH₄ emission and uptake significantly increased by 42% and 11% under W, respectively. An increase in CO₂ concentration did not affect CH₄  emission in wetlands while enhanced CH₄ emission in rice paddies (39%, P<0.05). Increased precipitation inhibited CH₄ uptake (-21%, P<0.05), whereas decreased precipitation had a significantly positive effect on CH₄ uptake (26%, P<0.05) in uplands. The overall effects of four global change drivers were -9% for CH₄ uptake and 13% for CH₄ emission averaged across different ecosystem types. The interactive effect of multiple factors on CH₄ fluxes generally was antagonistic. In addition, the responses of CH₄ emission to global change drivers significantly shifted from negative to positive with the increases in wetness indices, soil clay content, and effects of global change drivers on belowground biomass (BGB) and methanogenic bacteria (mcrA) (P<0.05). In contrast, the effects of global change drivers on CH₄ emission switched from positive to negative with the increases in the responses of grain yield and aboveground biomass, respectively (P<0.05). CH₄ uptake increased with the increases of BGB, inorganic nitrogen, the ratio of carbon to nitrogen, and mcrA induced by global change drivers but decreased with the increase of NO₃- (P<0.05). The responses of CH₄ fluxes to N addition, W, and precipitation changes exhibited considerable variations in sensitivities and magnitudes. This synthesis showed an urgent need to consider the effects of changing multiple global change drivers on CH₄ fluxes for better understanding the methane-climate feedback.