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.