Response of soil greenhouse gas fluxes to warming: A global meta-analysis of field studies

Climate warming is predicted to stimulate soil greenhouse gas (GHG) fluxes. A growing number of field studies have investigated the effect of warming on GHG fluxes, but the global responses and temperature sensitivities of all three major biogenic GHGs (CO2, CH4 and N2O) to warming and the potential driving factors have not been well synthesized across different biomes. We conducted a meta-analysis of 2423 globally obtained observations across multiple terrestrial ecosystems (i.e., forest, shrubland, grassland, wetland, cropland and tundra) from 224 published studies. Globally, experimental warming significantly stimulated soil GHG fluxes; soil CO2 and N2O emissions increased by 9.9% and 17.9%, respectively, and CH(4 )emissions in the lowland regions and CH4 uptakes in the upland regions increased by 10.1% and 7.5%, respectively, under warming. These increases amounted to 2289.0 kg, 31.0 kg, and 0.5 kg ha(-1) year(-1) of CO2, CH4 and N2O emissions and 1.1 kg ha(-1) year (-1)of CH(4 )uptake per 1 degrees C globally, which translates to an additional emission of 30.3 Pg CO2 and 2.3 Pg CO2 equivalent (CO2-eq) of CH4 and N2O fluxes from terrestrial ecosystems annually under 1 degrees C warming scenarios. We found that the responses and temperature sensitivities of GHG fluxes to warming were highly biome-dependent, and climate and warming duration also impacted the responses and temperature sensitivities of GHG fluxes. Moreover, warming also affected plant biomass, soil moisture, soil microbial substrate availability, and microbial biomass and function, and changes in soil moisture and plant biomass were related to the changes in GHG fluxes under warming. Our results provide strong empirical support that warming significantly increases soil GHG emissions and aboveground biomass but has no effect on soil carbon. Studies focus only on soil GHG fluxes, with no measurements of soil and plant carbon response, would overestimate the net impact of warming on the terrestrial carbon budget.