Similar responses of soil carbon storage to drought and irrigation in terrestrial ecosystems but with contrasting mechanisms: A meta-analysis

Global climate models predict that future precipitation regimes will largely change across the globe due to the intensification of the global water cycle under climate warming, which may generate considerable impacts on ecosystem carbon (C) dynamics. Although substantial manipulative experiments have been conducted to probe the responses of ecosystem C processes to altered precipitation, how soil C storage responds to both drought and irrigation is still unclear across biomes and the globe. A meta-analysis of 179 published studies was carried out to examine responses of soil C storage and associated C fluxes and pools to drought and irrigation. Our results showed that, on average across all biomes, drought and irrigation similarly induced minor increases in soil C pool (SCP) by 1.45% and 1.27%, respectively. However, drought and irrigation oppositely affected both C fluxes and plant C pools as well as in agroecosystems (e.g., croplands and grasslands). The drought-induced increases in root: shoot ratio and decreases in heterotrophic respiration and soil C turnover rate mostly contributed to minor increase in SCP, while an increase in newly fixed C inputs in soil was more important under irrigation. In addition, the relative changes in precipitation intensity in manipulative experiments were positively correlated with response ratios of plant C pool (PCP), net primary production (NPP), microbial biomass C, ecosystem, soil and heterotrophic respiration. The drought-induced responses of SCP exhibited a positive correlation with experimental duration but not under irrigation and for other C pools and fluxes. These results indicate that more attention should be paid to the responses of C allocation and turnover rate to drought and irrigation, which should be incorporated into land surface models to better project effects of altered precipitation on ecosystem C cycling in terrestrial ecosystems.