Antagonistic and additive interactions dominate the responses of belowground carbon-cycling processes to nitrogen and phosphorus additions

While responses of belowground carbon (C)-cycling processes to nutrient additions have been widely studied, how interactions of nitrogen (N) and phosphorus (P) influence belowground C-cycling processes are still unclear. By conducting a meta-analysis of 1928 observations from 158 independent experimental studies, we quantified the direction and magnitude of different interactions (i.e., additive, synergistic, and antagonistic) between N and P additions on eight variables related to belowground C pools and fluxes. The results showed that the interactions were antagonistic for dissolved organic carbon (DOC), microbial biomass carbon (MBC), and heterotrophic respiration (Rh), and were additive for soil organic carbon (SOC), fungal biomass, and soil respiration (Rs). Synergistic interactions occurred less frequently than additive or antagonistic interactions, and were observed for bacterial and total microbial biomass (TPLFAs). Additive interactions dominated the responses of most belowground C pools and fluxes among different terrestrial ecosystems, whereas antagonistic interactions were found for DOC in subtropical forest, MBC in tropical/subtropical forest, SOC in cropland, and fungal/bacterial biomass in tundra. Synergistic interactions were rare and were found only for MBC in temperate forest and Rh in tropical forest and wetland. The responses of belowground C pools and fluxes to N and P additions were significantly dependent on environmental or experimental conditions. Our meta-analysis highlighted the importance of soil microbes in affecting belowground C-cycling processes, and identified some key interactions to be implemented into C-cycling models for predicting the impacts of increasing depositions of N and P on terrestrial C uptake in the future.