Nitrogen availability mediates soil organic carbon cycling in response to phosphorus supply: A global meta-analysis

Soil organic carbon (SOC) is the largest carbon (C) pool in terrestrial ecosystems, and plays an important role in mitigating climate change. With increasing nitrogen (N) loading, phosphorus (P) becomes more limiting for plants and soil microbes, and consequently has a strong impact on ecosystem C sequestration. However, the response of SOC to altered P supply and its mechanisms have not been systematically investigated at the global scale. We conducted a meta-analysis of 536 paired observations from 232 published experiments to explore the dynamics of SOC stock in response to experimental P additions. We found that P additions significantly increased SOC stock across the globe, but the P addition effect on SOC strongly depended on soil N availability. This was confirmed by larger effect sizes in ecosystems dominated by N-fixing species compared with those dominated by non-N-fixing species, under high N availability (soil C:N ratio <16) compared with low N availability (soil C:N ratio >16), and under N + P additions compared with P additions alone. Under high N availability, P additions significantly increased aboveground biomass and litterfall, but had no significant effect on soil microbial biomass and oxidase activity. Under low N availability, P additions not only increased aboveground biomass and litterfall but also increased soil microbial biomass and oxidase activity. However, the increases in aboveground biomass and litterfall did not induce a corresponding accumulation of C in the soil, possibly because P additions promoted microbial CO2 release that offset the increasing plant C inputs. The reactive N input reduced P-addition-induced C loss from the soil to increase SOC sequestration, and the P addition effect on SOC was largely enhanced with increasing experimental duration, partly due to N retention in the soil after P additions. Collectively, the findings suggest that future changes in global exogenous P loads will have divergent impacts on SOC sequestration, depending on soil N availability.