Phosphorus addition promotes plant nitrogen uptake mainly via enhancing microbial activities: A global meta-analysis

Nitrogen (N) addition drives ecosystems toward phosphorus (P) limitation has gained increasing recognition, whereas how P addition affects ecosystem N-cycling processes remains elusive. We performed a meta-analysis of 1826 observational data from 222 independent studies to evaluate the responses of plant N pools, soil N pools, and N fluxes to P addition at the global scale. The results suggested that P addition had an overall positive effect on plant N uptake and ecosystem N retention, which were mainly reflected in increases of plant N pools (i.e., aboveground/belowground biomass and belowground biomass-N) and microbial N transformation (i.e., net N mineralization and nitrification), and decrease of soil N leaching (i.e., nitrate-N and total dissolved N). The contribution of microbial processes to plant N uptake was greater in the mid−/high-latitude ecosystems (≥30°) than in the low-latitude ecosystems (<30°), and the sensitivities of most N-cycling variables to P addition were stronger in the most/moderately acidic soils (pH < 6.5) or strongly/intermediately weathered soils than in the neutral/alkaline soils or slightly weathered soils. Our meta-analysis highlighted the key role of soil microbes in promoting plant N uptake, and identified several important soil properties (i.e., soil N status, initial pH, and weathering degree) regulating the N-cycling responses to P addition and should be incorporated into the nutrient-cycling models of terrestrial ecosystems.