Nitrogen (N) effects on ecosystem carbon (C) budgets are critical to understand as C sequestration is considered as a mechanism to offset anthropogenic CO₂ emissions. Interactions between aboveground C and N cycles are more clearly characterized than belowground processes. Through synthesizing data from multiple terrestrial ecosystems, we quantified the responses of belowground C cycling under N addition.
Results/Conclusions

We found that N addition increased litter input from aboveground (+20%) but not litter input from fine root. N addition inhibited microbial activity as indicated by a reduction in microbial respiration (-8%) and microbial biomass carbon (-20%). Although soil respiration was not altered by N addition, dissolved organic carbon (DOC) concentration was increased by 18%, suggesting C leaching loss may increase. N addition increased the C content of the organic layer (+17%) but not the mineral soil layer. Overall, our meta-analysis indicates that N addition will increase short-term belowground C storage by increasing C content of organic layer.  However, it is difficult to predict the response of long-term C sequestration since there is no significant change in mineral soil C content.