Minor stimulation of soil carbon storage by nitrogen addition

Background/Question/Methods: Anthropogenic nitrogen (N) fertilization and deposition adds 292 Tg N each year, largely to terrestrial ecosystems. N addition usually stimulates biomass growth and results in increases in plant carbon (C) pool sizes. Whether this stimulated biomass growth would lead to increased C storage in soils – the largest pool in the land – is a critical issue for the mitigation of climate change. Fossil fuel burning and deforestation emit about 10 Gt C per year to the atmosphere. Global analysis has shown that terrestrial ecosystems absorb a substantial portion of emitted C. If N addition is one key mechanism underlying contemporary terrestrial C sequestration, the land C sink would be likely to be enhanced as N deposition and fertilization are anticipated to increase in the future. However, impacts of C addition on soil C sequestration remain highly controversial. To reveal a central tendency, we synthesized results from nearly 300 studies using the meta-analysis approach.

Results/Conclusions: Our analysis showed that N addition did not significantly stimulate soil C storage in nonagricultural ecosystems including forests and grasslands but significantly in agricultural ecosystems.  Averaged effects of N addition on soil C storage were minor across all ecosystems. N-induced changes in soil C stocks were significantly regulated by soil C:N. Nitrogen addition tends to stimulate C storage in low C:N soil but increase C loss in high C:N soil. Our results indicate that N stimulation of C storage primarily occurs in plant pools but is highly limited in soil pools.