Nitrogen deposition reduces nitrogen limitation of terrestrial net primary production

Experimental evidence demonstrates that nitrogen (N) limitation of ecosystem net primary productivity (NPP) is widespread. Human acceleration of the N cycle should therefore stimulate NPP and reduce the degree of N-limitation. The aim of this investigation was to test the prediction that the degree of ecosystem N limitation decreases with increasing rates of N deposition. We examined the effect of ambient N deposition rates on plant responses to N addition observed in 126 independent field studies. We compiled nitrogen deposition data from the literature and public databases, and calculated a response ratio R for each study, equal to the ratio of plant growth in fertilized to control plots. To test our hypothesis, we performed regressions of R versus ambient deposition rates.

We found that most ecosystems are nitrogen limited (P<0.0001) and average growth response to nitrogen was 29% (95%CI: 22-35%). Background nitrogen deposition was negatively correlated to R overall (P<0.001). This negative correlation between background deposition and R supports our hypothesis and suggests that the strength of N limitation will decrease with increasing anthropogenic inputs. Negative correlations were also found within grasslands (P=0.04) and forests (P=0.01). Below deposition rates of 2 gN m^-2 y^-1, wetland R was negatively correlated to ambient N deposition (P=0.03), but there was no effect of N-deposition on R at higher rates of N deposition. These results suggest a strong interaction between the global nitrogen and carbon cycles, and a significant impact of anthropogenic pollution on biogeochemical coupling of the carbon and nitrogen cycles.