COS 90-3: Nitrogen deposition reduces nitrogen limitation of terrestrial net primary production
David S. LeBauer, UC Irvine and Kathleen K. Treseder, UC Irvine.
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.