Increased reactive nitrogen (N) deposition is traditionally expected to increase net primary production (NPP), but continued retention of N deposition may saturate the ecosystem capacity to store N and cause some environmenal and ecological problems, like soil acidification and the limitation of other nutrient. However, those dark side effects of nitrogen deposition have not been well quantified based on experimental evidences. We synthesized nitrogen deposition experiments in the world and conducted a series of meta-analysis.
Results/Conclusions
By compiling a dataset from 44 studies with at least three levels of N addition treatment, we found an universal saturation response of NPP to N addition gradient in terrestrial ecosystems. The N saturation threshold for NPP was at the N addition rates of 4-5 g m-2 yr-1 on average across all the ecosystems. However, the saturation response patterns and the N thresholds largely varied with ecosystem types and environmental factors. By synthesizing 106 studies that monitored soil pH and base cations under N enrichment, we quantified global soil acidification caused by N addition. On average, N addition significantly reduced soil pH by 0.26, but the magnitude varied with ecosystem types, N addition rate, N fertilization forms, and experimental durations. Environmental factors such as initial soil pH, soil carbon and nitrogen content, precipitation, and temperature all influenced the N responses of soil pH. Global soils are now at a buffering transition from base cations (Ca2+, Mg2+ and K+) to non-base cations (Mn2+ and Al3+). This calls our attention to care about the limitation of base cations and the toxic impact of non-base cations for terrestrial ecosystems with N deposition. By comparing the phosphorus limitation on biomass productions between the ambient and elevated N conditions, we found a stronger phosphorus limitation induced by N enrichment.
Overall, the results indicate that the beneficial effect of N deposition on ecosystem productivity will reduce quickly and the dark side effects should be cautious with continuous N enrichment.