Atmospheric nitrogen (N) is a principle agent of global change, and atmospheric N deposition is projected to continue increasing in coming decades. Many ecosystems are naturally N-limited; therefore, increased N from anthropogenic sources may alter species interactions in unknown ways. Numerous studies have investigated possible ecosystem responses; however, how plant-insect interactions are influenced in natural systems is still largely unknown. Despite the ecological and economic importance of insect outbreaks, there are major gaps in our understanding of how reactive N loading influences insect herbivory. We conducted a meta-analysis of terrestrial, non-agricultural studies that simulated N deposition at rates <150 kg N/ha/year. We extracted and divided data into two main categories: the effect of N deposition on plant responses, and on insect responses. Specifically we investigated if: 1) plant foliar N increases and defensive compounds decrease in response to N deposition; 2) insect herbivores benefit from alterations in foliar N and plant defenses.
Results/Conclusions
Nitrogen deposition had strong positive effects on foliar N; in 97% of the studies, foliar N concentrations increased due to N deposition. Conversely, 92% of the studies that measured the response of C-based defensive compounds such as phenolic glycosides and condensed tannins had significant decreases from N deposition. Few studies examined the effects on N-based plant defenses, demonstrating a need for future research. Furthermore, 99% of studies showed increases in herbivore populations when deposition rates were < 100kg N/ha/yr. However, some herbivore populations actually decreased at deposition rates >100kg N/ha/yr, suggesting that unrealistically high rates of experimental N addition are likely misleading. In conclusion, N deposition may have profound influences on how plant and insect herbivores interact, and results appear to be largely mediated through deposition rate and insect guild. If herbivore populations respond positively to N deposition as these results suggest, than outbreak densities may be a more serious concern in coming decades as rates continue to increase.