Assessing nitrogen deposition and its impacts on forest ecosystems

Global forest biomes are a large sink of anthropogenic nitrogen (N) inputs from atmospheric deposition. The spatiotemporal patterns of N deposition and its impacts on forest ecosystems have been intensively studied for several decades. However, uncertainties and knowledge gaps still remain due to the limitations of different research approaches. In this chapter, we present an overview of the methodologies to assess N deposition and its impacts in forest ecosystems and identify the uncertainties associated with each approach. Spatiotemporal patterns of N deposition have been studied using a combination of monitoring and modeling approaches, with largest uncertainties remaining in the quantification of dry deposition and canopy interactions with N deposition. Most of current knowledge of N deposition impacts is derived from experimental studies in which relatively high doses of N are applied to the forest floor. The conventional experimental approach generally ignores forest canopy interactions with N deposition, potentially leading to an underestimate of fertilization effect of N deposition on forest growth and carbon sequestration and an overestimate of the negative effects on understory plant diversity. Future experimental and modeling research efforts should further be made to better predict biogeochemical and ecological responses to decreasing N deposition in combination with other global change drivers. In view of the common design of existing N addition experiments, critical loads for N deposition might be overestimated in forest ecosystems and this should be considered in future N emission regulation policies.