Effects of Nitrogen Deposition on Forest Ecosystems

Anthropogenic activities have profoundly altered the global nitrogen (N) cycle. The substantial changes in anthropogenic N inputs to various systems have resulted in cascading effects. Forest covers approximately one-third of the global land area and provides essential ecosystem services. As a new N input, N deposition causes multiple ecological consequences in forest ecosystems, including both beneficial and detrimental effects. This chapter reviews observational, experimental, and modeling studies to summarize (i) the spatiotemporal patterns of N deposition in global forested regions, (ii) the methods to investigate the ecological effects of N deposition, and (iii) the current understanding of N deposition effects in global forests with a special focus on ecosystem N cycling, soil acidification, plant nutrition, carbon cycling, fluxes of non-CO2 greenhouse gases as well as plant and soil microbial diversity. In addition, the estimates of critical loads and their exceedances are summarized mainly based on results from Europe, the USA, and East Asia. Finally, major knowledge gaps are also identified and future research efforts are needed to gain more insights of (i) canopy interactions with N deposition, (ii) the effects of N deposition in urban forests, (iii) the ecosystem responses to declining N deposition, and (iv) the remediation of negative effects caused by N deposition.