Abstract: Global nitrogen deposition is generally increasing year by year, with substantial effects on carbon and nitrogen cycling in forest ecosystems. Soil microbes, soil enzymes, and fine roots are important belowground components, playing crucial roles in the material circulation and energy flow of forest ecosystems. Their responses to nitrogen deposition have become a hot topic in the research field of global climate changes. Here, we summarized the effects of nitrogen deposition on soil microbes, enzyme activities, fine root production and turnover in forest ecosystems. Results showed that nitrogen deposition caused soil acidification, preserved soil nutrients, and changed the distribution pattern of belowground carbon and nitrogen. Effects of nitrogen deposition on the structures of soil microbial communities were mainly reflected in the increase or no change of soil bacteria/fungi ratio and changes in the relative abundance of soil microbes. Effects of nitrogen deposition on the functions of soil microbes were mainly reflected in promoting or inhibiting soil microbial respiration, changing nitrogenfixation groups, and reducing nitrogen use rate of soil substrate, and were significantly affected by soil pH, soil available nutrients, and the time of nitrogen application. Nitrogen deposition could promote or inhibit soil enzyme activities, which were significantly affected by soil depth, soil pH, and soil limiting elements (such as carbon, nitrogen and phosphorus). Along with increasing nitrogen deposition, fine root biomass decreased, fine root turnover speeded up or slowed down, fine root decomposition shifted from stimulating to inhibiting, which were significantly affected by soil nutrient availability, soil nitrogen storage, and fine root chemical properties. Focusing on the key scientific issues needed to be solved urgently in this research field, eventually we discussed important directions for future research. This review has great significance for further clarifying the mechanisms underlying the impacts of nitrogen deposition on soil microbes, enzyme activities, and fine root in forest ecosystems, and thus for the protection and rehabilitation of forest ecosystems under the background of increasing nitrogen deposition.

Key words: soil microbes, soil enzyme activity, fine root biomass, fine root decomposition.