Research advances in nitrogen deposition effects on microbial processes involved in soil nitrogen cycling in tropical and subtropical forests

The rapid urbanization and industrialization have increased global atmosphere nitrogen（N） deposition year by year, and the N deposition amount in tropical and subtropical areas is higher than the global average value. However, majority of the soil from tropical and subtropical forests is almost N saturation. The elevated N deposition will continuously enhance the input of active N into soil, which would easily induce soil N excess and further destroy N cycling balance of the entire forest ecosystem. The tropical and subtropical areas in our country are characterized by rapid economic development, and the increase of N deposition has caused the ecological problems such as soil nutrients imbalance as well as forest degradation. Therefore, the responses and adaptive mechanisms of forest soil N cycling to N atmosphere deposition have attracted lots of attentions in academia. According to previous researches, each step in N cycling pathway is driven by specifically functional microorganisms. Thus, clarifying the effects of elevated N deposition on functional microorganisms in tropical and subtropical soil is important for the assessment on soil N cycling and the driving mechanisms in forests under global N deposition. Moreover, the study results can provide scientific bases for the forest restoration and environmental improvement of the tropical and subtropical areas in our country. Therefore, this research reviewed the responses of soil N cycling processes（e.g., N-fixation, nitrification, denitrification and anammox） and functional microbial abundance, activity and composition to the increased N deposition in tropical and subtropical forests. The correlations between functional microbial attributes and environmental factors（e.g., NH+4, NO-3, organic carbon, pH and soil water content） were summarized and discussed. Based on the review, we discussed the regulations of functional microorganisms on soil N cycling under N deposition, highlighted the effects of functional microorganisms on N cycling processes through altering microbial community abundance and composition, afterwards proposed the main defects of the existing studies as well as the key points in future studies.