Continuous accumulation of nitrous oxide (N₂O) in the atmosphere leads to global warming and ozone depletion. Forest ecosystems act as source and sink of atmospheric N₂O, posing a great uncertainty in budgeting of atmospheric N₂O. Exogenous nitrogen inputs into terrestrial ecosystems are an alternative explanation for this uncertainty. Therefore, exploring mechanisms involved in responses of N₂O emission from forest soils to increased atmospheric nitrogen deposition is of some important theoretical and practical significance. However, due to complexity of soil nitrogen cycling and high spatial heterogeneity of forest ecosystems, progress of the research on soil N₂O flux response to N addition has been quite slow. N₂O emission in forest soils is mediated by microbial communities, and nitrification, denitrification, nitrifier denitrification and chemical denitrification are the four main processes of soil N₂O production. Presently, which one of nitrification and denitrificatiopn is the leading contributor to soil N₂O emission under nitrogen enrichment is still controversial; and how N₂O emission responds to increased N deposition and what mechanism is involved in soil microbes driving the porcess are not well known. In this paper, a review is presented of the progresses of the study on identification of sources of N₂O in forest soils using the stable isotope labelling technique, laws of the responses of total N transformation in and N₂O emission from forest soils to nitrogen addition, as well as effects of increased N depostion on activity and composition of soil microbial community. Also, the paper points out weak links in the present studies and possible research priorities in the future. Generally, soil N₂O flux is influenced by many environmental factors including soil temperature, soil moisture, pH, Eh, and N availability. Increased nitrogen deposition may increase, decrease or have little effect on forest soil N₂O emission, depending on forest types, initial nitrogen content in soils, and dose and duration of nitrogen application. Overall, the response of N₂O emission from forest soils to increased atmospheric N deposition exibits a nonlinear pattern, including no significant response at the early stage, linear increase at the medium stage, and exponential increase at the late stage. The three-stage pattern depends on degree of "N saturation" of the forest ecosystems. Besides, significant relationships were observed between soil NO₃- content and abundance of denitrobacterial genes, between soil NH4+ content and abundance of nitrobacterial genes, and between soil N₂O flux and abundance of denitrobacterial genes. Nitrogen application leads to change in status of soil available N from N deficiency to N sufficiency, and hence changes in abundance and composition of nitrobacteria and dennitrobacteria, thus affectng soil N₂O emission. Moreover, as the monitoring of N₂O emission from forest soils and the researches on transformation of soil TN and dynanucs of N₂O producing bacterium communities are often carried out independently, making it hard to elaborate on the coupling relationship between soil microbial functional groups and soil N₂O emission. It is, therefore, suggested that future researches should focus on the following three aspects 1) to lay out and conduct some multi-dosage multi-form N fertilizer application experiments to work out equations for response of soil N₂O to N addition and threshold of N deposition that may cause significant change in N2O flux response curve; 2) to define relationships between soil N transformatioon and soil N₂O emission and explore relative contributions of soil nitrification and denitrification to soil N₂O generation by means of the ¹⁵ N-¹⁸ O labeling technique in combination of molecular biology, of which the findings may explain the difference between the forests in North China and in South China in source of soil N₂O; and 3) to quantify the coupling relationships of soil N₂O flux with major soil microbial functional groups, such as nitrifiers, denitrifiers, and ammonia-oxidizing bacteria, and use molecular biological and matagenomic methods and techniques to determine effects of N addition on abundance and composition of N₂O producing bacterial community. By so doing, it is expected that the mechanism of the non-linear response of N₂O emission to increased N depostion in forest soils could be fully understood.