Nitrogen (N) deposition and precipitation could profoundly inflfluence the structure andfunction of forest ecosystems. However, conventional studies with understory additionsof nitrogen and water largely ignored canopy-associated ecological processes and mayhave not accurately reflflected the natural situations. Additionally, most studies only madesampling at one time point, overlooked temporal dynamics of ecosystem response toenvironmental changes. Here we carried out a fifield trial in a mixed deciduous forestof China with canopy addition of N and water for 4 years to investigate the effects ofincreased N deposition and precipitation on the diversity and community compositionof arbuscular mycorrhizal (AM) fungi, the ubiquitous symbiotic fungi for the majority ofterrestrial plants. We found that (1) in the 1st year, N addition, water addition and theirinteractions all exhibited signifificant inflfluences on AM fungal community composition;(2) in the 2nd year, only water addition signifificantly reduced AM fungal alpha-diversity(richness and Shannon index); (3) in the next 2 years, both N addition and water additionshowed no signifificant effect on AM fungal community composition or alpha-diversity,with an exception that water addition signifificantly changed AM fungal communitycomposition in the 4th year; (4) the increment of N or water tended to decrease theabundance and richness of the dominant genus Glomus and favored other AM fungi.(5) soil pH was marginally positively related with AM fungal community compositiondissimilarity, soil NH4+-N and N/P showed signifificant/marginal positive correlation withAM fungal alpha-diversity. We concluded that the effect of increased N deposition andprecipitation on AM fungal community composition was time-dependent, mediated bysoil factors, and possibly related to the sensitivity and resilience of forest ecosystem toenvironmental changes.