Abstract: Impacts of simulated atmospheric nitrogen (N) deposition on soil fauna biomass were studied in a 16 month period from May 2003 to August 2004. Plots were built in pine forest (PF), pine and broad-leaf mixed forest (MF) and monsoon evergreen broad-leaf forest (MEBF) in Dinghushan of subtropical China. Dissolved NH4NO3 equivalent to the control (No addition), low ［50 kg/(hm2·a)］, and medium ［100 kg/(hm2·a)］ was sprayed during the testing period. The biomass of soil fauna group was decided by an experimental standard based on research performed a long time in this area. When the data was averaged throughout the period and across the forests, there were no significant effects of N treatment on soil fauna biomass. However, effects could be observed in the interactions between N and forest type, and the interactions between N and sampling date. There were obviously positive effects of N treatment on soil fauna biomass in PF, but negative effects under abundant N deposition in MEBF. After one year the distribution of soil fauna biomass in different forests was changed significantly, with MEBF>MF>PF before the treatments and PF>MEBF>MF after the treatments. Low N treatment was positive to soil fauna biomass to a certain extent, which increased soil fauna biomass by 44.33% in MEBF, 9.19% in MF and 60.66% in PF; but soil fauna biomass declined generally under the medium treatment, 32.55% in MEBF and 2.81% in MF. The results indicated that there might be a threshold in the effects of N deposition on soil fauna biomass.