Effects of long-term application of N fertilizer on microbial communities in Mollisol,in Northeast China,were investigated by determining phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) of soil microbes,NLFA/PLFA ratio,and PLFA composition in soil samples different in nitrogen treatment with or without phosphorus and potassium fertilizer.Treatments Nil-fertilizer and Fallow were set as controls.Results of PLFA determination show that application of nitrogen coupled with phosphorus promoted microbial growth and immobilization of nitrogen in the soil,while application of nitrogen coupled with phosphorus and potassium did not show any similar effect.Application of nitrogen alone,however,inhibited growth of fungi,probably due to the excessive consumption of nutrients such as available phosphorus.Comparison between different microbial groups in PLFA ratio demonstrates that fungi were better adapted to nutritional stress than bacteria,while the latter accumulated more rapidly in biomass than the former in Treatment N or Treatment NP.NLFA content and NLFA/PLFA ratio varied sharply between treatments different in N rate,suggesting that they could be used as indicators of bio-physiological conditions of soil microorganisms and nutrient level of the soil.