Effects of N addition on the plant and soil microbial community in alpine Kobresia tibetica meadow of Qinghai-Tibet Plateau

Effects of continuous six-year N addition on the quantitative properties of plant and soil microbial communities in alpine Kobresia tibetica meadows were investigated experimentally by using field surveys and phospholipid fatty acid(PLFA) analyses.The experiment included four manipulative treatments involving both N and litter:added N(+N) with the litter removed(LR),+N with the litter left intact(LI),LI without N addition(-N),and LR with-N.Soil samples were collected at the depths of 0-10 and 10-20 cm.Aboveground biomass(AGB),litter biomass,microbial community composition,and nutrient content were measured.The results showed that(1) fertilization did not have significant effects on AGB and litter biomass.(2) N addition significantly increased the abundance of the microbial group PLFAs at the soil depth of 0-10 cm,particularly the abundance of the bacterial PLFAs(B PLFAs) and gram-positive bacterial PLFAs(G~+PLFAs),but decreased microbial abundances at the soil depth of 10-20 cm.(3) The ratio of the normal saturated fatty acids/monounsaturated fatty acids and B PLFAs/fungal PLFAs(F PLFAs) increased with the increase in the soil depth.(4) G~+ PLFA and F PLFA contents were significantly and positively correlated to pH,available phosphorus,available nitrogen,and soil organic matter contents at 0-10 cm soil depth.The contents and abundance of fatty acids were mainly regulated and affected by initial nutrient level in the topsoil after short-term N addition.These results indicate that microbial communities in this alpine tundra are sensitive to both litter and N inputs,and that the vertical structures of these communities are particularly vulnerable.