Alpine grasslands with a high soil organic carbon (SOC) storage on the Tibetan Plateau are experiencing rapid climate warming and anthropogenic nitrogen (N) deposition;this is expected to substantially increase the soil N availability,which may impact carbon (C) cycling.However,little is known regarding how N enrichment influences soil microbial communities and functions relative to C cycling in this region.We conducted a 4-year field experiment on an alpine grassland to evaluate the effects of four different rates of N addition (0,25,50,and 100 kg N ha-1 year-1) on the abundance and community structure (phospholipid fatty acids,PLFAs) of microbes,enzyme activities,and community level physiological profiles (CLPP) in soil.We found that N addition increased the microbial biomass C (MBC) and N (MBN),along with an increased abundance of bacterial PLFAs,especially Gram-negative bacterial PLFAs,with a decreasing ratio of Gram-positive to Gram-negative bacteria.The N addition also stimulated the growth of fungi,especially arbuscular mycorrhizal fungi,reducing the ratio of fungi to bacteria.Microbial functional diversity and activity of enzymes involved in C cycling (β-1,4-glucosidase and phenol oxidase) and N cycling (β-1,4-N-acetyl-glucosaminidase and leucine aminopeptidase) increased after N addition,resulting in a loss of SOC.A meta-analysis showed that the soil C/N ratio was a key factor in the response of oxidase activity to N amendment,suggesting that the responses of soil microbial functions,which are linked to C tover relative to N input,primarily depended upon the soil C/N ratio.Overall,our findings highlight that N addition has a positive influence on microbial communities and their associated functions,which may reduce soil C storage in alpine grasslands under global change scenarios.