Abstract:Abstract:An experiment on 9 years of continuous nitrogen application was conducted to explore the effect of nitrogen addition on plant community structure in stipa baicalensis steppe, with different nitrogen addition levels of 0(N0)、15(N15)、30(N30)、50(N50)、100(N100)、150(N150)、200(N200) and 300(N300) kg N hm-2 a-1. The responses of community structure and important valve, diversity as well as biomass to different nitrogen addition in 9 years of continuous nitrogen application were explored.The results showed that: (1) Community structure was significantly responsed to long-term nitrogen addition with the lowest species number in high nitrogen application rate, while the highest important valve in domiant species under high nitrogen application rate were obserbed. (2) Plant diversity index in stipa baicalensis steppe was reduced with varying degrees in long-term nitrogen addition. The dominance index was increased with long-term nitrogen addition with the most prominent value in N300 level addtion (P<0.05). (3) The productivity of grassland vegetation (aboveground and belowground biomass) increased at first and then decreased after long-term nitrogen addition. The peak value of aboveground biomass was obserbed under N100 level addtion, while the maximum value of belowground biomass was appeared at N50 level (P<0.05). Root was characterized at 0-10 cm distribution.(4) RDA analysis showed that Carex duriuscula and Bupleurum scorzonerifolium were significantly positively correlated with soil pH(P<0.05) , and Thaictrum petaloideum and Potentilla bifurca were significantly positively correlated with soil organic matter (P<0.05).The community structure in stipa baicalensis steppe was greatly responsed to long-term nitrogen addition composition with diversity decrese and changes in community composition. Key Words: stipa baicalensis steppe; nitrogen addition; structure of plant community; plant diversity; soil environment