The plant subsystem in typical steppe grassland of the Loess Plateau was divided into four components: live shoots,standing dead matter,litter,and underground roots.The plant biomass of these four components and their C,N,and P contents were studied in grazed(GG) and fenced(FG) grasslands.The seasonal dynamics of each component biomass was correlated with their C,N and P contents both in grazed and fenced grasslands.There were significant positive correlations between plant biomass and C,N,and P contents,with correlation coefficients of 0.990,0.899,0.936(FG) and 0.990,0.891,0.936(GG),respectively.The differences in C,N,and P contents of each plant component were caused by the difference in plant biomass.The total biomass,aboveground and underground biomass,standing dead matter and litter,and their C,N,and P contents were significantly(P0.05) higher in fenced grassland than those in grazed grassland.The N and P contents in live shoots were highest in July,and significantly(P0.05) lower in fenced(1.291 8 and 0.083 7 g/m2 for N and P respectively) than in grazed(1.529 7 and 0.100 2 g/m2 for N and P respectively) grassland.The results indicate that the higher plant N,P storages in grazed grassland were mainly attributed to the increases of young organ biomass and their N,P contents,which further improve the utilization of grassland.