A field experiment was conducted from October 2000 to Spring 2004 to examine soil microbial biomass C and N,soil light fraction C and N among the soils of alfalfa grassland,alfalfa-crop system,and conventional field systems(CS) in the semiarid loess plateau region of northwest China.The results showed that light fraction C and N pools of the alfalfa-crop rotation systems were higher than in alfalfa grassland.Microbial biomass C and N,as well as their ratios to the total in soils of the alfalfa-crop systems were higher than in conventional systems.Metabolic quotient was lower in the alfalfa-crop rotation systems than in 14-year-alfalfa grassland(AS14) and CS.The high soil microbial biomass,high decomposition ability for recently added plant material,but not utilizing the available resources more economically,indicated that high organic matter circulation in AS14 just kept soil fertility and didn't increase soil fertilizer.The alfalfa-crop rotation systems not only improved circulation of organic matter in 14-year-alfalfa grassland,but also maintained soil fertilizer over conventional farming system,and increased soil N utilization.