Abstract: Pasture-crop rotation is regarded as a key planting system in the semiarid area of China's Loess Plateau.This paper studied the dynamics of soil nitrogen,phosphorus and organic matter within the two years of alfalfa-crop rotation.The results showed that in comparing with continuous alfalfa planting,alfalfa-crop rotation induced a decline of soil total nitrogen and organic matter contents by 5.4% and 19.5%,and 46.8% and 28.2%, respectively, in the first and second year,but no significant difference was found in soil total phosphorus.Soil nutrient availability was improved due to the rotation.In the first and second year of rotation,soil nitrate nitrogen and available phosphorus contents increased by 15.5% and 159.1%,and 44.5% and 48.0%,respectively.Planting spring wheat could maintain soil fertility,the second was fallow and planting potato,while planting corn accelerated the depletion of soil total nitrogen,organic matter and available phosphorus.As a result,in the alfalfa-crop rotation,planting spring wheat could be more available to the maintenance of soil fertility.

Key words: Japonica hybrid rice, Super high yield, Canopy structure