Abstract: To obtain some information about transformation of mineral nutrients by soil microorganisms,we have measured the amount and turnover rate of soil rnicrobial biomass in no-tillage,direct drilling and plow-ing soils.Microbial biornass in notillage top soil is 51.7% and 26.0% higher than those in plowing and direct drilling soils(0-7.5cm)；The turnover rates of soil microbial biomass are 1.17yr^-1 in plowing soil (0-15cm),0.66 in no-tillage and 0.74 in direct drilling soils(0-7.5cm),with minimal difference for tillage or depth in lower soil layers;The nutrients immobilized by microbes and their flux through microb biomass are greater than their uptake by crops. The principal function of the microbial biomass is thus to accumulate and conserve nutrients in a biologically active form during seedling period of crops when crops are growing rapidly and absorbing much nutrients from soil,and then to release nutrients to promote crop growth.

Key words: Ambrosia trifida, shading, phenotypic plasticity, morphological characteristics, photosynthesis characteristics.