Abstract: Rapid-growth poplars have been planted in large areas because they can bring about huge economic benefit.Soil organic carbon(SOC),total nitrogen(TN) and microbial biomass in soils of a farmland and two poplar woodlots,4-year-old and 7-year-old,in Dezhou,Shandong Province were selected for comparison.Results show that microbial biomass,SOC and TN were all concentrated in the soil layers of 0-5 cm and 5-10 cm,and significantly lower in the woodlot soils than in the farmland soil,especially microbial biomass.But no significant difference was found between the three sites when it went down to 10 cm below in soil depth.Significant positive linear relationship was found between SOC and microbial biomass carbon,and between soil TN and microbial biomass N.Obviously once farmland is turned into rapid-growth poplar plantation,soil fertility decreases.