Abstract: Taking the farmland and the poplar (Populus × liaoningensis) plantation stands with different successive rotation durations (the first rotation of tenyearold, the second rotation of sixyearold, and the second rotation of fifteenyearold) in Jixie Forest Farm in Xinmin City, Liaoning of Northeast China as test objects, an investigation was made on the soil nutrient contents, microbial biomass, and enzyme activities, aimed to evaluate the effects of returning farmland to forestland on the soil chemical and biological properties. After returning farmland to forestland, and with the increasing stand age and successive rotation duration, the soil pH presented an increasing trend, whereas the soil nutrient contents and microbial biomass increased after an initial decrease. The soil basal respiration and catalase activity were significantly higher in poplar stands than in farmland, whereas the soil urease and neutral phosphatase activities were in opposite trend. The soil fertility indices except pH all decreased with increasing soil depth. All the results indicated that the soil fertility tended to be declined at the early stage of afforestation, but gradually increased with the increasing stand age and successive rotation duration. These findings could offer references for the returning farmland to forestland in Northeast China.