【Objective】Biochar has been considered as a promising strategy to improve soil carbon (C) sequestration and soil fertility. The aim of this study was to investigate effects of amendment of maize stalk biochar and returning of maize stalk on soil organic carbon fractionation and microbial activities in an unfertilized field of upland red soil, in an attempt to provide certain theoretical guidance for using biochar as soil amendment to improve soil fertility and enhance soil C storage in agricultural soils. 【Method】A field experiment was carried out to investigate changes in soil organic carbon pools, activities of the enzymes involved in C, N and P cycling, and microbial substrate utilization rates as affected by amendment of biochar and/or returning of straw. The biochar used in the experiment was prepared out of maize stalk via pyrolysis at 450~500 °C near anaerobically, and amended with or without straw returned in the experiment upland field of Ferralsol low in fertility. Soil samples were collected from the experiment for analysis of soil labile and recalcitrant C fractions with the two-step sulfuric acid hydrolysis method, and for soil enzyme activities and microbial substrate utilization rates with the microplate fluorimetric assay and MicroResp, respectively. 【Result】Results show that 9 months after the experiment started, soil labile C content increased in the treatment of straw returning alone, but no much changes were observed in soil pH, total organic C and recalcitrant C contents. Soil organic C and recalcitrant C increased significantly in content in the treatments of amending biochar alone or with straw returned. β-glucosidase activity was enhanced by straw returning, but not by amendment of biochar. Soil basal respiration and microbial utilization rates of glucose, asparagic acid and syringic acid were significantly increased by straw returning, but were not much affected by biochar amendment alone or in combination with straw. Significant interactions were observed between biochar and straw on labile C content and soil basal respiration rate. Significant and positive correlations were also found of soil basal respiration with labile C content, β-glucosidase activity and microbial utilization rate of glucose. 【Conclusion】To sum up, all the findings in this experiment suggest that compared with straw returning, amendment of the biochar derived from maize stalk improves soil stable C pools and reduces soil C emission.