The forest litter and crop straw residues are important components in the agroforestry system and play a key role in ecosystem carbon and nutrient cycle. During decomposition, to a great extent, the carbon mineralization is controlled by soil microorganisms. However, the quality of mixtures with various compositions is different due to difference in chemical compositions and physical structure from single component. Moreover, frequent addition of residues is reported to be able to keep the easily available compounds in soils in a high level. All the factors mentioned above were considered to have important effects on soil microbial activity. However, the effects of residue composition and addition frequencies on carbon mineralization, microbial biomass carbon and nitrogen have been seldom reported in the soil of agroforestry system. Poplars are the major tree component of traditional agroforestry systems throughout the south temperate central area of China. In this paper, an experiment was designed to study the influnce of different residue compositions and addition frequencies on carbon mineralization, microbial biomass carbon and nitrogen in the soil of poplar-wheat agroforestry system. The clay loam and silty loam in the experiment were collected from Sihong and Dafeng County of northern Jiangsu Province. The six treatments in the experiment were W: 16 g/kg (soil) wheat straw residue; P1: 16 g/kg (soil) poplar leaf litter; P1-W: 16 g/kg (soil) mixture of poplar leaf and wheat straw residue with 1∶1; P2: adding 4 g/kg (soil) poplar leaf litter every 8 days;P2-W: adding 2 g/kg (soil) poplar leaf litter every 8 days and 8 g/kg (soil) wheat straw residue at the first day, and CK: the control (without any addition). The soils with different treatments were incubated under the temperature of 25℃ at daytime and 15℃ at night for 31 days. The results showed that, for all the residue addition treatments, the carbon mineralization rates were higher in the daytime with the temperature of 25℃ than those in the night with 15℃ (P < 0.05). For the soils, the silty loam showed higher carbon mineralization rates than clay loam(P < 0.05). After 8 days of incubation, the treatment P2 and P2-W showed higher carbon mineralization rates than the other treatments (P ＜0.05), and the peak values occurred at 9, 17, 25 days. Mixed residues of poplar leaf and wheat straw could promote the cumulated carbon mineralization at the end of incubation, and the cumulated carbon mineralization was significantly higher in the treatment of adding poplar leaf litter serially (P2) than that of adding all at the first day (P1) (P＜0.05). Compared to the treatments of poplar leaf litter (P1) and wheat straw residues (W), mixed residues of poplar leaf and wheat straw significantly increased microbial biomass carbon and nitrogen in soil (P < 0.05). Furthermore, the microbial biomass carbon and nitrogen were significantly higher in the treatment of adding poplar leaf litter serially (P2) than that of adding all at the first day (P1). However, compared to the CK, the content of soil inorganic nitrogen decreased significantly for all the residue addition treatments, especially in the mixed residues treatments (P1-W and P2-W). The results from this study suggested that residue composition and addition frequencies could enhance the decomposition process, and regulate carbon dynamic and nitrogen supply. Such study would provide an important information for a deeper understanding of the carbon and nitrogen cycle in agroforestry systems.