Abstract: Soil microbial biomass play an important role in the decomposition and transformation of residues returned to soil. In order to evaluate the effects of residue types and its carbon and nitrogen concentration on soil microbial biomass and respiration in black soil, different parts of corn and soybean straws were mixed separately with soil, and then incubated for 61 d at 25℃. The microbial biomass carbon (MBC) and nitrogen (MBN), mineral N and soil microbial respiration were measured after incubation, then the relationship between soil microbial biomass (MBC, MBN), respiration and C, N concentrations of different crop residue parts were analyzed. The results showed that the MBC under soybean roots treatments were highest, followed by bottom corn stover, soybean stalk, corn leaves, up corn stover, corn roots, the lowest were under soybean leaves. The MBN of different treatment in a descending order as:bottom corn stover, corn leaves, soybean roots, corn roots, up corn stover, soybean stalk, soybean leaves. Except for soybean leaves, the MBC/MBN of soil amended with residues were lower than blank control significantly. There were significant correlation between the carbon concentration of residues and MBC, MBN, MBC/MBN, microbial respiration rate and microbial metabolic quotient. MBC, microbial respiration rate and microbial metabolic quotient were negative related to the nitrogen concentration of residues. Moreover, a highly significant negative correlation was found between carbon concentration of residues and mineral nitrogen concentration of soil. Soil microbial respiration rate and microbial metabolic quotient among different controls were similar. A significant positive correlation was found between microbial respiration rate and microbial metabolic quotient. Those results suggested that straw amendments promote the growth and reproduction of the microbes with lower C/N, such as bacteria and actinomycetes, residues with higher nitrogen concentration lead to lower microbial biomass, respiration and metabolic activity at later stage of incubation, the microbial metabolic activity is a more important factor than the soil microbial biomass in residues decomposition and microbial respiration.