Abstract: Soil microbial biomass carbon and nitrogen were determined after adding fixed concentration of organic acids and changeable concentration of cadmium in pre incubated red soil and 14 day incubation at 25 ℃. The results show that soil microbial biomass carbon and nitrogen in the soil added with organic acids decreased with increasing concentration of cadmium. When the concentration of cadmium was higher than25 mg·kg^-1,the microbial biomass carbon and nitrogen in the soil added with low molecular weight of organic acids (oxalic acid,acetic acid,citric acid,tartaric acid) were lower than those without the addition of oranic acids,indicating that low molecular weight of organic acids could increase the toxicity of cadmium. On the contrary,the microbial biomass carbon and nitrogen in the soil added with low molecular weight of oranic acids were higher than those without the addition of organic acids when the concentration of cadmium was lower than25 mg·kg^-1,indicating that low molecular weight organic acids could decrease the toxicity of cadmium. The microbial biomass carbon and mitrogen in the soil containing humic acid were higher than those without the addition of organic acids,indicating that humic acid could decrease the toxicity of cadmium.However,the C:Nratio of soil microbial biomass was increased with incresing concentration of cadmium. Organic acids could increase microbial biomass carbon and nitrogen when soil was not added with cadmium.

Key words: Organic acid, Cadmium, Microbial biomass, Red soil, photosynthetic indicator, spatial heterogeneity, Larix olgensis, environmental factor