The loss of soil organic matter is a major concern in many areas of the world, especially in China, in rice paddies under warm-humid climatic conditions. Because carbon content is typically associated with soil quality, there is a need to preserve soil carbon pools. Biochar is a material that has shown promise as a means of soil amendment and carbon sequestration. However, it is unclear how the added biochar affects the distribution of organic carbon among different density fractions. In the current study, a field experiment was conducted to study the effects of biochar application on the organic carbon distribution among different density fractions and the soil microbial biomass of paddy soil. The five treatments were soil only (CK), soil and corn straw (CS), soil and biochar produced at 300℃ (300BC), soil and biochar produced at 400℃ (400BC), and soil and biochar produced at 500℃ (500BC). The different biochars were added to the field twice, at a concentration of 6 t/hm². The results show that biochar application increased the size of the light fraction proportion significantly, by 39.81%-41.20% compared to the CK treatment. This may be because of the low density of biochar. The added biochar increased the organic carbon content of the light fraction (LFOC) significantly, by 60.04%-69.66% compared to the CK treatment. LFOC in the different treatments was ordered as follows:500BC > 400BC > 300BC > CS > CK. The organic carbon content was highest in the 500BC treatment, with a value of 185.1 g/kg. This may be due to the higher carbon content of biochar produced at higher temperatures. With the increase in pyrolysis temperature, the volatile matter content decreased, the organic carbon content increased, and the stability of the organic carbon increased. Additionally, compared to the control, corn straw application also increased the organic carbon content. The organic carbon content was significantly lower in the straw amendment treatment than in the biochar treatments. None of the treatments had a significant effect on the heavy fraction organic carbon (HFOC). Straw incorporation improved the soil microbial biomass significantly, because straw contains a large quantity of soluble carbohydrates, which may stimulate the growth of soil microorganisms. The incorporation of biochar improved the soil microbial biomass significantly; however, the soil microbial biomasses measured in biochar treatments were lower than that of the CS treatment. The biochar treatments contained less soil microbial biomass because of the extremely low microbial availability of biochar. The slight increase in soil microbial biomass observed in the biochar treatments could be explained by a microbial response to the porosity of biochar and the presence of biochar carbon, although the concentration of soluble carbohydrates was reduced by the pyrolysis treatment. These results suggest that the application of biochar increased the organic carbon content of the soil, especially the LFOC. Compared to the treatment involving the incorporation of straw, biochar incorporation decreased the soil microbial biomass and microbial quotient, which improves soil carbon fixation. However, in the biochar treatments, the chemical and biological stability of LFOC was high due to the aromatic structure of biochar, which is different from the traditional theory that LFOC is labile and easily recycled. We concluded that the application of biochar was a more efficient method of carbon sequestration in paddy soil than the incorporation of straw, because of the lower soil microbial biomass present in the biochar treatments.