【Objective】Paddy soil has enormous potential for carbon sequestration and straw inputs in these soils strongly influence the microorganism-mediated turnover of soil organic carbon (SOC). This study aims to explore the microbial processes involved in the formation of SOC in response to straw inputs and to determine the regulatory effects of nitrogen and phosphorus on these microbial processes. 【Method】A 300-day laboratory experiment was conducted to investigate the effects of straw (S) and straw combined with nitrogen and phosphorus nutrient additions (S+NP) on soil microbial communities and their necromass accumulation processes, in a subtropical red loam rice soil. Phospholipid fatty acids (PLFA) and amino sugars (AS) were used as microbial biomarkers to indicate living microbial biomass community and necromass, respectively. 【Result】The results showed that compared to the control (CK) treatment, the S and S+NP treatments significantly increased the total soil PLFA content (P < 0.05) and the increase in fungal biomass (65.1%-30.1%) was greater than that of bacterial biomass (22.6%-34.3%). Fungal biomass was significantly higher in the S+NP treatment than that in the other treatments at the later incubation stage of 300 d, and the ratio of fungal/bacterial biomass (F/B) also significantly increased in straw plus high rate of nutrient supply (P < 0.05). Our results suggested that nitrogen and phosphorus application may affect soil microbial community structure over a longer time after straw amendment, which gradually shifted towards a fungal-dominated community structure with time. The accumulation of microbial necromass was significantly higher in both S and S+NP treatments than in the control throughout the incubation period (P < 0.05), and the dynamics of fungal necromass mirrored the total microbial necromass. However, bacterial necromass reduced remarkably at the end of the incubation and their content was significantly lower in the S+NP treatment than that in the S treatment (P < 0.05), suggesting its decomposition in the later stages of incubation. Further correlation analysis showed significant positive correlations between microbial biomass, microbial necromass and SOC (P < 0.05). This demonstrates the importance of microbial mediated mechanisms driving SOC formation and transformation in paddy fields following exogenous straw application. 【Conclusion】In conclusion, the impact of straw return on soil organic carbon pool in paddy fields is closely related to microbial community structure and its mediation of necromass accumulation process, which may be regulated by extraneous nutrient supply. Furthermore, supplementation with moderate amounts of nitrogen and phosphorus nutrients can facilitate the accumulation of organic carbon fractions of microbial origin over longer time scales, especially the fungal necromass carbon. This will be of significant importance in regulating microbial-mediated organic carbon sequestration processes through on-farm nutrient management practices.