【Objective】Crop straw input and nutrient supply may influence microorganism-mediated formation and transformation of soil organic carbon (SOC). However, the mechanisms by which straw and nutrient supply influence SOC transformation remain unclear. The objective of this study was to explore how and to what extent of straw-derived carbon (C) can be assimilated by microbes into necromass following crop straw amendment combined with nitrogen and phosphorus nutrients supply. 【Method】Using a 13C-labelled rice straw, the 300-day incubation experiment study examined the pattern of microbial assimilating straw-derived C in necromass (indicated by 13C-amino sugar dynamics) and its accumulation efficiency (CAE) as affected by nutrient supply levels. 【Result】The results showed that straw C could be transferred into microbial necromass rapidly as evidenced by the production of 13C-labelled individual and total amino sugars. There were higher amounts of 13C-amino sugars and CAE during the early stage of straw incubation (before 30 days), but a strong decline of total 13C-amino sugars, especially bacterial necromass (decreased by 18%-28% across treatments) was observed during the middle and later stages. The effect of nutrient supply on microbial assimilation of straw C processes was highly time-dependent, that is, no significant effect in the early stage and significant inhibition in the later stage. Specifically, nutrient supply did not have pronounced effects on straw C incorporation into amino sugars but significantly decreased 13C-amino sugars and CAE towards the end of incubation, suggesting an accelerated turnover of newly-formed amino sugars in treatments of straw combined with nitrogen and phosphorus supply. This might be related to potential changes of microbial strategies regarding nutrient acquisition and C allocation, highlighting the complex relationship between extraneous C and nutrient availability. Noteworthy, the total amino sugars (including 13C-labelled and unlabeled) were higher in nutrient supply treatments despite lower amounts of 13C-amino sugars, suggesting that crop straw addition with nutrient supply stimulated microbial transformation of native SOC components into necromass via microbial C pump mechanisms. 【Conclusion】This study highlights that straw input could stimulate microbial-derived C production and accumulation by accelerating microbial anabolisms via microbial C pump. The nutrient supply exerted an overall negative effect on the straw-derived microbial necromass accumulation in the long term, but stimulated native SOC-derived necromass contribution. It is suggested that the microbial necromass accumulation from newly added (fresh organic matter) and old soil carbon (inherent SOC) needs to be further investigated when evaluating the impacts of straw input on SOC formation and transformation. These findings advance the understanding of the mechanisms of microbial control over SOC formation and sequestration following extraneous organic matter input under varying nutrient conditions.