Soil microbivorous nematodes (bacterivores and fungivores) regulate the formation of microbial necromass carbon (MNC) through predation-driven shifts in microbial biomass and community composition, with straw return amplifying these effects via an increase in the abundance of microbivorous nematodes. However, how microbivorous nematodes affect MNC formation under straw return remains unclear. A 14-year field experiment was conducted in Southwest China to investigate the soil microbivorous nematode abundance, microbial biomass, and MNC content, along with their relationships across four levels of straw return: control (0 % return, CK), low (30 % return, S30), moderate (50 %, S50), and full (100 %, S100). Compared to the CK treatment, all straw return treatments significantly increased the contents of bacterial necromass carbon (BNC) by 29.1-55.3 %, fungal necromass carbon (FNC) by 35.5-49.8 %, and MNC by 33.4-51.6 %, but compared to the CK treatment, only the S100 treatment significantly increased the proportions of BNC, FNC, and MNC contributing to soil organic carbon (SOC). Compared to the CK treatment, the S100 treatment also significantly increased the abundance of r-strategist bacterivores by 163.4 %, bacterial biomass by 21.7 %, and the ratio of gram-positive (G+) bacteria to gram-negative (G-) bacteria by 9.3 %. Further correlation and aggregated boosted trees (ABT) analyses indicated that r-strategy bacterivores promoted BNC formation by influencing bacterial biomass and the ratio of G+ to G-. Moreover, the carbon use efficiency (CUE) of bacterivores was significantly positively correlated with the BNC and the ratios of BNC and MNC to SOC. Compared to the S30 and S50 treatments, the S100 treatment significantly increased the content of BNC, which was attributed to the elevated CUE of bacterivores in the S100 soil. Additionally, although straw return also significantly increased the abundance of fungivores, ABT analysis indicated that the effect of bacterivores on FNC was stronger than that of fungivores. Overall, 14 years of continuous straw return in a maize-wheat rotation system can increase the MNC content and its ratio to SOC by influencing the abundance of microbivorous nematodes and CUE, with the most pronounced effects observed under the S100 treatment.