Microbial necromass carbon plays a crucial role in the formation of soil organic carbon (SOC). This study conducted a Meta-analysis of 80 published papers to quantify how microbial necromass carbon contributed to SOC in different mycorrhizal forests. The differences in the contents of microbial necromass carbon and their proportions of SOC in arbuscular mycorrhizae-dominated (AM) and ectomycorrhizae-dominated (EM) forests were specifically examined, with a particular emphasis on bacterial and fungal necromass carbon. The results showed the contents of microbial, bacterial and fungal necromass carbon, as well as their contributions to SOC in the topsoils of EM forests were significantly higher than those in AM forests. However, in the subsoils, no significant differences were observed in the microbial necromass carbon contents between AM and EM forests. For both types of forests, the contents of microbial necromass carbon (including fungal and bacterial necromass carbon) in the topsoils were significantly higher than that in the subsoils, and the contents of fungal necromass carbon in the topsoils were significantly higher than that of bacterial necromass carbon. This implied the fungal necromass carbon was the primary source of microbial-derived carbon in the topsoils of two forest types. In the topsoils, the contents of microbial necromass carbon in EM forests ranged from 0.08 to 89.17 g/kg. The average contents of fungal and bacterial necromass carbon were 12.75 g/kg and 3.98 g/kg, contributing 27.78% and 10.68% to SOC, respectively. By contrast, the contents of microbial necromass carbon in the topsoils of AM forests ranged from 0.54 to 71.64 g/kg. The average content of fungal necromass carbon was 6.42 g/kg, and the bacterial necromass carbon was 2.31 g/kg, accounting for 22.65% and 8.84% of SOC, respectively. Environmental factors had significant impacts on the accumulation of microbial necromass carbon. Random forest model predictions indicated that the contents of SOC, total nitrogen (TN), and mean annual temperature (MAT) were important drivers of microbial necromass carbon in both AM and EM forests. Higher SOC and TN contents and lower MAT in EM forests, were found to stimulate the accumulation of microbial necromass carbon. Conversely, higher soil pH and MAT values in AM forests promoted the decomposition of microbial necromass. This study analyzed the contributions of microbial-derived carbon to SOC and revealed the key factors influencing microbial-derived carbon accumulations in AM and EM forests. The results provided a theoretical basis for understanding the roles of soil microbes in SOC stabilization and transformation in the AM and EM forests.