Natural wetlands store 20-30 % of soil organic carbon (SOC), and conversion of croplands to wetlands is thus an effective strategy for SOC restoration. However, the contribution of microbial necromass carbon to SOC change during this conversion process remains unclear. We collected samples of topsoil (0-20 cm) and subsoil (40-60 cm) from croplands, rewetlands (i.e., rewetting croplands) and natural wetlands in 20 different sites in Baiyangdian Lake to compare the changes of SOC fractions (particulate organic carbon, POC; minerals associated organic carbon, MAOC) and the microbial contribution to SOC. Both SOC and POC in the topsoil gradually decreased from croplands to wetlands, and the proportion of MAOC to SOC increased. The content of microbial necromass carbon (MNC) mirrored the decline in SOC, yet its contribution to SOC in the topsoil remained stable at 16.0 %-19.6 %. The relative contribution of MNC to POC gradually decreased in converting croplands to wetlands across the topsoil and subsoil, while its contribution to MAOC gradually increased in the subsoil. Depth-dependent regulation of SOC fractions was observed, with conversion indirectly affecting soil microbe and necromass accumulation through regulating soil properties, thereby influencing SOC fractions in the topsoil. In contrast, in the subsoil, soil properties had a more direct impact on SOC fractions, with microbial effects being less apparent. This study confirms the critical role of MNC in regulating SOC and its fractions during converting croplands to wetlands, and highlights the potential to enhance SOC stability.