Abstract: 【Objective】Biocrusts are critical surface covers in desert ecosystems that play an essential role in enhancing soil organic carbon (SOC) sequestration through various biological and physicochemical processes. Despite extensive research on biocrust functions, the contribution of microbial necromass carbon (MNC) under biocrust (BSCs) coverage to SOC and its influencing factors remains unclear. This study aims to address this knowledge gap by analyzing the role of cyanobacterial, lichen, and moss crusts in the Mu Us Desert.【Method】Soil samples were collected from the cyanobacterial, lichen, and moss crust layers, as well as from the underlying soil (0–5 cm depth), to investigate their physicochemical properties and amino sugar contents as proxies for MNC. Contributions of fungal necromass carbon (FNC) and bacterial necromass carbon (BNC) to SOC were evaluated, and their relationships with soil pH, nitrogen content, exchangeable calcium ions, soil moisture, and particulate organic carbon (POC) versus mineral-associated organic carbon (MAOC) fractions were assessed.【Result】The results revealed that: (1) MNC constituted approximately 57.7%, 47.9%, and 22.5% of SOC in cyanobacterial, lichen, and moss crusts, respectively, while in the underlying soil, MNC contributed 40.7%, 40.2%, and 28.5% of SOC for the respective crust types. (2) Across all crust types, FNC contributed significantly more to SOC than BNC, with average contributions of 28.4%±10.7% and 11.2%±4.8%, respectively. (3) MNC, especially FNC, had a stronger influence on the POC fraction compared to MAOC, suggesting its dominant role in labile carbon pools. (4) Positive correlations were found between FNC and BNC and soil nitrogen contents (ammonium, nitrate, and total nitrogen) as well as SOC. Conversely, significant negative correlations were observed with soil pH, exchangeable calcium ions, and moisture content. These findings indicate that soil properties strongly regulate the dynamics of MNC in biocrust-covered soils. (5) The spatial variability of MNC contributions highlights the critical role of crust type and underlying soil characteristics in shaping microbial-derived SOC.【Conclusion】This study highlights that the contribution of MNC to SOC diminishes from cyanobacterial to lichen to moss crusts, with FNC consistently being the dominant component. MNC primarily contributes to the POC fraction, underscoring its role in maintaining active carbon pools. Soil nitrogen content, SOC, pH, exchangeable calcium ions, and soil moisture emerged as key factors influencing the accumulation and decomposition of MNC. These insights enhance our understanding of microbial-mediated soil carbon cycling and sequestration mechanisms in arid ecosystems. Furthermore, the findings underscore the importance of preserving biocrust integrity to sustain carbon storage functions in desert landscapes. The results provide a scientific foundation for devising carbon management strategies aimed at mitigating desertification, enhancing carbon sequestration, and fostering sustainable development in desert regions.