Microbial-derived carbon is key to mineral-associated organic carbon accumulation in arid cropland soils

Soil organic carbon (SOC) pools in croplands are vital components of terrestrial ecosystem carbon sink. Identifying the sources and stabilization mechanisms of SOC in climate-sensitive, extensive arid croplands is crucial for understanding the global soil carbon sequestration capacity. We selected 37 typical farmland sites spanning approximately 2000 km across Xinjiang, China. Using lignin phenols and amino sugars as biomarkers for plant and microbial necromass, to quantify microbial-derived carbon (MDC) and plant-derived carbon (PDC). This enabled us to clarify their distribution patterns, contributions to SOC, and the factors influencing them. Overall, MDCdominated SOC accumulation in Xinjiang farmlands (average contribution: 26%), being 3.3 times higher than PDC(average: 7.9%). Among the microbial sources, fungal-derived carbon (FDC)contributed 2.3 times more than bacterial-derived carbon (BDC). Comparisons between northern and southern Xinjiang revealed that northern Xinjiang soils contained 64% more MDCthan southern Xinjiang soils, and the contribution of MDCin northern Xinjiang was 1.6 times higher than that in southern Xinjiang. However, PDCshowed no significant regional difference. The ratio of mineral-associated organic carbon (MAOC) to particulate organic carbon (POC) in northern Xinjiang was 2.8 times higher than that in southern Xinjiang, indicating greater SOC stability and a stronger MAOC-SOC correlation in the north. MDC was negatively correlatedwith mean annual temperature (MAT) and positively correlated with mean annual precipitation (MAP), whereas PDC had no significant correlation with climate. These results indicate a preferential accumulation of microbial carbon under cold and humid conditions. MDC showed a strong positive correlation with MAOC but not with POC, confirming its role as the primary precursor of stable MAOC formation. In conclusion, SOC stabilization in China’s arid northwest is primarily driven by a "microbial-mineral" coupling mechanism, wherein cold and humid conditions effectively promote MDC conversion into MAOC and enhance the stability of carbon pools.