Purpose <p>Soil carbon sequestration is intricately linked to microbial metabolism. However, the impacts of straw management combined with potassium (K) fertilization on soil organic carbon (SOC) pool and microbial metabolic limitation remains poorly understood in a wheat-cotton cropping system.</p> Methods <p>An eight-year (2015–2022) field experiment was conducted to investigate the effects of four types of straw management, including straw removal (CK), cotton straw returning (SC), wheat straw returning (SW), and cotton and wheat straw double returning (SCW), with three K rates (0, 150, and 225&#xa0;kg K₂O ha^− 1) on SOC functional fraction, SOC stability (the ratio of mineral-associated organic carbon to particulate organic carbon), and microbial metabolic limitation using vector modeling in sandy loam topsoil (0–20&#xa0;cm).</p> Results <p>Straw returning significantly increased SOC and its fractions while reducing SOC stability. K rate primarily altered unprotected SOC fractions, but having no significant effect on SOC stability. Straw returning, rather than K rate, alleviated microbial carbon and phosphorus limitations, with the order of mitigative effect being SCW &gt; SW &gt; SC. Meanwhile, microbial carbon and phosphorus limitations were key predictors of SOC fractions and stability. This study revealed that dual regulatory mechanisms of straw management rather than K fertilizer to promote SOC sequestration via directly increasing the content of SOC fractions and indirectly mitigating soil microbial metabolic limitation to regulate SOC stability.</p> Conclusion <p>Cotton and wheat straw double returning was most beneficial to improve SOC sequestration and alleviate microbial metabolic limitation. However, K fertilization has almost no effect on the wheat-cotton fields.</p>