Background and aims <p>Mineral-associated organic carbon (MAOC) plays a significant role in the sequestration and stabilization of soil organic carbon (C) in croplands. Although straw return is widely practiced, how straw quality influences the conversion of straw-derived C into MAOC remains controversial.</p> Methods <p>We conducted a 180-day laboratory incubation experiment using maize (bract, leaf, sheath, stalk and root) and soybean (leaf, stalk, pod and root) straws at 15&#xa0;°C and 25&#xa0;°C. Straw C loss, MAOC accumulation, microbial biomass C, and microbial necromass C (amino sugars) production were examined.</p> Results <p>Soybean straw systematically decomposed more rapidly (with a 32.5% mean increase) and contributed more to MAOC accumulation (with a 28.9% mean increase) compared to maize straw, irrespective of temperature. The accumulation of MAOC was positively correlated with microbial biomass and necromass. Although fungal necromass remained dominant across all straw tissues, soybean straw generally exhibited a lower fungal‑to‑bacterial necromass C ratio than maize straw. Moreover, most soybean tissues exhibited higher total microbial necromass C than maize tissues, despite having lower microbial necromass accumulation efficiency. Additionally, elevated temperature accelerated straw decomposition, but did not significantly affect microbial necromass C and MAOC accumulation.</p> Conclusions <p>This study reveals that soybean straw is generally more conducive to MAOC formation than maize straw, and elucidates the mechanisms by which straw quality regulates microbial transformation. These findings enhance our understanding of how straw incorporation affects farmland C pools and provide a scientific basis for agroecosystem management.</p>