Straw return plus zinc fertilization increased the accumulations and changed the chemical compositions of mineral-associated soil organic carbon

Agricultural zinc (Zn) fertilization can markedly promote organo-Zn complex formation, especially when com-bined with straw. However, how soil organic carbon (SOC) conversion responds to Zn and/or straw plus Zn has received little attention. Here, we explored the SOC, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), as well as the OC chemical compositions' responses in a field experiment site created in 2016. The following treatments were control (CK, no straw return and no Zn fertilization), straw return (St), Zn fertilization (Zn), and straw return combined with Zn fertilization (St+Zn). The results of our three-year study indicated that MAOC content under St+Zn increased by 4.4-6.4%, which led to a slight SOC accumulation (3.7-7.4%) compared to that of St alone. After including straw return with Zn fertilization, the Zn content of particulate organic matter and that of mineral-associated organic matter (ZnMAOM) both increased and thus helped MAOC formation. The chemical compositions of POC and MAOC were affected mainly by straw return and Zn fertilization, respectively. Both the Zn and St+Zn treatments had increased relative hydroxyl C contents and reduced O-alkyl C contents in MAOC, compared to that of CK and St, and the ZnMAOM content best explained the changes. In both the POC and MAOC fractions, the aromatic C to aliphatic C ratio decreased in Zn treatments compared to those in CK. Altogether, Zn's effects on SOC and its fractions were closely linked to straw return, and Zn aids in MAOC and SOC accumulations when applied with straw.