Long-term warming offsets the beneficial effect of elevated CO2 on mineral associated organic carbon in Mollisols.

The stability of soil organic carbon (SOC) is fundamentally important to the carbon-climate feedback because soils act as a major carbon source or sink under climate change. The uncertainty of SOC stability in farming soils in response to climate change necessitates mechanistic studies on microbial attributes to the change of SOC. Here, we used open-top chambers to simulate elevated CO2 (eCO2) and warming for 12 years in a soybean-grown Mollisol. We did not find the change of SOC stock under eCO2 or warming. Although eCO2 resulted in the accumulation of mineral-associated organic carbon, this effect diminished under warming. The amplicon sequencing of 16S gene indicated a significant change in microbial community composition under warming or eCO2. The metagenomic sequencing demonstrated that warming increased the abundances of microbial genes related to decomposition of labile carbon such as hemicellulose and pectin. The warming-induced stimulation of microbial catabolic metabolisms on organic carbon decomposition might have accelerated SOC turnover, which may offset the increased mineral-associated organic carbon of the Mollisol under eCO2. Long-term eCO2 and warming might not significantly alter the SOC stock or stability but accelerate carbon cycling in farming Mollisols.