Long-term elevated CO2 and warming enhance microbial necromass carbon accumulation in a paddy soil

Soil microbial necromass plays a critical role in soil organic C (SOC) sequestration, while the long-term response of microbial necromass to climate change remains largely unclear. Here, we used amino sugars as biomarkers and examined their variation after 8 years of continuous manipulation of elevated CO2 (eCO2), warming, and their combined interaction in a paddy soil. Our results showed that eCO2 increased the concentrations of all amino sugar compounds by 6.5–28.9% while warming had no effect on the accumulation of glucosamine and galactosamine but increased muramic acid concentration by 22.1–29.1%. Elevated CO2 increased the contribution of microbial necromass C to SOC storage, mainly by increasing fungal-derived C, whereas warming increased the bacterial-derived C proportion in SOC. Furthermore, the combined effect of eCO2 and warming yielded the highest total microbial necromass and SOC accumulation, although the ratio of fungal to bacterial necromass C in SOC remained unchanged. Structural equation models showed that root biomass had an indirect positive effect on total amino sugar concentration, mainly through increased microbial biomass, whereas N-acetylglucosaminidase activity had a direct negative effect on total amino sugar accumulation. These differential responses of microbial necromass to climate change may further alter the sequestration of SOC. This study is only based on one sampling time, and future research should involve more sampling times so as to have the temporal dynamics of the studied properties. Our findings emphasize the contribution of the microbial-derived C to soil C stock under long-term elevated CO2 and warming in a rice-wheat rotation system, which reveals an important mechanism of microbial-mediated C sequestration under climate change.