Bacterial necromass determines the response of mineral-associated organic matter to elevated CO2

Microorganisms regulate soil organic matter (SOM) formation through accumulation and decomposition of microbial necromass, which is directly and indirectly affected by elevated CO₂ and N fertilization. We investigated the role of microorganisms in SOM formation by analyzing ¹³C recovery in microorganisms and carbon pools in paddy soil under two CO₂ levels, with and without N fertilization, after continuous ¹³CO₂ labelling was stopped. Microbial turnover transferred ¹³C from living microbial biomass (determined by the decrease in phospholipid fatty acids) to necromass (determined by the increase in amino sugars). ¹³C incorporation in fungal living biomass and necromass was higher than that in bacteria. Bacterial turnover was faster than necromass decomposition, resulting in net necromass accumulation over time; fungal necromass remained stable. Elevated CO₂ and N fertilization increased the net accumulation of bacterial, but not fungal, necromass. CO₂ levels, but not N fertilization, significantly affected ¹³C incorporation in SOM pools. Elevated CO₂ increased ¹³C in particulate organic matter via the roots, and in the mineral-associated organic matter (MAOM) via bacterial, but not fungal, necromass. Overall, bacterial necromass plays a dominant role in the MAOM formation response to elevated CO₂ because bacteria are sensitive to elevated CO₂.