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 C-13 recovery in microorganisms and carbon pools in paddy soil under two CO2 levels, with and without N fertilization, after continuous (CO2)-C-13 labelling was stopped. Microbial turnover transferred C-13 from living microbial biomass (determined by the decrease in phospholipid fatty acids) to necromass (determined by the increase in amino sugars). C-13 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 C-13 incorporation in SOM pools. Elevated CO2 increased C-13 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.