Assessing the accumulation efficiency of various microbial carbon components in soils of different minerals

Heterotrophic soil microbes are increasingly recognized as a key mediator transforming labile organic carbon (OC) into relatively stable soil carbon (C) in the form of microbial necromass (dead cells) and extracellular compounds associated with minerals. However, the accumulation of microbial necromass relative to labile OC consumption and its regulating factors remain poorly understood, although it has vital implications for soil C sequestration and modeling. Here by mimicking microbial C accumulation in constructed model soils using fructose as the sole OC substrate, we present a benchmark comparison of microbial C accrual versus OC mineralization under declining substrate availability and mineralogy. By quantifying various microbial components including biomass (living cells; indicated by phospholipid fatty acids), necromass (indicated by amino sugars) and total microbial C (including biomass, necromass and extracellular compounds; estimated as the difference between added and residual substrate C minus respiration) in a simple soil system, we compare microbial metabolic quotient (qCO2; i.e., microbial respiration rate per unit of biomass), amino sugar accumulation efficiency (AAE; i.e., ratio of amino sugars to respiration) and microbial C accumulation efficiency (mCAE; i.e., ratio of total microbial C to total microbial C and respiration), and assess their regulating factors. We find that while clay (bentonite) promotes microbial respiration, it enhances the rate as well as efficiency of amino sugar accumulation without affecting qCO2 or mCAE. On the contrary, ferrihydrite increases qCO2 and decreases AAE but promotes labile OC preservation via inhibiting microbial growth in the alkaline model soil. Hence, amino sugar accrual is more efficient in clay-rich model soils while labile OC is less consumed in model soils containing iron (hydr)oxide. Furthermore, while mCAE was correlated with qCO2 in all but the model soil with 6% clay, AAE was correlated with qCO2 only in model soils with 12% clay when the mineral treatments were considered separately. Collectively, our findings suggest that mCAE and AAE heavily depend on mechanisms preserving microbial C components but not solely on the metabolic efficiency and is mediated by soil mineral content as well as composition. Parameters considering microbial C preservation such as mCAE or AAE warrant further study for modelling and managing the formation of microbial derived stable soil OC.