Coupling energy balance and carbon flux during cellulose degradation in arable soils

Abstract

Microbial carbon use efficiency (CUE) is an important metric for understanding the balance between anabolic and catabolic metabolism, while energy use efficiency (EUE) provides insight into microbial energy requirements. They are linked by the ratio between released heat and respiration (calorespirometric ratio, CR), which can be used to describe the efficiency of microbial growth. In this study, microbial C and energy use during the degradation of $$-labeled cellulose in eight different soils was investigated experimentally and simulated using a process-based model. Our results show close agreement between the cumulative C and energy balances during the incubations, with a total C and energy release equal to 30-50% of the amount added as cellulose. Both energy and C fluxes indicated that a positive priming effect of soil organic matter (SOM) increased the release of heat and CO₂ by 10 - 32% relative to the added substrate. The CR-CUE relationship indicated that growth on cellulose was energy limited during the early but not the later stages of the incubation, especially in soils with high SOM content. We partly observed systematic differences between estimates for CUE based either on the $$ label or on the calorespirometric ratio. Both approaches were constrained by technical and methodological limitations and agreed best during the phase of microbial growth in the SOM-rich soils, with CUE values between 0.4-0.75 indicating efficient aerobic growth. During early stages or after transition to a maintenance phase, both estimates were less meaningful for cellulose degradation, a substrate with a lower turnover rate than glucose. Still, the coupled heat and mass balances during cellulose degradation in combination with process-based modeling provided additional information on growth yields as well as the contribution of SOM priming to microbial growth compared to considering mass balances alone.