Bioenergetics of microbial maintenance metabolism in soil

Both active and dormant soil microorganisms spend a substantial fraction of their carbon and energy resources to maintain themselves, yet microbial maintenance metabolism remains inadequately represented in models of soil carbon cycling. The calorespirometric ratio (CR) of heat to CO$_2$ release has been established as a powerful tool to investigate the bioenergetics of microbial growth in soil, but its application to non-growth metabolism has not been systematically explored. Here, we use dynamic modeling to assess how maintenance processes influence the coupling between microbial carbon and energy use as reflected by the CR and how maintenance alters the relationship between the CR and microbial carbon use efficiency (CUE). We find that maintenance metabolism reduces apparent CUE, while its effects on the CR depend on the energy content of the soil organic matter (SOM) or biomass compounds consumed to fuel the maintenance reaction. A compilation of literature data on the CR in different soils reveals highly variable values, indicating that soil microbes utilize a wide range of substrates and metabolic pathways to meet their maintenance demands. ... mehrIn arable soils, we find a close linear relationship between maintenance CR and average SOM energy content, while there is no clear pattern in forest soils. Compiled CR observations after glucose addition display a pronounced drop in CR at the onset of the retardation phase, suggesting a shift towards the use of energy poor substrates. We present a bioenergetic framework to incorporate maintenance metabolism in process-based models of soil microbial carbon use, and our compiled data show how maintenance processes affect the coupling between carbon and energy cycling both in unamended soils as well as after the addition of labile substrates.