INVESTIGATING THE EFFECT OF SOIL RESPIRATION AND MICROBIAL CARBON USE EFFICIENCY ON SOIL CO2 AND O2 CONCENTRATIONS

Soils represent the largest terrestrial carbon pool, and changes in temperature and precipitation driven by a changing climate leave the fate of ~1600 Gt of carbon uncertain. Carbon cycling dynamics are now more important than ever to understand, and this necessitates novel approaches. This research seeks to better inform the application of soil respiratory quotient (RQ) measurements to track microbial metabolisms and provide estimates of carbon use efficiency (CUE). RQ’s are the ratio of carbon dioxide produced to oxygen consumed through aerobic respiration, and they can serve as an indicator of carbon source through stoichiometric demand of the oxidation state of carbon within various compounds. Carbon use efficiency represents the proportion of carbon retained for biosynthesis of total carbon taken up and is therefore critical in understanding the fate of soil carbon. The typical RQ framework represents purely aerobic respiration corresponding to a CUE of zero. The metabolism of sugars and carbohydrates produces an RQ value of 1.0. Though this can occur during periods of “waste respiration”, non-zero CUEs represent all other periods of metabolism, meaning that RQ’s other than 1.0 are possible during glucose metabolism, and have been documented in previous works. We aim to extract additional context from these RQ values, which are observed to be higher or lower than expected based on carbon source. To investigate the effects of CUE on RQ values, we constructed an automated gas sampling apparatus, measured incubation headspace gases, and carried out chloroform fumigation extractions to measure biomass carbon, and stoichiometrically model this relationship. Here we provide RQ values at 2-Hr resolutions and present modelled relationships between RQ and CUE for the metabolism of glucose and production of a generic microbial stoichiometry. This relationship allows for the calculation of biomass production at the same temporal resolution and suggests short term changes in RQ values may correspond to shifts in CUE rather than carbon source. This work has implications for future applications of RQ values in field and lab settings and expands the applications of RQ measurements to indicate CUE and carbon source. Similarly, this modeled relationship can be derived for other compounds of interest.