Abstract
Soils store more carbon than the biosphere and atmosphere combined, and the efficiency to which soil microorganisms allocate carbon to growth rather than respiration is increasingly considered a proxy for the soil capacity to store carbon. This carbon use efficiency (CUE) is measured via different methods, and more recently, the 18O-H2O method has been embraced as a significant improvement for measuring CUE of soil microbial communities. Based on extrapolating 18O incorporation into DNA to new biomass, this measurement makes various implicit assumptions about the microbial community at hand. Here we conducted a literature review to evaluate how viable these assumptions are and then developed a mathematical model to test how violating them affects estimates of the growth component of CUE in soil. We applied this model to previously collected data from two kinds of soil microbial communities. By changing one parameter at a time, we confirmed our previous observation that CUE was reduced by fungal removal. Our results also show that depending on the microbial community composition, there can be substantial discrepancies between estimated and true microbial growth. Of the numerous implicit assumptions that might be violated, not accounting for the contribution of sources of oxygen other than extracellular water to DNA leads to a consistent underestimation of CUE. We present a framework that allows researchers to evaluate how their experimental conditions may influence their 18O-H2O-based CUE measurements and suggest the parameters that need further constraining to more accurately quantify growth and CUE.
Highlights
Soil microbes serve as key modulators of the global carbon cycle, transforming the massive amounts of carbon found in soil into biomass and waste products such as carbon dioxide
Whereas observations assumed that 100% of the oxygen in new DNA came from extracellular water (Spohn et al, 2016b; Zheng et al, 2019), our simulations assumed that only 70% of the new DNA oxygen did
carbon use efficiency (CUE) is an essential descriptor of soil carbon cycling, with important ramifications for both the ecology and
Summary
Soil microbes serve as key modulators of the global carbon cycle, transforming the massive amounts of carbon found in soil into biomass and waste products such as carbon dioxide This partitioning of carbon into growth as opposed to respiration—or carbon use efficiency (CUE)— is increasingly measured in soils as a sensitive integrator of microbial response to the impacts of human activity such as elevated temperature (Frey et al, 2013), altered soil moisture (Herron et al, 2009; Zheng et al, 2019), land use change (Malik et al, 2018; Bolscher et al, 2020), and nutrient availability (Spohn et al, 2016). 13C methods measure substrate use efficiency on a specific compound and do not capture differences in the efficiency of microbial growth on the wide repertoire of substrates available in the soil
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