Estimating microbial carbon use efficiency in soil: Isotope-based and enzyme-based methods measure fundamentally different aspects of microbial resource use

How carbon partitions between microbial biomass and CO2 (carbon use efficiency, CUE) is key in all soil carbon cycling models. Traditional methods to estimate CUE focus on the physiological partitioning of specific substrates, typically labeled with isotopes. However, an alternative approach (Sinsabaugh et al., 2016) is based on community-level resource capture using assays of extracellular enzymes; although this uses the same name (CUE), it measures something distinctly different from the isotopic methods. Rather, it assesses how microbes shift resource use in response to substrate stoichiometry.