Interpreting differences in microbial carbon and nitrogen use efficiencies estimated by isotope methods and the ecoenzyme stoichiometry model

Abstract

Isotope methods and ecoenzyme stoichiometry are the most commonly used approaches to estimate carbon and nitrogen use efficiencies (CUE and NUE) of microbial communities. Isotope methods estimate metabolic efficiencies by tracking the allocation of substrate between catabolic and anabolic fates, whereas ecoenzyme stoichiometry estimates resource use efficiencies based on balancing substrate availability with microbial element requirements. These differences affect interpretation, which has often been overlooked, leading to misconceptions and erroneous conclusions. In this paper, we clarify key differences in frameworks for interpreting results from the two methods with respect to potential driving factors. Differences in element use efficiencies determined by isotope methods primarily reflect changes in substrate biochemical transformations within microbial communities, whereas ecoenzyme stoichiometry mirrors shifts in the match between substrate stoichiometry and microbial community element requirements. Specifically clarifying these differences provides a better understanding and complementary interpretation of microbial CUE and NUE.