Is enzymatic stoichiometry a reliable indicator of microbial limitations in carbon, nitrogen, or phosphorus?

The enzymatic stoichiometry approach is based on the assumption that the metabolic and stoichiometric requirements of soil microorganisms are reflected in the production of enzymes targeting specific nutrient resources, making enzyme activities a useful tool for assessing microbial nutrient limitations. In this approach, β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG) (occasionally combined with leucine aminopeptidase, LAP), and acid/alkaline phosphatase (AP) are used as proxies for the broader suite of enzymes responsible for catalyzing complex substrates containing carbon (C), nitrogen (N), and phosphorus (P), respectively, because these enzymes catalyze the terminal reactions that release products containing C, N, and P. The fundamental premise of the approach can thus be reformulated as follows: BG:NAG and BG:AP ratios reflect the relative limitations of C versus N and C versus P, respectively, with higher ratios indicating stronger C limitation. Previous meta-analyses have suggested that the enzymatic stoichiometry approach is unreliable, as N and P fertilization often leads to reduction in BG:NAG and BG:AP, respectively, contradicting the predictions of the approach. However, some researchers question the validity of assessing enzymatic stoichiometry after the artificial addition of nutrients via fertilizers. For providing more convincing evidence, this study conducted a meta-analysis to assess the impact of incorporating various C sources, largely representative of those found in natural ecosystems-glucose, cellulose, and plant residues-on the BG:NAG or BG:AP ratios. The results demonstrated that a considerable number of data points exhibited elevated values (i.e., C addition accelerated C limitations), undoubtedly contradicting the fundamental premise of the approach. These findings highlight the unreliability of enzymatic stoichiometry as an indicator of microbial nutrient limitation.