Does ecoenzymatic stoichiometry really determine microbial nutrient limitations?

Recently, an increasing number of studies use ecoenzymatic stoichiometry for determining nutritional status or nutrient limitations of microbes. According to the ecoenzymatic stoichiometry theory, the ratios of β-1,4-glucosidase (BG) and β-1,4-N-acetylglucosaminidase (NAG) (BG:NAG) or BG and NAG + leucine aminopeptidase (LAP) (BG:(BG + LAP)) reflect microbial carbon (C) vs nitrogen (N) limitation, with larger ratios indicating C limitation. However, several studies reported that the ratios did not reflect the C vs N limitations. In this paper, I propose a new conceptual model to distinguish when BG:NAG (or BG:(BG + NAG)) reflects microbial C vs N limitation and when not: If cellulose is a predominant C source (relative to chitin, peptidoglycan, and protein), BG:NAG (or BG:(BG + NAG)) reflects the C vs N limitation as the enzymatic stoichiometry theory suggests, while if chitin, peptidoglycan, and protein are dominant C sources, C vs N limitation cannot be determined by BG:NAG (or BG:(BG + NAG)).