Enzymatic Stoichiometry and Vector Characteristics Can Indicate Microbial Resource Limitation: Empirical Evidence from Experiment with Multiple Nutrient Addition

Theory and models of enzymatic stoichiometry have been typically used to assess microbial resource limitation, such as the ratios between the activities of carbon (C), nitrogen (N) and phosphorus (P) acquiring enzymes and vector characteristics (that is, vector length and angle). However, the validity of using these stoichiometry indicators to infer microbial resource limitation has been questioned by recent some studies. To test the validity of using these indicators in peatland ecosystems, we conducted a laboratory incubation experiment by adding available C, N and P individually as well as in combination to create various resource limitation scenarios. Results showed that the activity of C- (beta-D-glucosidase, BDG) and P-acquiring enzymes (phosphatase, PHO) significantly decreased by 87% and 80% with the addition of C and P, respectively. However, the activity of N-acquiring enzymes (N-acetyl-beta-glucosaminidase and leucine aminopeptidase, NAG + LAP) increased by 11.5% following N addition, introducing a bias when using the ratios of lnBDG:ln(NAG + LAP) and ln(NAG + LAP):lnPHO to assess microbial resource limitation. Although the ratios of lnBDG:lnPHO could better indicate C or P limitation, it was not valid to indicate C and P co-limitation. Overall, any individual enzymatic stoichiometric ratio only based on two enzymes (for example, BDG:NAG) could not validly indicate microbial resources co-limitation. However, vector characteristics provided an intuitive indication of microbial C and N co-limitation, as well as C and P co-limitation, but not N and P co-limitation. In summary, we recommend prioritizing the more intuitive vector characteristics as a basis for evaluating microbial resource limitation in peatland ecosystems.