Does the ratio of β-1,4-glucosidase to β-1,4-N-acetylglucosaminidase indicate the relative resource allocation of soil microbes to C and N acquisition?

The ratio of β-1,4-glucosidase (BG) to β-1,4-N-acetylglucosaminidase (NAG) activity (BG:NAG ratio) is often used as an indicator of the relative resource allocation of soil microbes to C acquisition compared with N (the ecoenzymatic stoichiometry hypothesis). An increasing number of recent studies have used this index to assess the nutrient status of microbes. However, the validity of this index for assessing the nutrient status of microbes is not well tested. In this study, we collected published data and tested that validity by investigating whether N fertilization elevated the BG:NAG ratio, to test the assumption derived from the ecoenzymatic stoichiometry hypothesis that microbes reduce their allocation to the N-acquiring enzyme (NAG) under N-enriched conditions. Of the observations, 54% (82/151) did not support the hypothesis because those studies showed lower BG:NAG ratios in N-enriched soils than under ambient conditions (i.e., N was not enriched), especially when the ambient BG:NAG ratio was higher than 2.0 (77%, 59/77). This suggests that the BG:NAG ratio does not necessarily indicate the microbial status for C or N limitation. Rather, we hypothesized that NAG targets chitin or peptidoglycan, which accumulate at later stages of decomposition and that the variation in BG:NAG explains the decomposition stage. A negative correlation of BG:NAG ratio with polyphenol oxidase activity, which generally increases with decomposition, supported our hypothesis.