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

Abstract

The ratio of {beta}-1,4-glucosidase (BG) to {beta}-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. 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, assuming that microbes reduce their allocation to the N-acquiring enzyme (NAG) under N-enriched conditions. Of the data points, 54% (82/151) did not support the hypothesis because those studies showed lower BG:NAG ratios in N-enriched soils than under ambient conditions, especially when the ambient BG:NAG ratio was higher than 2.0 (77%, 59/77). This suggests that the BG:NAG ratio does not always indicate the microbial status for C or N limitation. Rather, we hypothesized that the decomposition stage explained the variation in BG:NAG because N addition accelerates decomposition, and the BG:NAG ratio is lower at later stages of decomposition due to the dominance of NAG-targeting C (chitin or peptidoglycan). A negative correlation of BG:NAG ratio with polyphenol oxidase activity, which increases with decomposition, supported our hypothesis.