Microorganisms regulate the quantity and quality of soil organic matter (SOM) via extracellular enzyme activity (EEA). As extracellular enzymes are essential in decaying distinct substrates in soil, recent soil carbon models have increasingly incorporated the Michaelis-Menten kinetics of microbial EEA (km for enzyme’s affinity to substrate, Vmax for maximum enzyme activity) in simulating SOM dynamics. However, we lack the knowledge about how individual EEA varies when multiple substrates simultaneously change, a common ecological context when aboveground vegetation changes. Here we used Japanese knotweed (polygonum cuspidatum) invasion as a model system to explore how the Michaelis-Menten kinetics of common microbial extracellular enzymes (AP, acid phosphate; BG, β-glucosidase; NAG, N-acetyl-glucosaminidase; PER, peroxidase) vary with corresponding SOM component (organic phosphorus for AP, cellulose for BG, chitin for NAG, and lignin for PER) across soil depth. We hypothesized that invasion will increase the Vmax and km of oxidative enzyme, but decrease the Vmax and km of hydrolytic enzyme and that increasing soil depth will alleviate the invasion effects on the enzyme kinetics and EEA.
Results/Conclusions
At 0-5 cm, invasion increased the Vmax of all enzymes except for AP, while at deeper soils, the Vmax of hydrolytic enzymes (AP, BG, and NAG) generally decreased, but the Vmax of PER increased. We found no invasion effect on km of any enzymes, but depth decreased the km of NAG. At 0-5 cm, we observed a greater abundance of total SOM, chitin, and lignin under invasion. Microbes under invasion, thus, were likely to maximize their resource acquisition from the relatively abundant substrates by producing more enzymes (similar km but higher Vmax) at 0-5 cm. At deeper soils, microbial enzyme production under plant invasion exhibited trade-offs between hydrolytic vs. oxidative enzyme productions. Generally, the production of hydrolytic enzymes decreased (similar km but lower Vmax), but that of oxidative enzyme increased (similar km but higher Vmax).
These results highlight that invasion preferentially increased microbial decay of recalcitrant carbon, implying enhanced vulnerability of relatively stable carbon under invasion. Also, our data suggest that the effects of plant invasion on EEA are not confined to surface, but penetrated into deeper soils. Soil carbon models may need to incorporate differential responses of distinct enzyme-substrate pair to changes in substrates landscape when simulating SOM dynamics.