Plant invasion alters the michaelis-menten kinetics of microbial extracellular enzymes and soil organic matter chemistry along soil depth.
Microbial extracellular enzymes decompose distinct components of soil organic matter (SOM), thus influencing its stability. However, we lack the knowledge about how the kinetics of individual enzymes vary when multiple substrates change simultaneously. Here we used Japanese knotweed (Polygonum cuspidatum) invasion as a model system to explore how the Michaelis-Menten kinetics (Vmax and km) of microbial extracellular enzymes vary with corresponding SOM components across soil depth (0-5, 5-10, and 10-15 cm). We hypothesized that invasion will increase the Vmax (maximum enzyme activity) and km (substrate concentration at half Vmax) of oxidative enzymes but decrease the Vmax and km of hydrolytic enzymes, and that increasing soil depth will alleviate the invasion effects on the enzyme kinetics. The invasion of knotweed, which input litter rich in recalcitrant compounds, altered soil chemistry including an increase in lignin and fungal biomass compared to the adjacent non-invaded soils. The Vmax of peroxidase, the oxidative enzyme that degrades lignin, increased in the invaded soils (0-5 cm) compared to the non-invaded soils. Among the hydrolytic enzymes, the Vmax of N-acetyl-glucosaminidase which degrades chitin from fungal cell walls increased in the invaded soils (0-5 cm). However, there was no associated change in the km of peroxidase and N-acetyl-glucosaminidase under invasion, suggesting that microbes modified the enzyme production rates, not the types (isozyme) of enzymes under invasion. The Vmax of all enzymes decreased with depth, due to the reduced substrate availability. These results highlight that the addition of relatively recalcitrant substrates due to plant invasion altered the kinetics of microbial extracellular enzymes with implications for SOM chemistry in the invaded soils.