Nonlinear responses of the V-max and K-m of hydrolytic and polyphenol oxidative enzymes to nitrogen enrichment

The kinetics of extracellular enzymes produced by soil microorganisms regulate soil organic matter decomposition and other ecosystem functions. Atmospheric nitrogen (N) deposition creates a soil environment with elevated reactive N, which is expected to affect the kinetic parameters (V-max and K-m) of hydrolytic and poly-phenol oxidative enzymes, as well as soil microbial respiration. We measured the V-max and K-m of seven soil hydrolytic enzymes and polyphenol oxidase (PPO), as well as soil microbial respiration and physiochemical properties, in a temperate steppe after 15 years of multi-level N addition treatments. As the N addition increased, the V-max of most carbon (C) -degrading and N-degrading hydrolytic enzymes decreased, and this was associated with a decline in soil pH. In contrast, the V-max of acid phosphatase (AP) increased with increasing N addition. Higher N addition was associated with lower K-m of most hydrolytic enzymes, expect for AP and beta-xylosidase (BX). There was a quadratic relationship between N addition and the V-max and K-m of PPO, which reached a maximum with the addition of 8 g ;N m(-2) y(-1), and decreased at higher N addition levels. Structural equation modeling indicated that the decline in soil microbial respiration with increasing N deposition was directly mediated by the BG kinetics, due to the fact N-induced acidification negatively impacted the V-max and K-m of BG. Our empirical data on soil enzyme (i.e., V-max, K-m) and its relationship with microbial respiration should be useful for modelling how microbes and substrates interact to regulate soil carbon cycling under N enrichment