Examining activity–pH relationships of soil nitrogen hydrolytic enzymes

Abstract

Nitrogen (N) depolymerization and mineralization in soils are catalyzed by extracellular enzymes, notably proteolytic and chitinolytic enzymes. However, there is limited knowledge of pH optima of these N-hydrolytic soil enzymes, potentially missing insights to soil pH effects on N cycling and requiring assumptions on pH optima in enzyme activity assays. We evaluated pH optima of five N-hydrolytic enzymes (casein protease, leucine aminopeptidase, glycine aminopeptidase, alanine aminopeptidase, N-acetyl-β-glucosaminidase) in soils under long-term (145-year) fertilization and crop rotation treatments, as well as a restored prairie, on an Aquic Argiudoll. We additionally tested the pH dependency of three types of non-enzymatic interferences in order to assess the relative importance of controls in determining N-hydrolytic enzyme activity pH optima. For all enzymes, pH–activity relationships varied by soil and exhibited secondary pH optima, though primary pH optima generally agreed with values reported for purified, non-soil enzymes. Enzyme activity pH optima did not reflect soil pH, though soil pH ranged 6.2–7.4 across 145-year treatments. Nonenzymatic interference was generally pH-dependent and soil-specific for protease and N-acetyl-β-glucosaminidase. Though omitting controls for dissolved organic matter tended to have the largest effect on pH optima misestimation, controlling for all three sources of interference had appreciable effects on estimated pH optima values. This study provides a benchmark of empirically determined pH optima of multiple hydrolytic enzymes that catalyze soil N depolymerization. We demonstrate that soil N-hydrolytic enzyme activities exhibit pH optima that generally vary most by enzyme type, and specifically between proteolytic versus chitinolytic enzymes. Because pH optima of these soil enzyme activities can vary among soils differing in management and land use, pH optima should be determined a priori.