Prescribed pH for soil β-glucosidase and phosphomonoesterase do not reflect pH optima

Abstract In soil enzyme assays, buffers are most commonly employed to constrain the assay to a specific pH at which activity is assumed to be maximized (i.e., pH optima). The assumed pH optima prescribed for the assay of soil β-glucosidase and phosphomonoesterase activities are based on early studies from a limited set of soils and have subsequently been found to be invalid in broader contexts. Here, we used the USDA-NRCS national soil survey database to preselect soils with a broad range of edaphic characteristics—namely soil pH, organic matter content, and clay content—to robustly assess pH optima of β-glucosidase and phosphomonoesterase. β-Glucosidase pH optima occurred within a narrow range and were independent of soil or climatic properties, including soil pH. Conversely, phosphomonoesterase pH optima showed a broad range of values that were closely linked to soil pH. For most samples, the mismatch between the prescribed and optimal pH (maximal activity) corresponded to activity differences of