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

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 <10% for β-glucosidase and <50% for phosphomonoesterase. Dichotomization of acid and alkaline phosphomonoesterase activity is likely an artifact of methodological decisions rather than an accurate representation of two distinct soil enzymes. Thus, current prescriptions of buffer pH values assumed as pH optima for assaying β-glucosidase and phosphomonoesterase are incorrect. Instead, pH optima of soil enzyme activities should be empirically determined for specific soils.