Modified universal buffer does not necessarily maintain soil enzyme assay pH

Buffers are widely used when performing soil enzyme assays, either to measure specific soil enzyme activities based on a standard protocol or to determine the pH optima of a soil enzyme. However, few studies have tested whether the existent buffer-making protocol they followed actually obtains the target pH during their enzyme assays. To test the pH stabilization capacity of modified universal buffer (MUB) commonly used in soil enzyme assays, four different soils with varying buffering capacity (proxied by contrasting soil pH and clay content) were used for phosphomonoesterase assays using a 1:5 (m/v) soil:buffer ratio, as per common practice for chromogenic enzyme assays. The original MUB working solution deviated from the target pH by as much as 1.6 units. Deviations were greatest when the soil and buffer had contrasting pH. Increasing MUB concentration by fourfold did not mitigate this pH discrepancy to a satisfactory level (± 0.1 unit). For the standardly used MUB concentration (1 ×), the deviation between the target pH and the assay pH was greatest when the difference between the target pH and the soil pH was > 1 unit. Therefore, forcing pH to a specific value in soil enzyme activity assays (1:5 soil:MUB) appears to be difficult for many soils, and increasing MUB concentration risks reducing enzyme activities due to higher ionic strength. Thus, the use of MUB to assay enzyme activity presents a tension between maintaining the target pH and maximizing the sensitivity of the assay.