Matrix-specificity of para-nitrophenol quantification in chromogenic assays of soil enzyme activity

The choice of matrix used in the assay of soil enzyme activities and corresponding standards can affect absolute values, sensitivity and precision of assayed activities. We quantified the extent to which phosphomonoesterase (PME) and arylsulfatase (SUL) activities of nine soils (3 soil series × 3 land uses) were misestimated due to matrix (assay + alkalization solution) mismatch of assay solutions and para -nitrophenol ( p NP) standards used for converting absorbance measurement to p NP concentration. Using soil-free matrices, we additionally quantified limit of detection (LOD) and quantification (LOQ), method detection limit (MDL) and precision of assays for commonly used matrices: (i) modified universal buffer (buffer universal ) + NaOH, (ii) buffer universal + Tris, (iii) acetate buffer (buffer acetate ) + NaOH and (iv) water + Tris. Using p NP standards without buffer (i.e., in water) misestimated PME activity assayed in buffer universal by up to 0.017 µmol g -1 h −1 regardless of alkalization solution, and misestimated SUL activity assayed in buffer acetate by up to 0.066 µmol g -1 h −1 , when alkalized with Tris but not NaOH. The LOD, LOQ, MDL and variation of p NP detection were buffer universal + NaOH > buffer acetate + NaOH > water + Tris ≥ buffer universal + Tris. Alkalization with Tris minimized matrix and dissolved organic matter interferences for PME activity assayed in buffer universal . We recommend preparing p NP standards in the same matrix as in soil assays and using Tris when assaying PME activities in buffer universal for greater sensitivity and precision.