Abstract
Soil enzymes play essential roles in catalyzing reactions necessary for nutrient cycling in the biosphere. They are also sensitive indicators of ecosystem stress, therefore their evaluation is very important in assessing soil health and quality. The standard soil enzyme assay method based on spectroscopic detection is a complicated operation that requires the removal of soil particles. The purpose of this study was to develop a new soil enzyme assay based on hydrodynamic electrochemical detection using a rotating disk electrode in a microliter droplet. The activities of enzymes were determined by measuring the electrochemical oxidation of p-aminophenol (PAP), following the enzymatic conversion of substrate-conjugated PAP. The calibration curves of β-galactosidase (β-gal), β-glucosidase (β-glu) and acid phosphatase (AcP) showed good linear correlation after being spiked in soils using chronoamperometry. We also performed electrochemical detection using real soils. Hydrodynamic chronoamperometry can be used to assess the AcP in soils, with a detection time of only 90 s. Linear sweep voltammetry was used to measure the amount of PAP released from β-gal and β-glu by enzymatic reaction after 60 min. For the assessment of soil enzymes, the results of hydrodynamic voltammetry assay compared favorably to those using a standard assay procedure, but this new procedure is more user-friendly, rapid and simple.
Highlights
Enzymes are released in soils by microbes and plant roots
A new soil enzyme assay based on hydrodynamic electrochemical detection in a microliter droplet using the rotating disk electrode (RDE) system was developed
Since the developed enzyme assay was not affected by the existing colored dissolved organic matter and soil particles, unlike a regulatory assay no filtration procedure was required to remove soil particles
Summary
Enzymes are released in soils by microbes and plant roots. They can degrade complex substrates into low molecular weight compounds in soils [1,2]. The evaluation of enzyme activity in the soil is very important in order to assess the biochemical function, organic matter fraction, nutrient cycle, and decomposition of xenobiotics. Regulatory soil enzyme assays are based on the colorimetric determination of p-nitro-phenol (PNP) that is released by enzyme reactions when a soil sample is incubated in an optimum buffer solution containing substrate-conjugated PNP at optimum temperature [6,7,8]. Regulatory assays are not suitable to evaluate a sample that contains a high concentration of colored dissolved organic matter, because these assays are based on spectroscopic detection. Regulatory assays require long enzymatic reaction times (1 to 24 h) with substrates and involve complicated methods for soil particle removal by filtration or centrifugation
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