Measurement of arylsulphatase activity in agricultural soils using a simplified assay

A simplified colorimetric assay for arylsulphatase (arylsulphate sulphohydrolase, EC 3.1.6.1) was tested with four Danish agricultural soils (clay content 4–12%). The assay was based on hydrolysis of p-nitrophenyl sulphate in soil-buffer solution, but the assay was done: (i) with sample extraction by centrifugation rather than filtration; (ii) without toluene as bacteriostatic and plasmolytic agent; and (iii) with mechanical shaking of assay mixture. For the four soils, arylsulphatase activity measured after centrifugation represented 98–103% of the activity measured after filtration. Time courses (1–5 h) of p-nitrophenol evolution in the absence of toluene were linear and showed that bacteriostatic agents were not needed. Shaking increased the amount of product (24–79%) and the precision of the assay (1.5–4-fold). Unexpectedly, it was found that toluene (as a plasmolytic agent) had little effect on the arylsulphatase activity during incubation with shaking, whereas a significant increase (22–31%) occurred during static incubation. This obscured the interpretation of the arylsulphatase activity as solely extracellular when measured in the absence of toluene. With the simplified assay, the arylsulphatase activity in the four soils (0–50 cm) ranged from 0.8 to 30.5 μg p-nitrophenol g−1 dry weight soil h−1 and was correlated with the soil organic matter content (r=0.871).