Optimization of Methods for Analysis of Phosphatase, β-Glucosidase and Arylsulfatase in Soils.

Because of the readiness of response of microorganisms to changes in soil environment, some hydrolytic soil enzymes have been widely proposed as sensitive indicators of soil degradation. However, costs of several enzymatic analyses in soil laboratories are still high compared with the traditional routine soil fertility tests. This study aimed to evaluate the feasibility of modifications in the commonly used methods for the analysis of phosphatase, β-glucosidase and arylsulfatase in soils intending to reduce time, reagents, and laboratory space necessary for these analyses. The modified method consisted of (1) reduction in 50% the quantity of soil and reagents; (2) use of 15 ml test tubes instead of 50 ml Erlenmeyers; (3) use of water bath instead of incubators to sample incubation, and (4) substitution of the filtering step by centrifugation. In addition, we evaluated the possibility to reduce the concentration of the enzyme substrates from 50 mM to 25 mM. Comparisons among methods were done using 30 soil samples collected from different soil types with soil organic carbon varying from 6.5 to 21.5 mg kg^-1. Results for all three enzymes showed high correlation between the original and modified methods. However, the average enzyme activity using the modified method was 11% and 30% higher, respectively for phosphatase and both β-glucosidase and arylsulfatase, compared with the original methods. These differences were attributed to the use of water bath for incubation of samples, which results in faster temperature equilibrium of the reaction medium, i.e, more incubation time under enzyme optimum temperature. Reducing the substrate concentration resulted in lower enzymatic activity for all three enzymes, but for β-glucosidase and arylsulfatase, results were highly correlated with those at 50 mM. We conclude that the modified methods proposed for the analysis of phosphatase, β-glucosidase and arylsulfatase in soils produce results comparable to the original methods, and may significantly reduce the costs of these analyses, opening the opportunity of their inclusion in routine soil tests.