Soil Enzyme assays using P-nitrophenil derivates: an Inter-laboratory comparison

Reproducibility and reliability of soil enzyme tests needs to be validated by comparing results among labs. The Italian Group of Enzymology organized an inter-laboratory comparison: five labs participated. Beta-glucosidase- acid- and alkaline phosphatase were chosen as enzyme activities to be determined: p-nitrophenyl derivatives, due to wide utilization, were chosen as substrates. Procedure for each enzymatic activity was discussed, considering practical and available factors such as available instruments. Nine soils with contrasting physico-chemical properties were carefully dried and sieved to ensure homogeneity. They were sent to each laboratory. Results of tests were evaluated using the z-score. Systematic error and random error of a specific lab were calculated as the average z-score and standard deviation of z-score for a given enzyme in that lab. Due to large correlation between the parameter and the error term, only z-scores below 1 were considered as satisfactory Two labs had acceptable systematic errors for all three enzymes, i.e. average z-score always less than 1. Although the number of labs participating was small, some indications emerged: i- systematic error appear, to be enzyme-dependent; each lab tends to over- or underestimate any enzymatic activity; ii- random errors were enzyme-dependent, some labs have random errors lower than others; iii-beta-glucosidase is the enzyme which measurement is, in general, less variable among labs. This inter-laboratory comparison improved awareness of soil enzyme tests reliability performed in their lab; it is the start to identify critical steps for more reliable protocols.