Soil enzyme assays using p-nitrophenyl derivatives: an inter-laboratory comparison

Reproducibility and reliability of soil enzyme assays need to be validated by comparing results among different labs. The Italian Group of Enzymology organized a ring test among five labs. Beta-glucosidase, acid- and alkaline phosphatase activities were chosen as they are all measurable by using substrates with wide applicability, i.e. p-nitrophenyl derivatives. The five labs preliminarily agreed and standardised the specific methodological procedure for each enzymatic activity, also taking into account factors such as practicability and equipment availability. Nine soils with contrasting physico- chemical and biological properties were adequately sampled, pre-treated and sieved, in order to ensure representativeness and homogeneity. Then they were sent to each laboratory. Results of ring test were evaluated using the Z-score procedure. Systematic error and random error of a specific lab were calculated, respectively, as the average Z-score and standard deviation of Z-score for a given enzyme in that specific lab. Due to large endogeneity (correlation between the parameter and the error term), only Z-scores below 1 were considered as satisfactory. Only two labs showed acceptable systematic errors for all three enzymes, i.e. average Z-score always less than 1. Even though the number of lab partecipating to the ring-test was small, some trends emerged: i) systematic errors appear to be mainly soil-dependent, i.e. generally each lab among soils indifferently overestimated or underestimated any enzymatic activity; ii) random error was more lab-dependent than enzyme-dependent; iii-beta-glucosidase is the enzyme which measurement is, in general, less variable among labs. This inter-laboratory comparison improved participants awareness about soil enzyme assays meaning and is also the starting point to identify critical steps for setting up more reliable protocols.