Limitations of soil enzymes as indicators of soil pollution

Soil enzyme activities are considered to be sensitive to pollution and have been proposed as indicators for measuring the degree of soil degradation. In this work we found that in three galician soils exposed to various degrees of pollution by tanning effluent, hydrocarbons or landfill effluent, the changes in the activities of individual enzyme did not allow precise quantification of soil degradation. Thus, the enzymatic activities in polluted soils with respect to that in control soils was between 37 and 260% for phosphomonoesterase, between 16 and 250% for β-glucosidase, between 28 and 194% for urease and between 24 and 251% for dehydrogenase. The degree of degradation was, however, clearly shown in all cases by the ratio Nc/Nk, where Nk is Kjeldahl nitrogen and Nc is a function of microbial biomass C and nitrogen mineralization capacity combined with three enzyme activities (phosphomonoesterase, β-glucosidase and urease). This ratio, Nc/Nk, exhibited all the attributes of a good pollution indicator and, in particular, was able to discriminate between the effect of the pollutant and any prior degradation of the sites. It is concluded that quantification of soil degradation can require that information on enzyme activities be supplemented with information on other biochemical soil properties.