Limitations of soil enzymes as indicators of soil pollution
C. Trasar-Cepeda
a,
, M.C. Leiro´s
b
, S. Seoane
b
, F. Gil-Sotres
b
a
Departamento de Bioquı´mica del Suelo, Instituto de Investigaciones Agrobiolo´gicas de Galicia, Consejo Superior de Investigaciones Cientı´ficas, Apartado 122, E-15080 Santiago de Compostela, Spain
b
Departamento de Edafologı´a y Quı´mica Agrı´cola, Facultad de Farmacia, Universidad de Santiago de Compostela, E-15706 Santiago de Compostela, Spain Accepted 3 May 2000
Abstract
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 b-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 NcNk, where Nk is Kjeldahl nitrogen and Nc is a function of microbial biomass C and
nitrogen mineralization capacity combined with three enzyme activities phosphomonoesterase, b-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. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Soil enzymes; Soil contamination; Soil biochemical properties
1. Introduction
The wide-spread pollution of soils is an increasingly urgent problem because of its contribution to environmental
deterioration on a global basis Bezdicek et al., 1996; Dick, 1997; Lal, 1997; van Beelen and Fleuren-Kamila´, 1997.
Until relatively recently, soil was widely regarded as just an environmental filter ensuring the quality of both water
and atmosphere. However, in the context of the pursuit of sustainability, it is now recognised that soil is not only an
effective de-contaminant of potential pollutants but that its chemical, physical and biological quality must be main-
tained Hornick, 1992; Parr et al., 1992. From the point of view of sustainability, a high-quality soil is a soil that
is capable of producing healthy and abundant crops; de- contaminating the water passing through it; not emitting
gases in quantities detrimental to the environment; and behaving as a mature, sustainable ecosystem capable of
degrading organic input Doran and Parkin, 1994; Gregor- ich et al., 1994; Brookes, 1995; Pankhurst et al., 1995. This
view clearly implies that diagnosis of soil pollution should be carried out on the basis of observed alterations in the soil
properties controlling the behaviours described above, ideally in a way that allows any loss of soil quality to be
quantified as well as identified qualitatively Larson and Pierce, 1991; Doran and Parkin, 1994.
The physical, chemical, biochemical and biological prop- erties of a soil are all important for its behaviour Arshad
and Coen, 1992; Parr et al., 1992. Characterisation of this behaviour should focus on the properties that are most sensi-
tive to environmental stress. Since this sensitivity is a feature of many biological and biochemical properties
Dick and Gupta 1994; Vanhala and Ahtiainen 1994, it is these that may be considered as most appropriate for the
purpose of soil quality evaluation Pankhurst et al., 1995; Yakovchenko et al., 1996; Elliott, 1997. Visser and Parkin-
son 1992 have suggested that the biological and biochem- ical properties that are most useful for detecting the
deterioration of soil quality are those that are most closely related to nutrient cycles, including soil respiration, micro-
bial biomass, nitrogen mineralisation capacity and the activ- ities of soil enzymes. In particular, enzyme activities are
especially significant because of their major contribution to the ability of the soil to degrade organic matter Franken-
berger and Dick, 1983. Furthermore, Kandeler et al. 1996 have indicated that the composition of the microbial
Soil Biology Biochemistry 32 2000 1867–1875
0038-071700 - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 0 0 0 0 1 6 0 - 7
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Corresponding author. Tel.: 134-981-590958 ext. 14; fax: 134-981-
592504. E-mail address: ctrasarcesga.es C. Trasar-Cepeda.
community determines the potential of that community for enzyme synthesis, and thus any modification of microbial
community due to environmental factors should be reflected on the level of soil enzymatic activities. In addition, enzyme
activities have the further advantage of being easy to deter- mine without expensive, sophisticated instruments Dick,
1997.
In spite of the above considerations, published findings on the influence of pollutants on soil enzyme activities indicate
that polluted soils are a system of great complexity. For example, the behaviour of dehydrogenase activity, which
is only present in viable cells Skujins, 1978; Trevors, 1984 and may therefore be considered as a direct measure
of soil microbial activity Garcı´a and Herna´ndez, 1997, is very variable. Dehydrogenase activity appears to depend on
the type of pollutant; for example, it is high in soils polluted with pulp and paper mill effluents McCarthy et al., 1994
and low in soils polluted with fly ash Pichtel and Hayes, 1990. Also significant is the concentration of pollutant
higher levels of dehydrogenase at low doses of pesticides and vice versa Barnah and Mishra, 1986, and the type of
soil Doelman and Haanstra, 1979; Kandeler et al., 1996. Similarly complex behaviour is exhibited by all the hydro-
lytic enzymes that have been investigated, which has thrown doubts on the possibility of their use as reliable indicators of
soil quality Dick, 1997.
With a view to determining which of a variety of enzyme activities, if any, might be useful as indicators of the quality
of acid soils rich in organic matter, we studied how these parameters were affected in soils that had at some time been
subject to pollution by hydrocarbons, tanning effluent or urban waste landfill effluent.
2. Materials and methods
