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
2.1. Soil sample collection Soil samples were collected from three sites at which
different kinds of pollution had been detected by environ- mental protection agencies see Acknowledgements. At
each site, samples were collected from apparently more affected, less affected and unaffected areas, as detailed
below. However, the fact that the pollution events were uncontrolled, together with the topographical characteristics
of the polluted sites, meant that it was impossible to know exactly the quantity of pollutant to which the soil was initi-
ally exposed.
At each sampling point, 10–15 cores of the top 0–7 cm of soil were taken at random and pooled in the field. The
samples were transported to the laboratory in insulated bags and sieved 4 mm mesh. A sub-sample of each sieved
soil was air-dried to determine general soil properties and the remainder was stored at 4
8C. Analyses of biochemical properties enzyme activities, microbial biomass C, and N
mineralization capacity were performed within 2 weeks of sample collection.
2.2. Tanning effluent In November 1993, an area of meadow soils loamy
texture located in Carballo A Corun˜a, Spain was polluted by effluent from a local tanning factory pH 4.2, suspended
solids 252 mg l
21
, Cr
3 1
21 mg l
21
, total Pb content 0.34 mg l
21
, COD 813 mg O
2
l
21
. In February 1998 samples were taken at a point adjacent to the effluent outlet
T1, at a nearby point at which the soil was covered with hairs deposited from the effluent T2, at a point somewhat
further removed where the soil surface was hair-free T3, and at a pollution-free point far from the effluent source
T4.
2.3. Landfill effluent In April 1997, the meadow soils loamy texture adjoin-
ing an urban waste landfill near Corun˜a Spain were reported to have been polluted by landfill effluent of pH
9.74 with a total C content of 548 mg l
21
and a total inor- ganic N content of 106 mg l
21
mainly as ammonium. In September 1997 samples were taken from two polluted
points close to the effluent outlet L1 and L2, from a shal- low depression in which the effluent had remained stagnant
for several days L3, and from an unaffected point L4.
2.4. Hydrocarbons On May 15th 1998 an area of meadow soil sandy loam
texture near Santiago de Compostela A Corun˜a, Spain was polluted by hydrocarbons leaking from a faulty oil pipe-
line. On June 10th 1998, samples were collected from a point close to the leak with clear traces of hydrocarbons
on the surface H1, from two rather more distant points with no surface signs of hydrocarbons H2 and H3, and
from an unpolluted point far from the effluent source H4.
2.5. Soil physical and chemical properties The methods described by Guitia´n and Carballas 1976
were used to determine the following soil properties: pH in water 1:2.5, soil:water ratio, pH in 1 M KCl 1:2.5,
soil:solution ratio, water field capacity at 33 kPa in a Richard’s membrane-plate extractor; total C content
by potassium dichromate oxidation and total N content by Kjeldahl digestion.
2.6. Biochemical properties Nitrogen mineralization capacity. Soil 25 g was spread
out in a plastic pot, when necessary moistened to reach field capacity, and incubated for 10 days at 25
8C in a 1 l airtight jar containing 25 ml of water to maintain the humidity of the
atmosphere. Inorganic N was extracted with 2 M KCl 1:5 soil:solution ratio and determined according to Bremner
C. Trasar-Cepeda et al. Soil Biology Biochemistry 32 2000 1867–1875 1868
1965. The nitrogen mineralised was estimated from the difference between the inorganic N content before and
after incubation. Results are expressed in mg kg
