3.2. Landfill effluent At the site polluted by landfill effluent Table 2, the
phosphomonoesterase activities of samples L1, L2 and L3 were, respectively, 121 11
; 49 4 and 37 2 of the
activity in the control sample L4. All the other activities were lower in polluted samples than in the control, and in all
these cases the fall was greatest for sample L2. However, there were marked quantitative differences among the trends
displayed: for b-glucosidase, the fall to 94 20 of control in L1 was not significant, but there was a drastic fall to 16
3 and 18 2 of control in L2 and L3, respectively; for urease, the fall was moderate in L1
66 9 of control,
marked in L2
40 4 and only slight in L3
81 9; while for dehydrogenase, the marked fall in L2
42 4 of
control was accompanied by only slight falls in L1
89 15 and L3
82 5
: When the activities are expressed relative to C and N
contents Fig. 1 they exhibit patterns similar to those described above. However, in contrast to the absolute
values, for L1 the phosphomonoesteraseN value is lower than in the control
88 8 and the phosphomonoester-
aseC value is only slightly higher than in the control
106 10
: The b-glucosidaseC
82 17 and b-glucosidase
N
68 15 values for L1 are lower than those for L4. 3.3. Hydrocarbons
With the sole exceptions of the phosphomonoesterase and urease activities in sample H3, all the enzyme activities
were significantly higher than control values in all the hydrocarbon polluted samples Table 2. The greatest
increases were shown by sample H1, which had values ranging from 194 18 of control for urease to 260
25 of control for phosphomonoesterase. Urease and phos- phomonoesterase activities were somewhat lower, though
still very high, in sample H2
188 38 and 171 9 of control, respectively, but were both lower than the control
values in H3
80 12 and 83 4 ;
respectively. b- Glucosidase and dehydrogenase behaved quite similarly in
all three samples, with values relative to the control of, respectively, 250 7 and 251 13 in H1, 164 15 and
155 9 in H2, and 135 12 and 132 10 in H3; like urease and phosphomonoesterase activities, they decreased
in the order H1
. H2 . H3. When the activities are expressed relative to C and N
contents Fig. 1, the phosphomonoesterase and urease activities of H3 were again the only cases of values lower
than the corresponding control. In contrast, the lowest non- control values for b-glucosidase and dehydrogenase were
recorded for H2.
4. Discussion
The results, which show that enzyme activities were, in general, altered by all three kinds of pollution, suggests the
possibility of using these enzymes as indicators of pollution. However, pollution indicators should possess the following
attributes Doran and Parkin, 1994; Elliott, 1997:
1. sensitivity to the presence of pollutant; 2. ability to reflect different levels of pollution;
3. consistency in the sign of the change undergone in response to any given pollutant i.e. that independently
of the dose the indicator always increases or decreases; and
4. sensitivity to the greatest possible number of pollutants. Furthermore, for quantification of environmental pollution
damage, a good indicator should be capable of: 5. discriminating between the effect of the pollutant and any
prior degradation of the polluted soil; and
C. Trasar-Cepeda et al. Soil Biology Biochemistry 32 2000 1867–1875 1872
Table 3 Checklist for possession of desirable attributes by potential pollution markers
Enzymatic activity Sensitivity to the
presence of pollutant
Consistency in sign of change for a given
pollutant Discrimination between
prior and pollution- induced degradation
Differentiation of pollutants by
degradation levels Absolute values
Dehydrogenase Yes
Yes No
No Urease
Yes No
No No
b -glucosidase
No No
No Phosphomonoesterase
Yes No
No No
Values relative to total C and N contents DehydrogenaseC
Yes Yes
No No
UreaseC Yes
No No
No b
-glucosidaseC Yes
Yes No
No PhosphomonoesteraseC
No No
No No
DehydrogenaseN No
No No
No UreaseN
Yes No
No No
b -glucosidaseN
Yes Yes
No No
PhosphomonoesteraseN Yes
No No
No
6. differentiating among pollutants according to the different degrees of soil degradation they cause.
Table 3 is a checklist showing which of the enzyme activity measures satisfied requirements 1, 3, 5 and 6 in this study. The
only absolute activity to exhibit a consistent response to pollu- tion was dehydrogenase, which was always reduced by
tanning and landfill effluent and always increased by hydro- carbons. However, dehydrogenase failed to discriminate
between pollution and prior degradation, or to discriminate between different pollutants. Although prior degradation
might in principle be estimated by comparison of the control sample with high-quality reference soils native soils under
climax vegetation, this is in practice prevented by the wide geographic variability of enzyme activities in the latter Table
2 lists the observed ranges of the activities considered here in Galician soils under climax oakwood. Natural variability
within and between soils, and the influence of soil type on the stress induced by any given pollutant, have been pointed
out by Nannipieri et al. 1990, Brendecke et al. 1993 and Howard 1993 as decisive impediments to the use of simple
parameters such as single enzyme activities for diagnostic purposes. Furthermore, although Barriuso et al. 1988,
Kandeler et al. 1996 and other authors have suggested that better results might be obtainable with enzyme activities
expressed with respect to C or N since C and N contents are often significantly altered by pollution, in this study the only
improvement so achieved was to make b-glucosidase activity behave consistently either increasing or decreasing for any
given pollutant. In short, the individual enzyme activities are of very limited utility as degradation markers.
A possible way to increase the potential of soil enzymes as indicators of soil contamination would be its use in
combination with other biochemical properties, developing more complex expressions in the same way that did other
authors Beck, 1984; Stefanic et al., 1984; Perucci, 1992; Sinsabaugh, 1994; Stefanic, 1994; Yakovchenko et al.,
1996. However, these indices, too, have severe limitations, since they have been designed for specific situations or
purposes such as the evaluation of alterations in soil ferti- lity, and their general validity is questionable.
Previous experiences of our group Trasar-Cepeda et al., 1998 have shown that in soils that function correctly, i.e.
undisturbed native soils under climax vegetation Dick, 1994; Doran et al., 1994, a biochemical equilibrium exists
represented by a balance between the organic matter content and its biological activity, and this equilibrium is disturbed
by distorting agents such as pollutants Leiro´s et al., 1999.
For climax soils of Galicia NW Spain this equilibrium can be expressed by an equation which defines the total N
content of the soil as a function of microbial biomass C, N mineralisation capacity, and three enzymatic activities:
phosphomonoesterase, b-glucosidase, and urease Trasar- Cepeda et al., 1998:
Total N
×10
23
:
38 microbial biomass C 1 1
: 40
N mineralization capacity 1 13
: 60 phosphomonoesterase
18 :
90 b-glucosidase 1 1
: 60 urease
For these climax soils, the NcNk ratio is 100, Nc being the total N obtained from biochemical properties by using
the above equation, and Nk the total N content of the soils measured by Kjeldahl’s method. Furthermore, both labora-
tory experiments and the evaluation of soils disturbed by diverse accidents or management practices have shown that
the degree of disturbance is reflected by the modifications in the ratio NcNk.
Table 4 lists the values of Nk, Nc and NcNk the last mentioned as a percentage, for each of the samples studied
in this work. The fact that for the control samples NcNk is not 100 but ranges from 56 to 79 reflects pre-pollution
degradation of the soil at all three sites, especially the site later polluted with hydrocarbons; these results are not
surprising in view of the negative effects of agricultural practices on soil biochemical quality Leiro´s et al., 1999.
For the samples polluted by tanning effluent or landfill efflu- ent, intense degradation is indicated by very low NcNk
values, ranging from 15 T3 to 28 T1 in the former case, and from 19 L2 to 35 L1 in the latter, and
significantly different from the NcNk values in their corre- sponding controls. Quite different behaviour is observed
among the samples polluted by hydrocarbons: only H1 the only sample taken at a point with surface traces of
hydrocarbons has a NcNk value differing significantly from the control one, and the value of NcNk for H1 is
furthermore greater than the control value, not less. Similar rises in NcNk values have in other situations been inter-
preted as reflecting a transitory state of high microbiological and biochemical activity Leiro´s et al., 1999 which, in the
present case, would be due to the hydrocarbon pollutants
C. Trasar-Cepeda et al. Soil Biology Biochemistry 32 2000 1867–1875 1873
Table 4 Kjeldahl N contents Nk, Nc values and the NcNk ratios of the soil
samples Soil sample
Nk Nc
NcNk Tanning effluent
T1 1.267
0.356 28
T2 1.803
0.443 25
T3 1.039
0.159 15
T4 control 0.756
0.569 75
Landfill effluent L1
0.653 0.227
35 L2
0.583 0.111
19 L3
0.549 0.168
31 L4 control
0.475 0.376
79 Hydrocarbons
H1 0.470
0.346 74
H2 0.600
0.330 55
H3 0.410
0.213 52
H4 control 0.390
0.218 56
constituting a degradable substrate capable of stimulating the proliferation of part of the soil microflora Joergensen et
al., 1995; Braddock and McCarthy, 1996. By contrast, the values of NcNk for samples H2 and H3 show that at these
points the amount of pollution was insufficient to alter the biochemical quality of the soil.
The above results illustrate how, unlike the individual enzyme activities discussed previously, the ratio NcNk
allows pre-pollution degradation to be evaluated and distin- guishes this from degradation caused by pollution. This
facilitates comparison of the effects of different pollutants at different sites. The soils examined in this study, for exam-
ple, can be ranked by NcNk ratio Fig. 2. The tanning and landfill effluents had very similar effects, reducing the Nc
Nk value by 63–80 in the former case and by 61–76 in the latter, while at the site polluted by hydrocarbons pollu-
tion had a negligible effect on soil biochemical quality at the points where samples H2 and H3 were taken, and increased
the NcNk value by 32 where H1 was obtained.
5. Conclusions
