37
5.2 Vanadium toxicity and bioavailability (Papers II-IV)
5.2.1 Threshold values
The toxicity assays showed a clear negative response to increasing vanadium
concentration in the soils. Hence no hormesis effects were observed for any of
the toxicity assays performed.
Microorganisms
The EC50 values obtained in the substrate-induced respiration assay ranged
from 200 to 580 mg added V kg
-1
and the EC10 values from 8.4 to 58 mg
added V kg
-1
soil (Table 5). Many of the threshold values were uncertain,
however, with large standard error. In the potential nitrification rate assay,
EC50 values ranged from 28 to 690 mg added V kg
-1
soil, which is a 24-fold
difference between soils. Most of the EC10 values were within the range of the
vanadium background concentrations. The potential nitrification assay is
known to be a sensitive endpoint (Broos et al., 2005). The response in the
untreated control soils varied by a factor of up to 20 between soils. This
demonstrated that the conditions in the soil itself had a strong influence on the
microbial populations. Inhibition of nitrification and nitrogen mineralisation, in
the short term, has previously been observed at a dose of 250 mg V kg
-1
soil
(Liang & Tabatabai, 1978; Liang & Tabatabai, 1977) but as indicated here the
inhibiting vanadium concentration may span a much wider range in different
soils.
Table 5. Vanadium toxicity threshold values (EC10 and EC50) for microorganisms in five
different soils. Values are based on the added vanadium concentration ± standard error of the
mean.
Substrate-induced respiration
Potential nitrification rate
Soil
a
Control
b
EC10
EC50
Control
b
EC10
EC50
(µg glucose g
-1
d
-1
)
(mg V kg
-1
) (mg V kg
-1
) (µg NO
3
-N g
-1
d
-1
)
(mg V kg
-1
) (mg V kg
-1
)
G
46 ± 5
58 ± 26
580 ± 97
11.5 ± 0.7
19 ± 4
130 ± 11
P
321 ± 13
10 ± 4
200 ± 28
2.3 ± 0.2
14 ± 3
100 ± 8
S
502 ± 70
24 ± 11
320 ± 57
4.7 ± 0.1
190 ± 30
690 ± 46
T
190 ± 13
8.4
c
320 ± 133
10.2 ± 0.2
35 ± 8
330 ± 30
Z
25 ± 2
26
e
± 15
220
e
± 50
2.1
d
± 0.1
2.2
d
± 0.7
28
d
± 4
a
See Table 1 for abbreviations.
b
Microbial response in uncontaminated control soil with standard deviation (n=3).
c
Standard error > threshold value.
d
Value based on 28 observation days, see text.
e
Threshold value based on nominal vanadium concentration.
38
Plants
The plant assays performed on the freshly spiked soils produced EC50 values
that varied between 18 and 510 mg added V kg
-1
soil (Figure 9). Tomato shoot
growth was the most sensitive to increasing vanadium addition, while barley
root elongation was the least sensitive. The latter finding was unexpected
considering that vanadium is accumulated in plant roots (Yang et al., 2011; Gil
et al., 1995; Kaplan et al., 1990a). The reason may be the relatively short
period of time (5 days) over which the assay was conducted. Comparing with
other plant species grown in a standard soil with different V
2
O
5
additions
(Smith et al., 2013) the EC50 values determined here were within the same
range. However, as for the microbial assays, it was found that the variation
increased when different soils were tested. In contrast to the microbial assays,
the threshold values in the plant assays correlated to the soil type. The lowest
threshold values were obtained in the sandy Zwijnaarde soil for all three plant
assays and the highest values in the more clayey Säby soil.
In comparison with the freshly spiked soils, ageing of the soils increased the
threshold values for plants by a factor of between 1.3 and 2.9. This resulted in
EC50 values ranging from 46 to 780 mg added V kg
-1
soil (Figure 9). The
ageing process is known to reduce the bioavailability of other elements
(Smolders et al., 2009). In the case of vanadium, prior to this thesis work,
ageing has only been briefly mentioned (Martin & Kaplan, 1998).
Soils amended with BF slag did not exert any negative impact on barley
shoot growth up to the highest addition of 29 weight-% BF slag. However, the
added vanadium concentrations in the BF slag-amended soils were within the
range of threshold values determined for the freshly spiked soils. Hence, the
bioavailability was much lower when the vanadium was added by BF slag.
Figure 9. Range of vanadium EC50 values obtained for plant assays conducted on freshly spiked
(black) and aged soils (red). Markers represent the EC50 value determined in the respective soil.
The soil marked with * had an EC50 value larger than the stated value.
*