26
been spiked with vanadate(V) 5-10 months earlier (aged) (Paper III) and soils
amended with blast furnace (BF) slag containing relatively large amounts of
vanadium (Paper IV). Freshly spiked soils were subjected to five different
toxicity assays (Table 3); two microbial tests (respiration and nitrification), and
three plant tests (barley root elongation, barley shoot growth and tomato shoot
growth). The three plant assays were also performed on the aged soils and the
barley shoot growth assay was conducted on the BF slag-treated soils.
4.2.1 Soil treatments
The freshly spiked and aged soils were amended with different initial
concentrations of dissolved vanadate(V) (0, 3.2, 10, 32, 100, 320, 1000 and
3200 mg V kg
-1
soil). The freshly spiked soils were amended one week before
the assays were carried out. The aged soils were kept outdoors in plastic pots
with free drainage before starting the toxicity assays. Two soils, Pustnäs and
Säby, were aged for approximately 10 months at an experimental facility in
Sweden (Figure 4) and one soil, Ter Munck, was aged for five months at a
facility in Belgium. The Pustnäs and Säby soils were also amended with two
different BF slags (M-kalk and Merit 5000). These are two commercially
available products,
produced in the SSAB Oxelösund steelworks
. M-kalk is an
air-cooled blast furnace slag that is used as a soil amendment. Merit 5000 is a
ground granulated blast furnace slag that is used in concrete. Both slags had a
total vanadium concentration of approx. 800 mg kg
-1
and were added at
concentrations of 0.1, 1, 10 and 29 weight-% BF slag kg
-1
dry soil. These
corresponded to vanadium additions of 8-230 mg V kg
-1
soil. The aged and BF
slag-amended soils were again air-dried after the ageing period, sieved and
then stored until the assays were conducted. Before starting the toxicity assays,
all soils were wetted to half field capacity and then incubated for 1 week at 20
ºC in the dark.
Table 3. Summary of toxicity assays performed for the different vanadium soil treatments.
Soil treatment
Microbial assays
Plant assays
Respiration
Nitrification
Root elongation
Shoot growth
Barley
Barley
Tomato
Freshly spiked
√
√
√
√
√
Aged
√
√
√
BF slag
√
√
27
4.2.2 Toxicity assays
Microorganism
The two assays of soil microbial response, using respiration and nitrification as
indicators, were performed following the standard procedure according to
OECD 217 (OECD, 2000) and ISO 14238 (ISO, 1997), respectively. The
respiration assay was conducted by adding 5 g of soil to plastic vials (three
replicates per treatment), which were spiked with
14
C labelled glucose. The
vials were then placed in bottles containing 5 mL 1 M NaOH to trap respired
CO
2
. After 24 h, the NaOH was sampled, a scintillation cocktail was added,
and the
14
CO
2
concentration was measured by beta scintillation counting (Tri
Carb 2800 Tr; Perkin Elmer). The respiration rate was calculated based on the
amount of labelled glucose respired per g of soil and day.
The potential nitrification rate (PNR) was evaluated after adding 100 mg
kg
-1
NH
4
-N to 100 g of wetted soil, with three replicates per vanadium
treatment. The soils were then stored in the dark at 20 ºC and three subsamples
were taken from each soil after 0, 7 and 28 days. The subsamples were
extracted with 1M KCl and the NO
3
-
concentration in the extracts was
measured calorimetrically (SA40; Skalar). The PNR was estimated by the
increase in NO
3
-
during the first seven days and expressed as µg NO
3
-N g
-1
soil
day
-1
. Due to the low nitrification activity in the Zwijnaarde soil, the calculated
PNR was based on the NO
3
-
concentration after 28 days.
Plants
The two different plant toxicity assays performed were a root elongation assay
according to ISO 11269-1 (ISO, 1993) and a plant shoot assay according to
ISO 11269-2 (ISO, 2005). All plants were grown in plastic pots containing
Figure 4. Ageing of the Pustnäs and Säby soils, after
vanadate(V) addition, during the Swedish winter
(above) and summer (right).
28
approximately 500 g of soil and with a 1 cm layer of inert plastic beads placed
on the soil surface to reduce water losses. During the growing period, the pots
were placed in a growth chamber that was set to a 16 h light (20 ºC) and 8 h
dark (16 ºC) cycle (Figure 5). Water losses from the soils were monitored and
replaced on a daily basis.
The root elongation test was conducted with barley by planting 10
germinated seeds just below the soil surface in each pot (three replicates per
treatment). The pots were then placed in the growth chamber for five days,
after which the seedlings were carefully removed from the soils. The longest
root of each seedling was measured and a mean value was calculated for each
pot.
Two different plant shoot assays were performed; tomato and barley shoot
growth, with four replicates per treatment. The soils were fertilised with 50 mg
P kg
-1
and 100 mg N kg
-1
one week before commencing the assays to avoid
nutrient deficiency. Twenty tomato seeds or 10 germinated barley seeds were
placed just below the soil surface and the pots placed in the growth chamber.
When 70% of the seedlings had emerged above the surface (8-11 days for
tomato and 3 days for barley), they were reduced to five shoots per pot and left
to grow for 12-14 days. After the growing period, the aerial parts of the plants
were cut and weighed, air-dried at 70 ºC and then weighed again.
4.2.3 Soil and plant vanadium
The pseudo-total vanadium concentration in the soils (soil vanadium) was
determined by aqua regia digestion, which was performed in duplicate for the
freshly spiked and aged soils and in one replicate per treatment for the BF slag-
treated soils. The dissolved vanadium concentration in the soils was established
by soil solution extractions according to Merckx et al. (2001). The extractions
were performed in duplicate on the freshly spiked and aged soils, which were
wetted to just below field capacity and incubated at 20 ºC for three days. Soil
solution was then collected from approximately 50 g of soil that had been
centrifuged at 3000 g for 15 minutes. The amount of dissolved vanadium in the
Figure 5. Barley plants growing in the growth
chamber during the plant shoot assay.
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