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J Sci Food Agric 1997, 73, 446È454
Plant Availability of Heavy Metals in Soils
Previously Amended with Heavy Applications of
Sewage Sludge
P S Hooda*
Department of Geography, Queen Mary and WestÐeld College, University of London, Mile End Road,
London E1 4NS, UK
D McNulty
Biomathematics and Statistics, Scotland, Hannah Research Institute, Ayr KA6 5HL, UK
B J Alloway
Department of Soil Science, University of Reading, PO Box 233, Reading RG6 6DW, UK
and M N Aitken
Department of Environmental Science, SAC, Auchincruive, Ayr KA6 5HW, UK
(Received 25 July 1996 ; accepted 2 October 1996)
Abstract : Plant uptake is one of the major pathways by which sludge-borne
potentially toxic metals enter the food chain. This study examined the accumulation of Cd, Cu, Ni, Pb and Zn in wheat, carrots and spinach grown on soils from
13 sites previously amended with sewage sludge. Winter wheat, carrots and
spinach were grown consecutively under Ðeld like conditions. The results showed
that plant availability of heavy metals di†ered widely among the crop species.
The accumulation of Cd, Ni and Zn in the plants showed the greatest increases
compared to their background levels. The Cu and Pb accumulation in the plants
grown on sludge-amended soils showed only small increases compared to those
grown on uncontaminated soils. Multiple regression analysis of various soil
properties showed that the surest way to control the accumulation of metals in
food plants is by controlling their concentrations in the soils. Furthermore, soils
with a non-acidic pH and a clayey texture tended to achieve better control of
metal accumulation in food plants compared to those with an acidic reaction
and a coarse texture. Metal concentrations in the plants generally correlated well
with those extracted from soils in 0É005 M DTPA, 0É05 M EDTA-(Na) , 1 M
2
NH NO and 0É05 M CaCl . The EDTA, however, proved to be a more reliable
4
3
2
and consistent test in predicting the accumulation of metals in the plants. The
results also showed that liming soils to pH 7 e†ectively reduced the metal contents in carrots and spinach, but liming to pH 6É5 had little e†ect on metal
concentrations in wheat grain.
Key words : plant availability, sludge-amended soils, metal contents, wheat,
carrot, spinach, soil extractants, pH, soil type and liming.
* To whom correspondence should be addressed at : Institute of Environmental and Biological Sciences, Lancaster University,
Lancaster, LA1 4YQ, UK.
446
J Sci Food Agric 0022-5142/97/$09.00 ( 1997 SCI. Printed in Great Britain
Plant availability of heavy metals in soils
INTRODUCTION
Currently some 42% of the total sewage sludge produced in the UK is diposed of by application to agricultural land (DoE 1993). With increasing environmental
concern, which has outlawed sea disposal by 1998 (CEC
1991), sewage sludge utilisation in agriculture is likely to
increase in future years. Sewage sludge contains considerable amounts of N and P. In addition, it may also
contain high concentrations of potentially toxic elements (PTEs), particularly heavy metals (eg Cd, Cu, Ni,
Pb, Zn). While sewage sludge recycling to agricultural
land is generally recognised as a fertiliser resource
(Coker et al 1987), the contamination of soils with
sludge-borne heavy metals continues to be an area of
concern because of their persistence in the soils and
their increased uptake by crop plants many years after
sludge applications have ceased (McGrath 1987).
Plant uptake is one of the major pathways by which
sludge-borne PTEs enter the food chain (Chaney 1990).
Although a large body of research exists on factors controlling the uptake of these elements by crop plants (see
the review by Alloway and Jackson 1991), the conclusions drawn have tended to be based on studies conducted on soils recently treated with sludge. After the
terminal sludge application, the soil will gradually
establish a new biochemical equilibrium due to the
decomposition of sludge-added organic matter and, in
many cases, acidiÐcation of the soil. Consequently, the
plant availability of sludge-borne elements may change
after the biodegradation of sewage sludge in soils
(Robertson et al 1982 ; Hooda and Alloway 1993).
Liming soils o†ers a means of minimising the risk of
food chain contamination by reducing the plant uptake
of sludge-borne heavy metals (Williams et al 1987 ;
Jackson and Alloway 1991 ; Smith 1994). Several
studies, however, have indicated that liming may not
always have a signiÐcant e†ect, and that the e†ectiveness of liming could also vary depending on the soil,
metal, pH value of the limed soils and crop species
(Hemphill et al 1982 ; Pepper et al 1983 ; Kuo et al 1985 ;
Eriksson 1989). Furthermore, the e†ect of liming on
plant metal uptake from recently sludge-applied soils
may also be confounded due to a possible progressive
acidiÐcation of the soil during the period of sludge biodegradation.
Research on the behaviour of metals in soils which
have stabilised after sludge applications is therefore
needed to understand the long-term e†ects of sludge
disposal on agricultural land. This paper reports studies
conducted on 13 sludge-amended soils which had
equilibrated in the Ðeld for several years after sludge
applications. The speciÐc objectives of the studies were
to :
(i)
compare the uptake behaviour of Cd, Cu, Ni,
Pb and Zn by wheat, carrots and spinach ;
447
(ii)
examine the relation of various soil properties
to metal contents in the plants ;
(iii) test the suitability of four soil extractants in predicting metal uptake by crop plants ; and
(iv) study the e†ect of liming on plant metal contents.
MATERIALS AND METHODS
Samples of 13 soils, c 50 kg, previously amended with
large quantities of sewage sludge over many years were
collected from various sites in the UK. In addition,
similar samples of nine uncontaminated soils were also
included for measuring the background levels of metals
in the food crops. All soil samples were passed through
a coarse sieve (\5 mm) and divided into two equal
halves before being put into polyethylene tubs. The tubs
were kept in the Ðeld in a rural location near Brentwood in Essex (UK). Winter wheat (T riticum aestivum
L), carrots (Daucus carota L) and spinach (Spinacia
oleracea L) were grown consecutively between 1989 and
1991. All crops received a pre-sowing basal dressing of
an inorganic fertiliser supplying 60, 26 and 50 kg N, P
and K ha~1, respectively.
Of the 13 sludge-amended soils, nine (soil numbers
1È9, Table 1) were acidic in reaction, and one replicate
of these acidic soils was limed to pH 7 in 1987. Winter
wheat was grown on soils which had been limed two
years previously, and the pH of these soils averaged 6É5
when measured after harvesting the wheat crop. Soils
which showed a decrease in pH values were re-limed to
pH 7 again before sowing of carrots and spinach.
All crops were harvested at maturity. The edible parts
of wheat (grain), carrot (roots) and spinach (leaves) were
thoroughly washed with de-ionised water before being
dried in an oven at 65¡C. The samples of plant material
were Ðnely ground in a centrifugal ball mill before being
digested in concentrated AristaR grade HNO follow3
ing the procedure described in Jackson and Alloway
(1991).
Soil particle size fractions were quantitatively determined by the pipette method (Day 1965). Soil pH values
were measured in water (Avery and Bascomb 1974). The
contents of free iron oxides in soils were extracted with
sodium dithionate (Avery and Bascomb 1974), and
hydroxylamine hydrochloride was used for the extraction of hydrous manganese oxides (Chao 1972). Soil
organic matter and cation exchange capacity were
measured using the standard procedures described in
Hesse (1971).
The “plant availableÏ metal concentrations in soils
were extracted using 0É005 M DTPA (Lindsay and
Norvell 1978), 0É05 M EDTA-(Na) (Anon 1986), 1 M
2
NH NO and 0É05 M CaCl (Alloway and Morgan
4
3
2
P S Hooda et al
448
TABLE 1
Heavy metal contents and selected physico-chemical characteristics of soilsa
Soil number
Metal concentration (mg kg~1)
Cd
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
4É35
10É40
0É71
12É30
5É07
7É30
8É07
3É35
1É57
2É45
2É40
7É40
3É82
0É49
0É73
0É49
0É49
0É52
0É40
0É30
0É46
0É46
Cu
Ni
86É4 65É2
199É8 121É4
148É0 49É5
164É9 45É2
72É5 13É1
119É2 44É3
884É0 103É7
41É1 51É6
187É4 53É2
210É5 42É0
68É4 32É8
102É2 40É8
85É0 11É2
30É8 21É8
29É9 21É9
15É6 18É5
19É4 28É1
126É5 39É2
24É2 30É2
14É5
4É8
16É7
8É5
16É5 29É2
Pb
Clay
(%)
OM
(%)
pH
CEC
(cmol kg~1)
c
31É0
29É8
46É1
44É2
18É0
33É3
26É6
28É0
81É0
45É3
41É5
20É5
19É5
40É9
30É5
28É5
60É5
20É0
48É7
20É3
28É6
32É8
5É6
5É6
4É3
17É5
3É3
5É9
13É8
4É5
11É0
22É5
5É0
4É1
4É6
38É2
5É7
4É5
11É5
18É1
4É7
4É1
3É8
4É0
6É65
6É15
5É11
5É54
6É35
5É50
6É09
5É81
6É40
6É96
7É29
7É04
7É30
6É60
6É04
7É37
7É21
4É42
6É90
7É42
4É70
4É20
17É9
18É7
29É6
41É9
19É4
35É1
42É3
20É0
45É8
48É2
35É5
18É3
22É4
126É2
21É8
22É6
41É8
47É3
35É2
21É5
23É4
21É8
Zn
85É9 221É0
177É5 497É5
902É5 322É5
200É9 485É3
149É7 150É0
107É6 291É6
813É7 1125
63É5 346É0
200É8 462É5
450É5 370É0
42É0 170É0
92É6 260É0
85É0 202É5
75É2
82É3
79É5 128É7
22É3
37É3
44É1
75É8
268É5 158É0
32É5 126É2
36É0
95É2
45É0 182É3
23É5 124É6
FFeO
HMnO
CaCO L ast sludge
3
(%)
(mg kg~1)
(%)
application
1É55
1É24
1É15
0É57
1É04
2É16
1É12
1É30
3É11
2É66
2É08
4É70
0É46
1É27
1É20
2É09
0É80
2É45
2É21
0É46
1É27
3É07
444É8
315É2
195É6
75É9
128É8
95É6
139É9
91É7
70É4
292É0
95É6
598É7
292É9
86É2
581É9
357É8
370É1
108É8
73É7
329É8
83É2
266É7
1É5
1É9
2É7
41É5
2É8
2É5
5É9
1É9
3É2
4É9
2É9
3É0
64É0
65É2
2É0
26É0
24É9
1É2
2É4
8É2
1É5
1É0
1985
1985
1984
1984
1982
1979
1987
1978
1988
1984
1985
1979
1976
NCSS
NCSS
NCSS
NCSS
NCSS
NCSS
NCSS
NCSS
NCSS
a Abbreviations : OM, organic matter ; CEC, cation exchange capacity ; HMnO, hydrous manganese oxides ; FFeO, free Fe oxides ;
NCSS, no history of contamination with sewage sludge.
1985). For total metal concentrations, soil samples were
digested in a mixture of HClO and HF. The metal con4
centrations in food and soil samples were determined
using Ñame atomic absorption spectrometry or electrothermal atomisation atomic absorption spectrometry
for very low metal concentrations.
RESULTS AND DISCUSSION
The concentrations of metals in many of the soils used
in this study are higher than the maximum permissible
values under the current European Union regulations
(CEC 1986). For this reason the data from this study
may not be suitable for considerations of dietary metal
intake scenarios. However, sludge-amended soils which
have equilibrated in the Ðeld for many years provide a
valuable means of studying the soil-plant relationships
of PTEs. The soils used in these experiments di†ered
widely in physico-chemical properties, metal composition and history of sludge application (Table 1). The
results from this study were therefore expected to be of
wider applicability and more useful than those stemming from similar studies conducted on a few freshly
sludge-amended soils.
Metal concentrations in wheat, carrots and spinach
The concentration of metals in the dry matter of wheat
grain, carrot roots and spinach leaves produced on the
sewage sludge-amended and uncontaminated soils are
summarised in Table 2. While the concentrations of Cd
and Ni in wheat grain grown on the sludge-amended
soils were much greater than their concentrations in
those produced on the uncontaminated soils, Pb and
Zn concentrations in wheat grown on the sludgeamended soils were only marginally higher (Table 2).
The observed mean concentrations of 0É32 mg kg~1 for
Pb and 47É6 mg kg~1 for Zn in wheat grain from
sludge-amended soils, however are common for wheat
crops grown on soils not contaminated with sewage
sludges (Vigerust and Selmer-Olsen 1986 ; LuŽbben and
Sauerbeck 1991). The mean Cu concentration
(3É90 mg kg~1) in wheat grain produced on the sludgeamended soils appeared to be rather lower than its
background level (4É17 mg kg~1).
The mean concentrations of Cd, Ni and Zn in carrots
from the sludge-amended soils were very much higher
than those grown on the uncontaminated soils (Table
2). The concentrations of Cu and Pb in carrots produced on the sludge-amended soils were also elevated compared to their background levels, but the increases were
Plant availability of heavy metals in soils
449
TABLE 2
Contents of Cd, Cu, Ni, Pb and Zn (mg kg~1 DW) in wheat,
carrots and spinach produced on uncontaminated and sludgeamended soils, and their soil-to-plant transfer coefficients
Element
Uncontaminated
soils (n \ 9)
Sludge contaminated
soils (n \ 13)
Mean
T ransfer
ratioa
Mean
T ransfer
ratioa
W heat grain
Cadmium
Copper
Nickel
Lead
Zinc
0É24
4É17
0É58
0É23
47É62
0É451
0É189
0É051
0É006
0É412
0É68
3É90
2É15
0É32
58É35
0É112
0É031
0É034
0É002
0É133
Carrot roots
Cadmium
Copper
Nickel
Lead
Zinc
0É63
5É18
2É17
0É33
25É48
1É204
0É249
0É185
0É009
0É278
1É71
7É23
5É28
0É48
41É74
0É350
0É058
0É118
0É004
0É132
0É94
9É48
4É76
0É82
206É0
2É092
1É806
0É462
0É020
2É297
12É76
16É91
9É46
0É95
455É5
1É991
0É156
0É178
0É008
1É216
Spinach leaves
Cadmium
Copper
Nickel
Lead
Zinc
a Transfer ratio \ plant metal concentration/soil metal concentration.
relatively small compared to those observed for Cd, Ni
and Zn (Table 2). The observed average concentrations
of 7É23 mg kg~1 for Cu and 0É48 mg kg~1 for Pb in
carrots grown on the sludge-amended soils, however are
within the ranges reported for carrots grown on soils
not contaminated with sewage sludges (Keefer et al
1986).
Spinach leaves grown on the sludge-amended soils
accumulated substantially higher metal contents than
those grown on the uncontaminated soils. Spinach
grown on the sludge-amended soils, on an average,
absorbed more than 13-times as much Cd, more than
twice as much Zn and Ni and nearly twice as much Cu
compared to their background levels in plants on the
uncontaminated soils (Table 2). The concentration of
Pb in spinach leaves produced on sludge-amended soils
showed only a small increase compared to its background level (Table 2).
Spinach tissues accumulated the highest concentrations of all Ðve metals, while wheat grain accumulated
the lowest concentrations of Cd, Cu, Ni and Pb among
the three crops ; the lowest Zn concentration was in
carrot roots (Table 2). The sequence of metal concentrations in the plant tissues from sludge-amended soils was
Zn [ Cu [ Ni [ Cd [ Pb for wheat grain and carrot
roots, and Zn [ Cu [ Cd [ Ni [ Pb for spinach
leaves. The concentrations of metals in wheat, carrots
and spinach grown on sludge-amended soils in this
study clearly showed that metal uptake is plant-species
dependent. The Ðndings are consistent with those
reported in the literature (Schauer et al 1980 ; Keefer et
al 1986 ; Vigerust and Selmer-Olsen 1986 ; LuŽbben and
Sauerbeck 1991 ; Sauerbeck 1991 ; Smith 1994).
The metal uptake data also showed that soil-to-plant
transfer of metals varied widely among wheat, carrots
and spinach. Soil-to-plant metal transfer ratios (ratio of
the concentration of a metal in plants to its concentration in the soil) were much higher for spinach than for
wheat and carrots, and this was consistent with all Ðve
metals (Table 2). This implies that, for a given level of
metals in soils, spinach leaves accumulate far greater
amounts of metals than wheat grain and carrot roots.
Overall, the amounts of Cd, Ni and Zn in the food
plants showed the greatest increases compared to their
background levels. The concentrations of Cu and Pb in
the plants grown on the sludge-amended soils were also
elevated, but to a much less extent. This is primarily
because of their very low soil-to-plant transfer ratios
compared to those of Cd, Ni and Zn (Table 2). Plant
uptake is one of the major pathways by which metals
from sludge-amended soils enter the food chain. The
food-chain plants might absorb enough amounts of
heavy metals to become a potential health hazard to
consumers. In this context, the results showed that Cd,
Ni and Zn pose the greatest threat among the metals
studied because the levels of Cu and Pb were either
relatively low or did not show any appreciable increase
compared to their background concentrations in the
food crops.
Soil factors a†ecting metal accumulation in plants
The inÑuence of various soil properties on metal concentrations in wheat, carrots and spinach was evaluated
initially by stepwise multiple regression analyses and
involved all the soil variables mentioned in Table 1. Soil
total metal concentration was found to be the principal
factor controlling metal contents in the plants, and was
the only factor which consistently appeared in all
models. The inclusion of other soil variables, such as
pH, cation exchange capacity (CEC), organic matter,
iron and manganese oxides and texture (clay), in the
models gave results which were inconsistent and somewhat ambiguous. It is well known that many soil
descriptors (eg CEC and texture or CEC and organic
matter) are highly correlated and this may be one of the
reasons why the multiple stepwise regression gave
inconsistent results. To overcome the problems associated with correlated descriptors the soil variables were
mapped onto an uncorrelated set of predictors formed
by principal component analysis (PCA) of the soil variables. The stepwise regression was repeated using the
P S Hooda et al
450
new uncorrelated variables and the regression equation
back transformed to the original variables. The back
transformed regression equations were then used to
identify a small set of independent soil descriptors for
inclusion into Ðnal equations. All calculations were performed using Genstat 5 (release 3.1 copyright ; Lawes
Agricultural Trust, Rothamsted Experimental Station,
UK). The PCA results indicated that total metal, pH
and clay content were inÑuential in most stepwise
regression equations. Consequently it was decided that
the Ðnal regression equation ought to consist of three
soil descriptors ie total metal concentration, pH and
clay fraction. The multiple regression equations describing the relationships between metal concentrations in
plants and these three soil variables are given in Table
3. The results show that the relationship between metal
uptake in plants and the soil variables in the regression
model is strongly inÑuenced by metal and plant species.
For example, this model accounts for 62, 91 and 67%,
respectively, of the variation in Cd uptake data for
carrots, spinach and wheat (Table 3).
Regression coefficients with relatively small standard
errors of estimate (SE), ie with a high t-ratio (coefficient/
SE) indicate that a particular variable has a strong e†ect
on the uptake of a metal in a plant. The coefficients for
total metal concentration generally had the lowest SE
values (data not given) among the three soil variables in
the model. This means that the concentration of a metal
in soils is the principal factor for predicting its concentration in carrots, spinach and wheat. The relatively
small regression coefficients (Table 3) with somewhat
large SE values for clay fraction, as compared with pH,
suggest that the soil clay fraction does not predict metal
accumulation in plants as well as pH. Obviously the pH
is more important than clay content in regulating the
plant availability of these metals in the soils used in this
study. The e†ect of organic matter, CEC, free Fe oxides
and hydrous Mn oxides on metal accumulation in the
plants was less clear. This may be partly due to the
large variability in metal concentrations in these soils
(Table 1). Had the concentrations of the metals been
similar across all soils, these soil variables would possibly have also a†ected metal accumulation in the plants
in a more clear manner. These results however are consistent with a study conducted under similar conditions
involving soils from 13 long-term Ðeld experiments
(Sauerbeck 1991). The author found that organic matter
and texture are less important than the metal concentrations in soils and pH in regulating the availability of
Cd, Cu, Ni and Zn to several crop plant species.
From the above discussion it could be concluded that
although metal accumulation in plants is strongly
a†ected by their concentrations in soils, metal availability to plants varies with soil type. Plants grown on
soils with pH values in the neutral range and a clayey
texture tend to accumulate less metals than those grown
on soils with an acidic pH and a coarse texture. This
means disposal of sewage sludge on clayey textured
soils with either a neutral to alkaline pH or combined
with lime treatment might help achieve better control of
metal accumulation in food plants grown on sludgeamended soils.
TABLE 3
Multiple regression equationsa for plant metal content (mg kg~1) in relation to total soil metal
concentration (mg kg~1), pH and soil clay fraction (%)
Equation
Carrots
Cd \ 3É44
Cu \ 11É06
Ni \ 5É20
Pb \ 1É164
Zn \ 43É50
]0É284
]0É018
]0É218
]0É009
]0É068
Spinach
Cd \ 29É43
Cu \ 50É20
Ni \ 6É80
Pb \ 0É601
Zn \ 829
W heat
Cd \ 0É990
Cu \ 6É65
Ni \ 0É344
Pb \ 0É712
Zn \ 208
R2
P-value
[0É328 pH
[1É179 pH
[0É450 pH
[0É123 pH
[3É80 pH
[0É017 clay
]0É018 clay
[0É097 clay
]0É0004 clay
]0É027 clay
0É622
0É836
0É838
0É582
0É725
0É008
\0É001
\0É001
0É012
0É002
]1É919 soilÈCd
]0É014 soilÈCu
]0É185 soilÈNi
]0É0016 soilÈPb
]0É342 soilÈZn
[3É93 pH
[4É83 pH
[1É00 pH
]0É011 pH
[64É10 pH
[0É087 clay
[0É133 clay
]0É028 clay
]0É0023 clay
[4É82 clay
0É910
0É626
0É723
0É911
0É383
\0É001
0É007
0É002
\0É001
0É045
]0É183 soilÈCd
]0É002 soil [ Cu
]0É082 soilÈNi
]0É0008 soilÈPb
]0É019 soilÈZn
[0É156 pH
[0É503 pH
[0É051 pH
[0É047 pH
[24É86 pH
[0É003 clay
[0É021 clay
[0É029 clay
[0É0059 clay
[0É024 clay
0É672
0É664
0É761
0É759
0É595
0É012
0É013
0É004
0É004
0É025
soilÈCd
soilÈCu
soilÈNi
soilÈPb
soilÈZn
a Based on data from 13 sludge-amended soils.
Plant availability of heavy metals in soils
451
Soil extractants and metal concentrations in plants
The concentrations of Cd, Cu, Ni, Pb and Zn in the
edible parts of carrots, wheat and spinach were regressed against the concentrations of trace elements
extracted with the four chemical reagents (0É005 M
DTPA, 0É05 M EDTA-(Na) , 1 M NH NO and 0É05 M
2
4
3
CaCl ). Cadmium extracted with all the four extractants
2
correlated signiÐcantly with Cd contents in each of the
crops. However, Cd concentrations in spinach were best
predicted by the DTPA and those in carrots and wheat
by the EDTA (Table 4). Similarly, Cu contents in the
plant tissues were signiÐcantly related to soil Cu concentrations extracted by all the four reagents but DTPA
for carrots, NH NO for spinach and CaCl for wheat
4
3
2
gave the best relationships among the four extractants
employed (Table 4). For Ni concentrations in plant
tissues, all the four soil extractants gave highly signiÐcant relationships but the concentrations were best predicted by the CaCl for carrots, EDTA for spinach and
2
NH NO for wheat (Table 4).
4
3
The Pb concentrations in the plants were best predicted by the EDTA test (Table 4). Soil Pb concentrations extracted in CaCl and NH NO were not found
2
4
3
to be related to Pb contents in the crop plants. This is
partly due to the fact that the Pb concentrations
extracted by these neutral salt solutions were frequently
near the detection limit (0É05 mg dm~3) of the Ñame
atomic absorption spectrometer (FAAS) used. It would
appear, therefore, that the NH NO and CaCl extract4
3
2
ants may well be suitable for predicting Pb availability
to plants if a more sensitive analytical method became
available. The Zn concentrations in both spinach and
wheat were best predicted by those extracted from the
soils with the CaCl solution, only in carrots was the Zn
2
concentration better related to the EDTA-extractable
Zn (Table 4).
In general, the ability of the extractants to predict
plant-available metals depended on the crop species, the
metal and extractant used. Overall, the results show
that 0É05 M EDTA is a reliable test for predicting metal
availability to carrots, spinach and wheat from sludgeamended soils. This was because it was the only extractant which consistently correlated with all Ðve elements
in each of the crops and proved to be the best extractant in many cases (Table 4). EDTA and DTPA are the
most commonly used soil tests for assessing plantavailable metals, and, because of their greater extracting
strength, are likely to be more sensitive to reduced
metal solubilities at higher soil pH or to soils low in
metal contents. For a speciÐc metal or crop, however
0É05 M CaCl or 1 M NH NO may provide a better
2
4
3
prediction than EDTA or DTPA. For example, the Ni
concentrations in carrots and wheat were better predict-
TABLE 4
Linear regression equations of the form y \ a ] bx where y represents metal concentration in
the edible plant tissues (mg kg~1) and x is the soil metal concentration (mg kg~1) extracted by
one of the four extractants used
y
Cadmium
Carrot
Spinach
Wheat
Copper
Carrot
Spinach
Wheat
a
0É653
2É41
0É069
5É23
12É67
2É40
SE
b
SE
0É297
1É91
0É125
0É469
3É92
0É224
0É071
0É672
0É029
0É790
1É81
0É187
0É052
1É93
1É21
x
R2
P
EDTA
DTPA
EDTA
0É781
0É734
0É855
\0É001
\0É001
\0É001
0É008
0É543
0É218
DTPA
NH NO
4
3
CaCl
2
0É769
0É493
0É752
\0É001
0É004
\0É001
Nickel
Carrot
Spinach
Wheat
1É50
4É52
0É453
0É770
1É37
0É137
3É48
0É301
1É52
0É307
0É047
0É080
CaCl
2
EDTA
NH NO
4
3
0É914
0É763
0É973
\0É001
\0É001
\0É001
L ead
Carrot
Spinach
Wheat
0É327
0É698
0É179
0É097
0É089
0É061
0É002
0É004
0É002
0É0008
0É0007
0É0005
EDTA
EDTA
EDTA
0É433
0É711
0É653
0É009
\0É001
0É002
0É130
16É86
2É60
0É026
5É59
0É494
EDTA
CaCl
2
CaCl
2
0É663
0É403
0É728
\0É001
0É012
\0É001
Zinc
Carrot
Spinach
Wheat
26É62
299É10
37É78
5É04
56É50
5É42
a Based on data from 13 sludge-amended soils.
452
ed by 0É05 M CaCl and 1 M NH NO , respectively,
2
4
3
compared to the EDTA and DTPA (Table 4). Similarly,
soil Ni concentrations extracted by neutral salts such as
CaCl (0É05 M), MgCl (1 M) and NaNO (1 M) were
2
2
3
found to be better correlated with Ni contents in 13
crop species compared to other extractants (EDTA,
DTPA, NH OAc, CuCl ) (Sauerbeck and Hein 1991).
4
2
Indeed, metals extracted with a weaker extractant such
as 0É05 M CaCl or water would represent the most real
2
plant-available metals in soils. These extractants,
however, have the disadvantage of extracting metals in
very small concentrations which are often too low for
FAAS, and are difficult to determine by ETAAS
because of the chloride matrix interference and possible
contamination problems (Ure 1990). As a result, stronger extractants such as DTPA and EDTA have been
most widely used (Alloway and Jackson 1991). In this
study, 0É05 M EDTA-(Na) was found to be a better and
2
P S Hooda et al
consistent predictor of plant-available metals among
those tested.
E†ect of liming on metal contents in the plant tissues
Figure 1(A) shows the e†ect of liming on soil pH values.
The application of lime to soils signiÐcantly increased
soil pH values. The e†ects of liming on metal concentrations in wheat, carrots and spinach are summarised in
Figs 1(B)È(F). Liming the soils to pH 7 signiÐcantly
reduced the concentrations of all Ðve metals in carrot
roots (Figs 1(B)È(F)). The metal concentrations in
carrots grown on the limed soils were lower, on average,
by 39% for Cd and Ni, 44% for Zn, 48% for Pb and
28% for Cu compared to their concentrations in plants
grown on the unlimed soils. Liming the soils to pH 7
also decreased metal contents in spinach leaves. As with
Fig 1. E†ects of liming on soil pH values and accumulation of metals in wheat grain, carrot roots and spinach leaves. The error
bars represent the least signiÐcant di†erence (LSD, P O 0É05) between the two sets of data.
Plant availability of heavy metals in soils
carrots, the metal concentrations in spinach leaves produced on the limed soils were reduced by 39% for Cd,
27% for Cu, and more than 40% for Ni and Zn (Figs
1(B)È(F)). The Pb concentration in spinach leaves,
however showed a relatively small reduction of 19%
due to liming (Fig 1(E)).
Winter wheat was grown on soils which had been
limed two years previously, and the pH of these soils
averaged 6É5 when measured after harvesting the wheat
crop. This pH was still statistically higher than that of
unlimed soils (Fig 1(A)), but had no consequential e†ect
on the contents of Cd, Cu, Ni and Zn in wheat grain
(Figs 1(B)È(F)). In contrast, however the wheat grain
produced on the limed soils had signiÐcantly lower contents of Pb than those on the unlimed soils (Fig 1(E)).
Liming sludge-amended soils to pH P 6É5 is often
recommended to minimise plant uptake of PTEs. The
reduction in metal concentrations in carrots and
spinach due to liming the soils is consistent with the
Ðndings reported in the current literature (Eriksson
1989 ; Jackson and Alloway 1991 ; LuŽbben and Sauerbeck 1991 ; Smith 1994), but the liming had no e†ect on
metal contents in wheat grain, except for Pb. The results
indicate that while liming soils to pH 7É0 was e†ective
in reducing metal accumulation in carrots and spinach,
liming to pH 6É5 had little e†ect on metal accumulation
in wheat grain. This infers that either pH 6É5 was not
high enough to have a substantial e†ect on metal
uptake or that metal accumulation in wheat grain was
independent of soil pH. Wheat straw samples were not
analysed for their metal contents in the present study.
As a result, it is difficult to say whether liming to pH 6É5
was not sufficient enough to have a signiÐcant reduction
in wheat metal contents or metal translocation from
wheat vegetative parts to grain was independent of soil
pH. Results from a recent study, however suggested that
metal contents in reproductive plant parts, such as
grain, were independent of soil pH. For example, while
liming sludge-amended soils over a pH range of 3É9È7É6
substantially reduced Cd concentrations in potatoes,
ryegrass and oat straw, it had no e†ect on Cd contents
in oat grain (Smith 1994). The results from the present
investigation together with other recent studies (Smith
1994) indicate that liming is less likely to control metal
accumulation in the seeds of cereal crops such as wheat
and oat grain.
The Council of the European Communities has Ðxed
mandatory concentrations for PTEs in soils receiving
sewage sludge (CEC 1986). The maximum permissible
metal concentrations in the 1986 EC Directive on
sludge regulation (CEC 1986) apply to soils with pH in
the range of 6É0È7É0. Where sewage sludge is applied to
soils with pH values \6É0, Member States are required
to reduce the maximum permissible concentrations
accordingly to counterbalance the increased availability
of metals to crop plants. In the UK, lower metal limits
apply for Cu, Ni and Zn where sludge is applied to soils
453
with pH values \6É0 compared to pH band 6É0È7É0
(Statutory Instrument 1989). However, the maximum
concentration of Cd (3 mg kg~1) and Pb (300 mg kg~1)
in the EC Directive of 1986 are permitted in sludgeamended soils with a pH value as low as 5É0 (Statutory
Instrument 1989), without taking account of their
potentially increased bioavailability at lower pH values
(\6É0). The results in this study clearly demonstrate an
increased uptake of heavy metals at lower soil pH
values. This is particularly signiÐcant with regard to Cd
and Pb which potentially can accumulate in humans
following ingestion of contaminated foods. The current
UK Regulations for both these metals take no account
of the soil pH and are at the highest level of the
maximum range permitted in the European Union
regulations (CEC 1986). It would, therefore, be prudent
to base maximum Cd and Pb loading values on the soil
pH (as is the case with Cu, Zn and Ni).
REFERENCES
Alloway B J, Jackson A P 1991 Behaviour of trace metals in
sludge-amended soils. Sci T otal Environ 100 151È176.
Alloway B J, Morgan H 1985 The behaviour of Cd, Ni and
Pb in polluted soils In : Contaminated Soils, eds Assinnk
J W & van den Brink W J. Martinus Nijho† Publishers,
Dordrecht, pp 101È113.
Anon 1986 T he Analysis of Agricultural Materials. Reference
Book No 27, Ministry of Agriculture Fisheries and Food.
HMSO, London.
Avery B W, Bascomb C L (Eds) 1974 Soil Survey Methods.
Soil Survey Technical Monograph No 6, Harpenden : Soil
Survey of England and Wales.
CEC 1986 Council Directive of 12th June 1986 on the protection of the environment and in particular of the soils, when
sewage sludge is used in agriculture. Official J European
Communities, Directive 86/278/EEC, No L 181/6-12.
CEC 1991 Urban Waste Water Treatment Directive. Official J
European Communities, Directive 91/271/EEC, No L 375/18.
Chaney R L 1990 Public health and sludge utilisation. Biocycle 30 68È73.
Chao T T 1972 Selective dissolution of Mn oxides from soils
with acidiÐed hydroxylamine hydrochloride. Soil Sci Soc
Am Proce 36 764È768.
Coker E G, Hall J E, Carlton-Smith C H, Davis R D 1987
Field investigations into the manurial value of lagoonmatured digested sewage sludge. J Agric Sci 109 467È478.
Day P R 1965 Particle fractionation and particle size analysis.
In : Methods of Soil Analysis, ed Black C A. American
Society of Agronomy, Madison, Wisconsin, pp 545È567.
DoE 1993 Sludge Uses in 1990È91, UK Report to the Commission under Directive 86/275/EEC. DoE, HMSO,
London.
Eriksson J E 1989 The inÑuence of pH, soil type and time on
adsorption and uptake by plants of Cd added to soils.
W ater Air Soil Pollut 48 317È335.
Hemphill D D Jr, Jacobson T L, Martin L W, Kiemnec G L,
Hanson D, Volk V V 1982 Sweet corn response to application of three sewage sludges. J Environ Qual 11 191È196.
Hesse P R 1971 A T extbook of Soil Chemical Analysis, John
Murray, London.
454
Hooda P S, Alloway B J 1993 E†ects of time and temperature
on the bioavailability of Cd and Pb from sludge-amended
soils. J Soil Sci 44 97È110.
Jackson A P, Alloway B J 1991 Bioavailability of cadmium to
lettuce and cabbage in soils previously treated with sewage
sludge. Plant and Soil 132 179È186.
Keefer R F, Singh R N, Horvath D J 1986 Chemical composition of vegetables grown on agricultural soils amended with
sewage sludge. J Environ Qual 15 146È152.
Kuo S, Jellum E J, Baker A S 1985 E†ects of soil type, liming
and sludge application on zinc and cadmium availability to
Swiss chard. Soil Sci 139 122È130.
Lindsay W L, Norvell W A 1978 Development of DTPA test
for zinc, iron, manganese and copper. Soil Sci Soc Am J 42
421È428.
LuŽbben S, Sauerbeck D R 1991 The uptake and distribution
of heavy metals by spring wheat. W ater Air Soil Pollut
56È57 239È247.
McGrath S P 1987 Long-term studies of metal transfer following the application of sewage sludge. In : Pollutant T ransport and Fate in Ecosystems, eds Coughtrey P J, Martin
M H & Unsworth M H. Blackwell ScientiÐc, Oxford, pp
301È317.
Pepper I L, Bezdicek D F, Baker A S, Sims J M 1983 Silage
corn uptake of sludge applied zinc and cadmium as a†ected
by soil pH. J Environ Qual 12 270È275.
Robertson W K, Lutrick M C, Yuan T L 1982 Heavy applications of liquid-digested sludge on three Ultisols : 1. Soil fertility. J Environ Qual 11 450È454.
P S Hooda et al
Sauerbeck D R 1991 Plant element and soil properties governing uptake and availability of heavy metals derived from
sewage sludge. W ater Air Soil Pollut 57È58 227È237.
Sauerbeck D R, Hein A 1991 The nickel uptake from di†erent
soils and its prediction by chemical extractions. W ater Air
Soil Pollut 57È58 861È871.
Schauer P S, Wright W R, Pelcha J 1980 Sludge-borne heavy
metal availability and uptake by vegetable crops under Ðeld
conditions. J Environ Qual 9 69È73.
Statutory Instrument 1989 United Kingdom Statutory Instrument No 1263. The Sludge (Use in Agriculture) Regulations.
HMSO, London.
Smith S R 1994 E†ect of soil pH on availability of metals in
sewage sludge-treated soils. II. Cadmium uptake by crops
and implications for human dietary intake. Environ Pollut
86 5È13.
Ure A M 1990 Methods of analysis for heavy metals in soils.
In : Heavy Metals in Soils, ed Alloway B J. Blackie,
Glasgow, pp 40È80.
Vigerust E, Selmer-Olsen A R 1986 Basis for metal limits relevant to sludge utilisation. In : Factors InÑuencing Sludge
Utilisation Practices in Europe, eds Davis R D, Haeni H &
LÏHermite P. Elsevier Applied Science Publishers, London,
pp 26È41.
Williams D E, Vlamis J, Pukite J, Corey J E 1987 Metal
movement in sludge-amended soils : a nine year study. Soil
Sci 143 124È131.
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