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Soil Biology & Biochemistry 32 (2000) 2031±2043
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Mineralization of soil and legume nitrogen in soils treated with metalcontaminated sewage sludge
K.J. Munn a, J. Evans b,*, P.M. Chalk a,c
a

b

Department of Agriculture, University of Melbourne, Parkville, Vic 3052, Australia
NSW Agriculture, Agricultural Institute, Private Mail Bag, Wagga Wagga, 2650 NSW, Australia
c
Present address: International Atomic Energy Agency, P.O. Box 200, A-1400 Vienna, Austria
Accepted 16 May 2000

Abstract
Eighty percent of urban sewage sludge in southeastern Australia is destined to be reused on agricultural land to improve soil fertility.
However, this sludge is usually contaminated with industrial pollutants, in particular with heavy metals. As heavy metals are known to be
toxic to microorganisms, concern has been raised that treating soils with these sludges may adversely affect the mineralization of the organic
N in the soil, sludge or plant material incorporated into the amended soils.
In the absence of historically contaminated soils, dewatered sewage sludges with total heavy metal contents to 4658 mg kg 21 were ground

and mixed with different soils at rates up to 240 t ha 21. The soil±sludge mixtures were then `aged' through seven cycles of wetting and
drying, in the presence of plants, over a period of twelve months. Total heavy metal concentrations to 1026 mg kg 21 soil, with individual
metal concentrations (mg kg 21 soil) to Zn (481), Cu (249), Cr (187), Ni (86), Pb (80) and Cd (2.5), were achieved with the treatments.
Samples of the processed soils were incubated at the ®eld capacity water contents, with or without incorporated lucerne, before determining
the concentration of available N (nitrate 1 ammonium) in the soil.
More available N occurred in soils treated with sludge than in unamended soils, and available N increased with the amount of sewage
sludge added. The N in lucerne was mineralized in all treatments, and there were few cases in which the increase in available N due to lucerne
was reduced in sludge-amended soils. These cases involved nitrate loss and could not be ascribed to an effect of high concentrations of heavy
metals in soil.
Soil amendment with sludge increased the concentrations of total N, C and exchangeable cations, as well as pH, in the soil. Of these
factors, only total N and exchangeable cations were positively correlated with available N. Higher concentrations of C (or heavy metals) in
soil and higher pH were associated with less available N, but these effects were quantitatively inferior to the positive effects of total N and
exchangeable cations.
Based on the results of these studies the current limits on the allowable concentrations of heavy metals in soils, as de®ned by the New
South Wales Environmental Protection Agency, can be substantially increased without affecting the bene®ts in N obtained from the
incorporation of legume N with soil. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Sewage; Nitrogen; Mineralization; Legume; Heavy meta1

1. Introduction
Sewage sludge has an organic matter content of 40±60%

(dry weight basis; Ross et al., 1991), so that its application to
Australian agricultural soils, which usually contain less than
half the desirable concentration of organic matter, is likely
to improve the physical condition of the native soils. In
addition, the nutrient content of sludges (% of dry weight:
N, 2.1±8.2; P, 1.1±8.9; Ca, 0.9±4.3; Mg, 0.18±0.68, Ross et
al., 1991) indicates they may also increase soil fertility.
* Corresponding author. Tel.: 161-2693-81999; fax: 161-2693-81809.
E-mail address: jeffrey.evans@agric.nsw.gov.au (J. Evans).

However, urban sewage sludges are often contaminated with industrial organic chemicals and heavy
metals. These metals tend to accumulate in soil,
where they may then cause changes to microbial populations and their activities (Babich and Stotzky, 1985;
McGrath et al., 1988; Martensson and Witter, 1990).
The changes include reductions in microbial biomass
(Brookes and McGrath, 1984; Chander and Brookes,
1991a), which, according to Chander and Brookes
(1991b), may reduce the mineralization of organic
matter. In Australia, it would not be desirable to reduce
the capacity of soils to mineralize organic matter,

because cereal production depends on N mineralized

0038-0717/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0038-071 7(00)00105-X

2032

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

Table 1
Properties of the soils (0±10 cm) from different sites
Site

Rutherglen
Goulburn
Robertson
Condobolin
Wagga Wagga
Darlington Pt.
a

b
c
d
e
f

pH a

4.2
4.4
4.7
5.3
5.6
6.4

N b (g kg 21)

1.2
2.0
3.7

1.5
0.9
1.5

C c (g kg 21)

14
20
32
15
7
11

P d (mg kg 21)

28
16
5
22
55

47

Particle size e (g kg 21)

Exchangeable cations f (cmol kg 21)

Clay

Silt

Fine sand

Coarse sand

Al

Mg

Ca

K

Na

112
100
540
320
61
520

155
160
190
120
56
110

613
522

171
475
451
207

122
190
30
83
473
152

0.41
0.30
0.79
, 0.01
0.08
, 0.01

0.29

1.00
1.82
2.75
0.46
9.47

1.20
2.47
3.78
7.90
2.11
14.5

0.52
0.45
0.53
3.08
0.47
1.34

, 0.01
, 0.01
0.19
0.03
, 0.01
0.68

pH of a 1:5, w:v soil suspension in 0.01 M CaC12 at 258C.
Kjeldahl N.
Total carbon determined by combustion using a LECO FP-2000 Carbon Analyzer.
Bray P.
McIntyre and Loveday (1974).
Exchangeable cations in a 0.01 M BaC12 leachate (Vimpany et al., 1987) determined by inductively coupled-plasma-atomic emission-spectrometry.

from organic sources, particularly the roots and stubbles
of legumes.
Net N mineralization was inhibited by the addition of
heavy metals to an acidic silty loam (Chang and Broadbent,
1982) and by very high metal concentrations in other soils
(Tyler, 1975; Babich and Stotzky, 1985). However, Babich

and Stotzky (1985) also cite cases of either stimulation or no
effect of heavy metals on N mineralization. Similarly, Munn
et al. (1997) found no adverse effects on net mineralization
of legume and soil organic N from heavy metals accumulated in soil after treatment with metal-contaminated sewage
sludge. The soil studied by Munn et al. (1997) contained
maximal total heavy metal concentrations that were 50% of
those associated with reduced microbial biomass at Woburn
in the UK (Brookes and McGrath, 1984) or that resulted in
accumulation of organic matter in other soils in the UK
(Chander and Brookes, 1991a, b).
The New South Wales Environmental Protection
Authority (NSWEPA) has determined interim maximal
allowable concentrations of heavy metals for soils used
for food production in NSW. However, it wants to test
the appropriateness of these maxima for various
processes important for the effective, sustainable functioning of agricultural systems and to determine the
margins applying to the limits. Thus, following the
initial study of Munn et al. (1997), we investigated
whether the amendment of soils with sludge-associated
heavy metals signi®cantly compromised improving N
availability in soil by the addition of fresh legume material. A wider range of soils and sludges, greater
amounts of heavy metals and wider variation in heavy
metal composition were used than in the study of Munn
et al. (1997). The maximal amounts of heavy metals
added to soils with the sludges were suf®cient to test
the limits on heavy metals of NSWEPA guidelines. We
also de®ned some soil factors that affect production of
mineral N in soils treated with sludge.

2. Materials and methods
It was not possible to do this study using in situ, historically contaminated soils of the major agricultural soil
groups in NSW, because they do not exist. In lieu of this,
®nely ground dewatered sewage sludge (DWS), thorough
soil±sludge mixing, and several cycles of wetting and
drying over 12 months were used to `age' the amended
soils with sludge, in pots, before investigating the mineralization of N. Sewage sludges from several waste-water
treatment plants and several levels of application of the
sludges to soil provided a wide range of concentrations
and composition of heavy metals in several agricultural
soils.

2.1. Analytical methods
Total concentrations of soil N and C were estimated
with a LECO Carbon and Nitrogen Analyzer. Total
concentrations of heavy metals in soil and sludge
were recovered by microwave-assisted digestion (1
soil:10 mL reverse aqua regia (4 M) Ð 3 parts concentrated nitric acid:1 part concentrated hydrochloric acid).
Exchangeable cations were extracted with 0.01 M
BaCl2, and these and the heavy metals were quanti®ed
by inductively coupled-plasma-emission-spectrometry
(ICPAES; Fisons Instruments ARL 3520 B) (Vimpany
et al., 1987). P was estimated as Bray (No.1) (Bray and
Kurtz, 1945), pH in 0.01 M CaCl2 (1:5, soil/solution),
and electrical conductivity (EC) in water (1:5, soil/
water). Exchangeable inorganic N was extracted with
and
1 M KCl, and the concentrations of NH1
4
2
2
)
were
(subsequently
referred
to
as
NO
1NO
NO2
3
2
3
determined with an autoanalyzer (Bran and Luebbe
TRAACS 800). Particle-size analysis was determined
by the method of McIntyre and Loveday (1974).

2033

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043
Table 2
Sewage sludge composition
Sludge source

Malabar
Port Kembla
Quakers Hill
St Marys
Richmond
a
b
c
d

N a (g kg 21)

23.4
25.2
38.0
26.7
11.0

C b (g kg 21)

284
239
236
220
97

P c (mg kg 21)

Heavy metal concentration d (mg kg 21)

300
89
78
52
290

Zn

Cu

Cd

Ni

Pb

Hg

Cr

Total

2669
1767
512
498
704

1274
648
468
447
439

11
32
2
2
3

162
32
28
25
20

303
97
81
111
90

4
,2
,2
,2
27

235
66
488
66
54

4658
2612
1579
1149
1137

Kjeldahl N.
Total carbon determined by combustion using a LECO FP-2000 Carbon Analyser.
Bray P.
Total elements determined by microwave acid digestion and inductively coupled-plasma-atomic emission-spectrometry (Vimpany et al., 1987).

2.2. Soils and sewage sludges
The soils and their chemical and physical characteristics
are given in Table 1. The soil types (Stace et al., 1968)
included: a Red Podzolic (Rutherglen); a Yellow Podzolic
(Goulburn); a Krasnozem (Robertson); a Red-brown Earth
(Condobolin); a Red Earth (Wagga Wagga); and a Black
Earth (Darlington Pt.). Surface (0±10 cm) soil, sieved to
,2 mm, was used in the experiments.
The sludges and their chemical characteristics are listed
in Table 2. On receipt, the sludges were dried at 30±408C,

milled, and sieved to ,3 mm, which allowed the inclusion
of some larger particles; however, these were narrow
(,1 mm) ®brous strands. The pH values of all sludges
were similar (5.7±6.2), and their EC ranged from 1.35 to
2.29 dS m 21. Total C:total N ratios ranged from 6.2 to 11.9.
Extractable P concentrations were higher in the Malabar and
Richmond sludges, because these had not been amended
with Fe (ferrous sulfate) at the waste-water treatment
plant. Total heavy metal concentrations were greater in
the Malabar and Port Kembla sludges, except for Hg,
which was most concentrated in the Richmond sludge.

Table 3
Chemical characteristics of soils after amendment with Malabar dewatered sewage sludge (DWS)
Soil

Sludge level
(t DWS ha 21)

pH
(CaCl2)

Na
(g kg 21)

Cb
(g kg 21)

Pc
(mg kg 21)

Exchangeable cations d
(cmol kg 21)

Heavy metals e
(mg kg 21)

Rutherglen

0
60
240
0

4.2
5.3
6.3
4.4

1.2
2.4
5.2
2.0

11.7
19.3
44.8
18.0

28
198
385
16

2.6
11.4
nd f
1.8

48
202
898
45

60
240

5.4
6.1

2.9
5.9

24.1
62.8

162
381

11.0
20.0

218
961

Robertson

0
60
240

4.7
5.2
5.9

3.7
4.3
7.5

37.1
42.8
59.2

5
37
182

7.4
12.4
31.2

360
514
1026

Condobolin

0
60
240

5.3
5.6
6.3

1.5
2.2
6.8

13.6
21.3
51.4

22
131
353

11.2
16.2
23.1

89
211
759

Wagga Wagga

0
60
240

5.6
5.7
5.9

0.9
1.8
2.9

7.2
12.2
26.3

55
194
nd

3.4
5.8
12.7

45
154
461

Darlington Pt.

0
60
240

6.4
6.2
5.9

1.5
2.3
4.4

10.8
19.7
41.3

47
194
289

25.5
29.6
37.1

125
280
641

Goulburn

a

Kjeldahl N.
Total carbon determined by combustion using a LECO FP-2000 Carbon Analyser.
c
Bray P.
d
Exchangeable cations in a 0.01 M BaC12 leachate (Vimpany et al., 1987) and determined by inductively coupled-plasma-atomic emission-spectrometry.
e
Total elements extracted by microwave acid digestion (Vimpany et al., 1987) and determined by inductively coupled-plasma-atomic emission-spectrometry.
f
nd Ð not determined.
b

2034

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

Table 4
Chemical characteristics of Goulburn soil after amendment with different dewatered sewage sludges (DWS)
Sludge source

Sludge level
(t DWS ha 21)

pH
(CaC12)

N a (g
kg 21)

C b (g
kg 21)

P c (mg
kg 21)

Exchangeable cations d
(cmol kg 21)

Heavy metals e
(mg kg 21)

Nil

0

4.4

2.0

18.0

16

4.8

48

Malabar

60
240

5.4
6.1

2.9
5.9

24.1
62.8

162
381

11.0
20.0

218
961

Port Kembla

60
240

5.2
5.6

3.2
6.0

29.3
46.8

nd f
nd

8.5
16.9

162
814

Richmond

60
240

5.0
5.3

2.7
3.5

22.4
39.0

nd
224

6.5
10.8

91
250

St Marys

60
240

4.9
5.7

2.8
6.0

24.2
45.4

72
113

5.5
10.0

62
225

Quakers Hill

60
240

6.0
6.4

4.0
5.3

26.7
48.4

84
nd

nd
11.7

77
216

a

Kjeldahl N.
Total carbon determined by combustion using a LECO FP-2000 Carbon Analyser.
c
Bray P.
d
Exchangeable cations in a 0.01 M BaC12 leachate (Vimpany et al., 1987) and determined by inductively coupled-plasma-atomic emission-spectrometry.
e
Total elements determined by microwave acid digestion and ICPAES (Vimpany et al., 1987) and determined by inductively coupled-plasma-atomic
emission-spectrometry.
f
nd Ð not determined.
b

Only the Malabar sludge was applied to all soils; the other
sludges were applied only to the Goulburn soil. Levels of
application were equivalent to 0, 60, and 240 t DWS ha 21
(0±10 cm), but not all levels were used with all sludges.
Control soils received no sludge. The chemical characteristics of the soils after amendment with the Malabar sewage
sludge and of the Goulburn soil after amendment with
different sludges are given in Tables 3 and 4, respectively.

until the EC of the soils was below 1.0 dS m 21 after
which no further leaching was carried out. This was
done after symptoms of saline toxicity were observed
on seedlings in the B1 biocycle in soils amended with
the highest amounts of sludge. It would be expected
that leaching of salts would occur naturally in the
®eld via rainfall. After B7, EC ranged from 0.4 to
0.94 dS m 21 in soils treated with Malabar sludge and
from 0.2 to 0.84 dS m 21 in the sludge-amended Goulburn soil.

2.4. Reaction of soils and sludges (`aging')

2.5. N mineralization

Soil-sludge mixtures were added to pots (2 kg pot 21),
thoroughly wetted with sterile, deionized water, and the
pots placed in a glasshouse (15±188C/20±258C; night/
day). After one week, the pots were sown with a
mixture of legumes (Trifolium spp., Medicago spp.
and Vicia spp.) and annual ryegrass (Lolium rigidum).
The seedlings were grown for 5±6 weeks (Biocycle 1,
B1), during which time the pots received 50±100 ml of
sterile deionized water once or twice weekly, as
required for plant growth. After the sixth week, no
further water was supplied, and the soils were allowed
to dry and the plants to senesce. The soil from replicate
pots was combined and sieved (,2 mm) before dispensing to pots, watering, and establishing fresh legumes,
as before. Seven such biocycles (B1 to B7), each of six
weeks duration and each followed by one week during
which soils were allowed to dry, were completed.
Between the end of B1 and the commencement of B2,
soils were leached in situ with sterile, deionized water

Soil at the completion of B7 was used in the experiments.
Shoots of milled (,1 mm) lucerne (Medicago sativa) (N,
3.28%; C, 34%) were mixed thoroughly with soil at 2 g
lucerne 100 g 21 soil. Twenty-®ve grams of lucerne-treated
(luc 1 ) or untreated (luc 2 ) soil was placed into plastic
tubes and brought to the ®eld capacity water content, determined by allowing saturated soil to drain in contact with dry
soil for 48 h. The tubes were closed with loose-®tting caps and
incubated at 268C (1, 2 28C) for 10 days, when the soil was
dried at 408C (Raymont and Higginson, 1992) in Petri dishes.
2
The concentrations NH1
4 and NO3 in soil were determined
and expressed per unit of oven dry (1108C) soil, and the sum of
these concentrations was de®ned as the available N concentration (AN; mgN kg21 oven dry soil). Other tubes were used
to determine the content of water in the soils.
For each initial soil treatment (amount and source of
sludge), there were 3 replicate tubes for both luc 2 and
luc 1 , and soil treatments were completely randomized
within soils. The effect of soil type and sludge treatment on

2.3. Levels of sewage sludge

2035

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

Table 5
In¯uence of the amount of dewatered sewage sludge (DWS) and lucerne incorporation on the available N concentrations in soils amended with Malabar sludge
and incubated for 10 days at ®eld capacity. Column values with a common letter within soils are not signi®cantly different at P , 0:05. Lucerne effect was
signi®cant in all treatments
Site

Sludge level (t DWS ha 21)

Available N (mg kg 21)

Soil N (%)

Minus lucerne

Plus lucerne

Mean

Rutherglen

0
60
240

0.15
0.19
0.42
Mean sludge level
main effect: sed a
25.2; lsd b 55.9

44
63
110
73

494
486
645
541

269 b
274 b
377 a

Goulburn

0
60
240

0.19
0.24
0.40
Mean sludge level
main effect: sed
11.1; lsd 24.7

57
53
128
79

507
549
604
542

282 b
301 b
366 a

Robertson

0
60
240

0.31
0.37
0.62
Mean sludge level
main effect: sed
15.2; lsd 33.7

48
105
276
143

532
582
759
624

290 c
344 b
518 a

Condobolin

0
60
240

0.18
0.23
0.37
Mean sludge
level £ lucerne: sed
11.4; lsd 25.4

44 b
59 b
169 a
91

531 b
553 b
603 a
562

287
306
386

Wagga Wagga

0
60
240

0.09
0.15
0.25
Mean sludge
level £ lucerne: sed
26.0; lsd 57.9

23 b
36 ab
94 a
51

247 c
338 b
530 a
370

135
187
312

Darlington Pt

0
60
240

0.18
0.23
0.52
Mean sludge
level £ lucerne: sed
6.5; lsd 14.5

51 b
65 b
227 a
115

492 b
507 b
627 a
541

271
286
425

a
b

sed ˆ standard error of differences between means …P , 0:05†.
lsd ˆ least signi®cant difference …P , 0:05†:

the available N from lucerne was determined as the difference
in AN between luc 1 and luc 2 , denoted d AN. AN and d AN
were compared between sludge rates within soils. The effects
of different soil variables on AN were evaluated using multiple
linear regression analysis.

3. Results
3.1. Sewage sludge effects on soil at the end of biocycle 1
The addition of sewage sludge to soil increased the
concentrations of total N, C, heavy metals, extractable P,

and exchangeable cations, as well as pH (Table 3). The
increases in total N, about 3-fold at 240 t DWS ha 21,
occurred together with increases in C …r 2 ˆ 0:83†; so that
there was little change in the C: N, which ranged from 7 to
10 in all treatments. The increases in pH and exchangeable
cations were greatest in the more acidic soils, with the highest amount of sludge added increasing the pH by 2 units and
the exchangeable cations 10-fold (Table 3). In contrast, the
addition of sewage sludge to the Darlington Pt. soil
decreased the pH. Concentrations of extractable P were
increased 5 to 35-fold with the maximal amount of sludge:
least in the Darlington Pt. soil and most in the Robertson
soil.

2036

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

Table 6
In¯uence of the amount of dewatered sewage sludge (DWS) and lucerne incorporation on the availble N concentration in Goulburn soil amended with different
sludges and incubated for 10 days at ®eld capacity. Column values with a common letter within sludge level are not signi®cantly different at P , 0:05. Lucerne
effect was signi®cant in all treatments
Sludge source

Sludge level (t DWS ha 21)

Available N (mg kg 21)
Minus lucerne

Plus lucerne

Mean

Malabar

0
60
240
Mean sludge level main
effect: sed a 11.1; lsd b 24.7

57
53
128
91

507
549
604
577

282 b
301 b
366 a

Port Kembla

0
60
240
Mean Sludge level main
effect: sed 10.2; lsd 22.8

57
60
447
253

507
558
963
747

282 c
309 b
705 a

St Marys

0
60
240
Mean sludge
level £ lucerne: sed 14.1;
lsd 31.4

57 b
47 b
198 a
123

507 a
421 b
531 a
476

282
234
365

Richmond

0
60
240
Mean sludge level main
effect: sed 18.9; lsd 42.1

57
50
123
86

507
454
546
501

282 b
253 b
335 a

Quakers Hill

0
60
240
Mean sludge
level £ lucerne: sed 17.0;
lsd 41.6

57 b
nd c
295 a
176

507 b
nd
849 a
678

282
na d
572

a
b
c
d

sed ˆ standard error of differences between means …P , 0:05†:
lsd ˆ least signi®cant difference …P , 0:05†:
nd Ð not determined.
na Ð not available.

The increased concentrations of total heavy metals
were correlated with increases in organic C …r2 ˆ
0:83†: Treatment with the maximal amount of Malabar
sludge produced the highest heavy metal concentration, which was 1026 mg kg 21 soil in the Robertson
soil and 961 mg kg 21 soil in the Goulburn soil (Table
3). Sludge from Port Kembla, which had a comparatively high heavy metal concentration, increased the
concentrations of total heavy metals in the Goulburn
soil to 814 mg kg 21 soil at the highest sludge level
(Table 4).
3.2. Effects of biocycles on chemical composition of the soils
The chemical characteristics of the soils at the end of
B7 were not greatly different from those at the end of
B1. In soils receiving maximal sludge application, total
N was about 2 g kg 21 lower and exchangeable cations
were 1±6 cmol kg 21 soil lower after B7 than after B1,

but the differences were less with smaller additions of
sludge. The pH after B7 was lower than after B1 by ca.
0.5 units, with the least change in the Wagga Wagga
and Darlington Pt. soils. Extractable P was relatively
unchanged, except in the Robertson soil, where the
concentration was ca. 55% less after B7 than after
B1, and in both the Rutherglen and Goulburn soils treated with 60 t DWS ha 21, where it was ca. 25% less after
B7 than after B1.
Total heavy metal concentration did not differ greatly
between the end of the B1 and B7 cycles, except in the
Rutherglen and Goulburn soils with the maximal amount
of sewage sludge, it was ca. 25% less after B7 than after
B1. The largest difference occurred in the Goulburn soil
treated with Port Kembla sludge.
3.3. Available N in soils without (luc 2 ) lucerne
In the six soils amended with Malabar sludge and in the

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

2037

Fig. 1. The effects of amending soils with dewatered sewage sludge (DWS) on the concentrations of ammonium and nitrate in soil: 1a, different soils treated
with Malabar sewage sludge; 1b, Goulburn soil treated with different sludges. The bars on columns are the least signi®cant differences …P , 0:05† where the
differences were signi®cant.

Goulburn soil amended with different sludges, AN
values were greater in the amended soils than in the
control soils (Tables 5 and 6, respectively), irrespective
of the source of sludge and the heavy metal concentration. The differences resulting from levels of sludge
were signi®cant …P , 0:05† when the level was 240 t
DWS ha 21 or 60 t DWS ha 21 in two of the soils
(Wagga Wagga and Robertson).
For the combined soil and sludge treatments in luc 2 ,

AN and total soil N were linearly correlated …r2 ˆ 0:68; P ,
0:05†: However, a multiple linear regression involving the
additive effects of total soil N (%N), organic C (%C),
exchangeable cations (EX) and pH, explained signi®cantly
more …r 2 ˆ 0:85; P , 0:05† of the variation in AN than N
alone. Accordingly,
AN…luc2† ˆ 884N 1 5:5EX 2 68:5pH 2 45:4C 1 258 …1†
The increases in AN in response to soil treatment with

2038

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

Fig. 1. (continued)

sludge involved signi®cant …P , 0:05† increases in the
1
concentrations of both NO2
3 and NH4 (Fig. 1a and b;
luc 2 ). For most of the soil and sludge treatments, the
1
concentration of NO2
3 relative to NH4 was similar to, or
greater, in sludge-amended soil than in control soil, but
exceptions occurred with the Robertson soil, the Wagga
Wagga soil treated with Malabar sludge at 240 t
DWS ha 21, and the Goulburn soil treated with Richmond
sludge at 240 t DWS ha 21.
The percentage of the total soil N recovered as available
N ranged from 1.5 to 9.5%. There was no evidence that
higher percentages occurred in treatments with lower
concentrations of heavy metals in the soils.

with Malabar sludge and in the Goulburn soil amended with
different sludges are given in Tables 5 and 6, respectively.
Except for the Goulburn soil treated with sludge from St.
Marys, AN values increased in the sludge-amended soils.
The increases involved greater concentrations of NH1
4 , except
in the Darlington Pt. soil (Fig. 1a and b; luc 1 ). In soils treated
with 240 t DWS ha 21, NO2
3 concentrations were signi®cantly
lower than in control soils for: Rutherglen, Goulburn, Robertson and Darlington Pt. soils amended with Malabar sludge and
the Goulburn soil amended with St. Marys' sludge. The variation in AN in the soils treated with lucerne followed a similar
response to the soil factors affecting the production of AN in
the absence of lucerne (Eq. (1)), i.e.

3.4. Available N in soils with added (luc 1 ) lucerne

AN…luc1† ˆ 1080N 1 10EX 2 124:8pH 2 55:9C 1 904

AN values for the luc 1 treatment in the six soils amended

…r 2 ˆ 0:84; P , 0:05†:

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

2039

Fig. 2. Differences (log10 transform) in the available N concentrations in soils with and without lucerne: 2a, effects of amending soils with dewatered sewage
sludge (DWS) from Malabar; 2b, effects of amending Goulburn soil with different dewatered sewage sludges. The bars on columns are the least signi®cant
differences …P , 0:05† where the differences were signi®cant; nd not determined.

3.5. Net mineralization of lucerne N

d AN values in soils amended with Malabar sludge were
not signi®cantly different than in the control soils (Fig. 2a),
except for the Condobolin and Darlington Pt. soils, where
they were signi®cantly less only at the maximal sludge
level. In the Wagga Wagga soil, the d AN increased …P ,
0:05† with increasing amounts of sludge. The signi®cant
decreases in d AN were associated with concentrations of

1
NO2
3 rather than NH4 (Fig. 1a). For example, the difference
1
in the NH4 concentration between (luc 1 ) and (luc 2 ) in
the Condobolin control soil was 219 mg N kg 21 soil as
compared with 319 mg N kg 21 soil at 240 t DWS ha 21,
and the respective differences in NO2
3 concentration were
267 and 118 mg N kg 21 soil.
In the Goulburn soil amended with sludge from Malabar,
Port Kembla, Quakers Hill or Richmond, the d AN was
similar to, or greater than, the d AN in the control soil

2040

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

(Fig. 2b), but it was reduced with the sludge from St. Marys.
The decrease was the result of signi®cantly lower concentrations of NO2
3 with higher amounts of sludge (Fig. 1b).
The differences in concentrations of NH1
4 between (luc 1 )
and (luc 2 ) increased signi®cantly from 113 to 143 to
227 mg N kg 21 soil between 0, 60 and 240 t DWS ha 21,
whereas the respective concentrations in NO2
3 decreased
from 341 to 230 to 105 mg N kg 21 soil.
The variation in the d AN over all treatments was not
correlated with the concentrations of total heavy metals.
On average, the net increase in AN with lucerne addition,
about 12 mg N, was 73% of the total amount of N added as
lucerne N, i.e. 16.4 mg N.

sludge could not be ascribed to its total heavy metal concentration. The metal concentration in this treatment, even at
240 t DWS ha 21, was less than with other sludges: for
example, the Goulburn soil amended with Malabar sludge
at 240 t DWS ha 21 contained four times the concentration of
total heavy metals as the same soil treated with the same
amount of sludge from St. Marys. Similarly, in the other
cases in which the d AN was signi®cantly less than in the
control soil, i.e. Condobolin and Darlington Pt. soils with
the highest amount of sludge, the heavy metal concentration
was not as high as in other treatments where there were no
differences in the d AN between sludge-amended and
control soils.

4. Discussion

4.3. Soil factors affecting available N

4.1. Available N in soils without lucerne
Amending soil with sewage sludge has been shown to
increase the concentration of mineral N in soil (Hobson et
al., 1974; Boyle and Paul, 1989; Douglass and Magdoff,
1991; Lerch et al., 1992; Serna and Pomares, 1992).
However, these studies rarely involved sludges that were
high in heavy metals. When the concentrations of heavy
metals in soil are high, the metals may reduce the microbial
biomass (Brookes and McGrath, 1984) that is intimately
involved with N mineralization. In the present study,
which used soils `aged' with metal-contaminated sludges
that increased the concentrations of heavy metals in the
soils to 1024 mg kg 21 soil, greater amounts of available N
were found than in the control soils. In addition, as the
increases in available N in luc 2 treatments involved
increased amounts of NO2
3 ; net nitri®cation was not inhibited by the highest concentrations of heavy metals.
The small percentage (,10%) of total N recovered from
the soil±sludge mixtures as AN was not unexpected; for
example, Sommers et al. (1981) found less AN in sludgeamended ®eld soils after the ®rst few years following treatment. Much of the labile sludge N in our treated soils was
presumably mineralized and removed in the harvested plant
material over the six biocycles, leaving the more recalcitrant
N in the `aged' soil±sludge mixtures used in the incubation
experiments.
4.2. Available N from incorporated lucerne
In the majority of treatments, the increase in AN (d AN)
resulting from the addition of lucerne to soil occurred with
similar ef®ciency in sludge-amended soil as in control soil;
i.e. the variation in the concentrations of the heavy metals
had no apparent effect. For example, the d AN was similar
for the Goulburn soil amended with Malabar, Richmond or
Port Kembla sludge as with Quakers Hill sludge, despite the
large differences in the heavy metal concentrations in soil
resulting from these treatments. In addition, the reduction in
the d AN in the Goulburn soil amended with St. Mary's

AN was correlated with total soil N, similar to the results
of Barbarika et al. (1985) and Douglass and Magdoff (1991).
However, although the variation in total soil N between soils
and levels of added sludge had a predominant in¯uence on
AN, other soil properties inherent to the soil or resulting
from the addition of sludge affected the production of AN,
either positively (associated with an increase in exchangeable cations) or negatively (associated with an increase in
pH or organic C).
The reason for the positive effect of an increase in
exchangeable cations on AN was not resolved in this
study. The apparent negative effect of higher total C on
AN seems unlikely to be the result of immobilization of
mineral N because the C:N ratios of the soil±sludge
mixtures were low, in the range of 7±10. The effect may
have been the result of enhanced denitri®cation as the result
of more available C to support denitrifying microorganisms
in the sludge-amended soils (Lindemann and Cardenas,
1984). Alternatively, as total soil C was correlated with
heavy metal concentration …r 2 ˆ 0:72†; the effect may
have involved an adverse in¯uence of heavy metals on N
mineralization. However, Douglass and Magdoff (1991)
also reported a negative effect of total soil C on N mineralized from a range of farm manures, presumably with a low
content of heavy metals. The apparent negative effect on
AN of higher soil pH is unlikely to involve those heavy
metals in sludge thought to affect microbial biomass and
mineralization of organic matter, namely, Cu, Zn, and Ni,
because these metals are less available at higher pH
(Helmke and Naidu, 1996).
Regardless of the negative in¯uences on AN of soil C
(or metals) and pH, these soil properties were not suf®cient
to prevent increases in AN in response to greater additions
of sludge N. Thus, the net effect of changes in soil properties
favorable and unfavorable to the production of AN can be
estimated from Eq. (1). For the maximal rate of sludge
additions, the magnitude of the positive and negative effects
are shown in Fig. 3, which illustrates that the combined
positive effects of increased soil N and exchangeable cations

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

2041

Fig. 3. Comparative effects on the production of available N (AN) of the factors that had a positive (total soil N, and exchangeable cations) and negative (total
soil C and pH) in¯uence on AN. Data are for soils amended with dewatered sewage sludge (DWS) applied at 240 t ha 21.

on AN outweigh the combined negative effects of higher pH
and organic C.

4.4. Composition of AN
The composition of AN varied between the 2luc and
1luc treatments, and with the amount of sludge added to
soil. In particular, in some of the 1luc treatments with the
highest sludge level, the concentration of NO2
3 was
decreased as compared with the control. This decrease
was usually associated with a correspondingly higher
concentration of NH1
4 in the sludge-amended soils suggesting that the rate of nitri®cation in these treatments may be
slower than in control soils. As these responses were not
observed in the 2luc treatments, it is unlikely that they
resulted from sludge contaminants. The EC of the soils
with the highest amounts of sludge were suf®cient to
decrease the rate of, but not stop, nitri®cation, based on
the studies of McCormick and Wolf (1980). However, the
variations in EC were inconsistent with the variations in the
1
relative concentrations of NO2
3 and NH4 in both 1luc and
in 2luc treatments, so that EC was not an adequate explanation for the reduction in concentrations of NO2
3 referred
to above. In addition, in the two soil treatments in which the
d AN decreased, the lower concentration of NO2
3 at the high
sludge level could not be explained by a correspondingly
greater concentration of NH1
4 : This was most apparent in
the Goulburn soil amended with St. Marys' sludge, where
the reduction in the concentration of NO2
3 in the sludgeamended soil compared to control soil exceeded the
increase in concentration of NH1
4 between the control and

sludge-amended soil by 82 and 122 mg N kg 21 soil for
sludge levels of 60 and 240 t DWS ha 21, respectively.
The above responses may have been be caused by low
concentrations of O2 in the soils treated with both high
amounts of sludge and lucerne, resulting from greater
respiration. This would provide a consistent explanation
of both the apparent decrease in nitri®cation, an oxidative
reaction, and the apparent net loss of mineral N described
above, via denitri®cation, an anaerobic reaction. St. Marys'
sludge was the only anaerobically digested sludge. Subsequently, the concentrations of respirable carbon compounds
in soil amended with this sludge were perhaps greater, and
soil O2 concentrations lower, than in soils treated with the
aerobically digested sludges.
4.5. Metal concentrations
A primary aim of the current study was to determine
whether the NSWEPA guidelines for maximal concentrations of heavy metals in soil were appropriate to ensuring
available N bene®ts from the incorporation of legume N
with sludges of high metal content. In Table 7, the EPA
maximal allowable concentrations of heavy metals in soil
used for food production are compared to the maximal
concentrations of heavy metals measured in this study. As
the latter concentrations failed to prevent increases in AN in
response to the addition of sludge or lucerne to soil, the
NSWEPA heavy metal limits are well below metal concentrations that may severely reduce the mineralization of
organic N.
The heavy metals in the metal-contaminated soil at
Woburn (Table 7) caused a substantial loss of soil microbial

2042

K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043

Table 7
Comparative concentrations of heavy metals in soils (mg kg 21 soil)

Zn
Cu
Ni
Cr
Pb
Cd

NSW EPA a

This study b

Woburn c

Berkeley d

Luddington e

Lee Valley e

200
100
60
100
150
3

222±481
105±249
8±86
20±187
25±86
0.8±5.2

635
239
42
258
209
19

804
123
37
302
224
7.6

277
369
53
na f
na
0.9

857
66
95
na
na
5.9

a

Maximal allowable concentrations in soil used for food production.
Range at maximal sludge amounts.
c
Brookes and McGrath (1984).
d
Estimated from data in Boyle and Paul (1989) assuming 80% of the applied heavy metals were retained in surface (0±10 cm) soil of bulk density
1.4 g cm 23.
e
Chander and Brookes (1991b).
f
na ˆ not available.
b

biomass (Brookes and McGrath, 1984), probably resulting
from increased expenditure of C for the maintenance
requirements of microbial cells (Dhalin and Witter, 1998).
Brookes and McGrath (1984) suggested that Cu and Ni were
the most likely heavy metal elements responsible for this
reduction. As the maximal Cu and Ni concentrations in our
study were similar to (Cu) or exceeded (Ni) those at
Woburn, it is possible that the microbial biomass in the
soils that we amended with maximal amounts of sludge
was stressed by metal pollution. If so, this did not prevent
the production of mineral N, implying that N mineralization
is only partially dependent on the size of the microbial
biomass. Moreover, because the maximal Cu and Ni
concentrations in our study were 2.5 and 1.4 times the
NSWEPA maxima, respectively, there would appear to be
a substantial margin of `safety' inherent in the EPA limits
on Cu and Ni with respect to N mineralization. Of the other
elements at their maximal concentrations, i.e. Zn, Cr, Pb and
Cd, all except Pb exceeded the EPA limits but were less
concentrated than at Woburn: for example, in the Robertson
soil, the concentrations of these elements were, respectively,
73, 72, 41, and 13% of those at Woburn.
De®nitive concentrations of heavy metals affecting
mineralization of organic matter were reported by Chander
and Brookes (1991b) who showed that Cu and Zn concentrations of about 370 mg kg 21 soil and 860 mg kg 21 soil,
respectively, resulted in accumulation of organic matter,
whereas concentrations of 212 (Cu) mg kg 21 soil and 465
(Zn) mg kg 21 soil did not. Our results with different soils
and sludges support the latter observation and together with
the data in Chander and Brookes (1991b), suggest threshold
concentrations for Cu affecting N mineralization between
250 and 370 mg kg 21 soil, at least 2.5 times the NSWEPA
limit. A threshold concentration for Zn is suggested between
480 and 860 mg kg 21 soil. However, in a study by Boyle
and Paul (1989), more mineral N was produced from soil of
the heavy metal composition shown in Table 7 than from
similar soil with 1/4 the metal concentration. Therefore, the
metal concentrations in the Berkeley soil (Table 7) were not

critical, so that the threshold concentration for Zn may be
closer to 800 mg kg 21 soil, at least 4 times the NSWEPA
limit.
Acknowledgements
This research was carried out for the Organic Waste
Recycling Unit, NSW Agriculture, Richmond, New South
Wales, Australia, through funds supplied by Sydney Water.
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