Mineralization of soil and legume nitrogen in soils treated with
metal-contaminated sewage sludge
K.J. Munn
a, J. Evans
b,*, P.M. Chalk
a,caDepartment of Agriculture, University of Melbourne, Parkville, Vic 3052, Australia bNSW Agriculture, Agricultural Institute, Private Mail Bag, Wagga Wagga, 2650 NSW, Australia
cPresent 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 kg21were ground
and mixed with different soils at rates up to 240 t ha21. 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 kg21soil, with individual
metal concentrations (mg kg21soil) 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 (nitrate1ammonium) 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.q2000 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.
However, urban sewage sludges are often contami-nated with industrial organic chemicals and heavy metals. These metals tend to accumulate in soil, where they may then cause changes to microbial popu-lations 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 matterq2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 1 0 5 - X
www.elsevier.com/locate/soilbio
* Corresponding author. Tel.:161-2693-81999; fax:161-2693-81809.
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 accumu-lated 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 func-tioning 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 mate-rial. 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, histori-cally 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 minera-lization 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 concen-trated nitric acid:1 part concenconcen-trated 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 1 M KCl, and the concentrations of NH14 and
NO231NO22 (subsequently referred to as NO23) were
determined with an autoanalyzer (Bran and Luebbe TRAACS 800). Particle-size analysis was determined by the method of McIntyre and Loveday (1974).
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 pHa Nb(g kg21) Cc(g kg21) Pd(mg kg21) Particle sizee(g kg21) Exchangeable cationsf(cmol kg21)
Clay Silt Fine sand Coarse sand Al Mg Ca K Na
Rutherglen 4.2 1.2 14 28 112 155 613 122 0.41 0.29 1.20 0.52 ,0.01
Goulburn 4.4 2.0 20 16 100 160 522 190 0.30 1.00 2.47 0.45 ,0.01
Robertson 4.7 3.7 32 5 540 190 171 30 0.79 1.82 3.78 0.53 0.19
Condobolin 5.3 1.5 15 22 320 120 475 83 ,0.01 2.75 7.90 3.08 0.03
c Total carbon determined by combustion using a LECO FP-2000 Carbon Analyzer.
dBray P.
e McIntyre and Loveday (1974).
fExchangeable cations in a 0.01 M BaC1
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 m21. 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.
K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043
Table 2
Sewage sludge composition
Sludge source Na(g kg21) Cb(g kg21) Pc(mg kg21) Heavy metal concentrationd(mg kg21)
Zn Cu Cd Ni Pb Hg Cr Total
Malabar 23.4 284 300 2669 1274 11 162 303 4 235 4658
Port Kembla 25.2 239 89 1767 648 32 32 97 ,2 66 2612
Quakers Hill 38.0 236 78 512 468 2 28 81 ,2 488 1579
St Marys 26.7 220 52 498 447 2 25 111 ,2 66 1149
Richmond 11.0 97 290 704 439 3 20 90 27 54 1137
a Kjeldahl N.
b Total carbon determined by combustion using a LECO FP-2000 Carbon Analyser.
c Bray P.
d Total elements determined by microwave acid digestion and inductively coupled-plasma-atomic emission-spectrometry (Vimpany et al., 1987).
Table 3
Chemical characteristics of soils after amendment with Malabar dewatered sewage sludge (DWS)
Soil Sludge level
Rutherglen 0 4.2 1.2 11.7 28 2.6 48
60 5.3 2.4 19.3 198 11.4 202
Condobolin 0 5.3 1.5 13.6 22 11.2 89
60 5.6 2.2 21.3 131 16.2 211
240 6.3 6.8 51.4 353 23.1 759
Wagga Wagga 0 5.6 0.9 7.2 55 3.4 45
60 5.7 1.8 12.2 194 5.8 154
240 5.9 2.9 26.3 nd 12.7 461
Darlington Pt. 0 6.4 1.5 10.8 47 25.5 125
60 6.2 2.3 19.7 194 29.6 280
240 5.9 4.4 41.3 289 37.1 641
a Kjeldahl N.
b Total carbon determined by combustion using a LECO FP-2000 Carbon Analyser.
c Bray P.
d Exchangeable cations in a 0.01 M BaC1
2leachate (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-spectro-metry.
2.3. Levels of sewage sludge
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 ha21 (0±10 cm), but not all levels were used with all sludges. Control soils received no sludge. The chemical characteris-tics 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.
2.4. Reaction of soils and sludges (`aging')
Soil-sludge mixtures were added to pots (2 kg pot21), 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 dispen-sing 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
until the EC of the soils was below 1.0 dS m21 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 m21 in soils treated with Malabar sludge and from 0.2 to 0.84 dS m21 in the sludge-amended Goul-burn soil.
2.5. N mineralization
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 g21 soil. Twenty-®ve grams of lucerne-treated (luc1) or untreated (luc2) soil was placed into plastic tubes and brought to the ®eld capacity water content, deter-mined 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. The concentrations NH14 and NO23 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 concen-tration (AN; mgN kg21oven 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 luc2 and luc1, and soil treatments were completely randomized within soils. The effect of soil type and sludge treatment on
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
Malabar 60 5.4 2.9 24.1 162 11.0 218
240 6.1 5.9 62.8 381 20.0 961
bTotal carbon determined by combustion using a LECO FP-2000 Carbon Analyser.
c Bray P.
dExchangeable cations in a 0.01 M BaC1
2leachate (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.
the available N from lucerne was determined as the difference in AN between luc1 and luc2, denoteddAN. AN anddAN
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 ha21, occurred together with increases in C r20:83;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 high-est 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.
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 atP,0:05. Lucerne effect was
signi®cant in all treatments
Site Sludge level (t DWS ha21) Soil N (%) Available N (mg kg21)
Minus lucerne Plus lucerne Mean
Rutherglen 0 0.15 44 494 269 b
60 0.19 63 486 274 b
Goulburn 0 0.19 57 507 282 b
60 0.24 53 549 301 b
Robertson 0 0.31 48 532 290 c
60 0.37 105 582 344 b
Condobolin 0 0.18 44 b 531 b 287
60 0.23 59 b 553 b 306
a sedstandard error of differences between means P
,0:05. b lsdleast signi®cant difference P
The increased concentrations of total heavy metals were correlated with increases in organic C r2
0:83: Treatment with the maximal amount of Malabar
sludge produced the highest heavy metal concentra-tion, which was 1026 mg kg21 soil in the Robertson soil and 961 mg kg21 soil in the Goulburn soil (Table 3). Sludge from Port Kembla, which had a compara-tively high heavy metal concentration, increased the concentrations of total heavy metals in the Goulburn soil to 814 mg kg21 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 kg21 lower and exchangeable cations were 1±6 cmol kg21 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 trea-ted with 60 t DWS ha21, 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 (luc2) lucerne
In the six soils amended with Malabar sludge and in the
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 atP,0:05. Lucerne
effect was signi®cant in all treatments
Sludge source Sludge level (t DWS ha21) Available N (mg kg21)
Minus lucerne Plus lucerne Mean
Malabar 0 57 507 282 b effect: sed 10.2; lsd 22.8
253 747
St Marys 0 57 b 507 a 282
60 47 b 421 b 234
240 198 a 531 a 365
Mean sludge
level£lucerne: sed 14.1; lsd 31.4
123 476
Richmond 0 57 507 282 b
60 50 454 253 b
240 123 546 335 a
Mean sludge level main effect: sed 18.9; lsd 42.1
86 501
Quakers Hill 0 57 b 507 b 282
60 ndc nd nad
240 295 a 849 a 572
Mean sludge
level£lucerne: sed 17.0; lsd 41.6
176 678
a sedstandard error of differences between means P,0 :05: blsdleast signi®cant difference P,0
:05: c nd Ð not determined.
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 concentra-tion. The differences resulting from levels of sludge were signi®cant P,0:05 when the level was 240 t
DWS ha21 or 60 t DWS ha21 in two of the soils (Wagga Wagga and Robertson).
For the combined soil and sludge treatments in luc2,
AN and total soil N were linearly correlated r20:68;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 r20:85;P,0:05 of the variation in AN than N
alone. Accordingly,
AN luc2 884N15:5EX268:5pH245:4C1258 1
The increases in AN in response to soil treatment with
K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043
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,0:05where the
sludge involved signi®cant P,0:05 increases in the
concentrations of both NO23 and NH14 (Fig. 1a and b;
luc2). For most of the soil and sludge treatments, the concentration of NO23 relative to NH14 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 ha21, and the Goulburn soil treated with Richmond sludge at 240 t DWS ha21.
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.
3.4. Available N in soils with added (luc1) lucerne
AN values for the luc1 treatment in the six soils amended
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 NH14, except
in the Darlington Pt. soil (Fig. 1a and b; luc1). In soils treated with 240 t DWS ha21, NO23 concentrations were signi®cantly
lower than in control soils for: Rutherglen, Goulburn, Robert-son and Darlington Pt. soils amended with Malabar sludge and the Goulburn soil amended with St. Marys' sludge. The varia-tion 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.
AN luc1 1080N110EX2124:8pH255:9C1904
r20:84; P,0:05:
K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043
3.5. Net mineralization of lucerne N
dAN 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, thedAN increased P,
0:05 with increasing amounts of sludge. The signi®cant
decreases in dAN were associated with concentrations of
NO23 rather than NH14 (Fig. 1a). For example, the difference
in the NH14 concentration between (luc1) and (luc2) in
the Condobolin control soil was 219 mg N kg21 soil as compared with 319 mg N kg21 soil at 240 t DWS ha21, and the respective differences in NO23 concentration were 267 and 118 mg N kg21soil.
In the Goulburn soil amended with sludge from Malabar, Port Kembla, Quakers Hill or Richmond, the dAN was
similar to, or greater than, the dAN in the control soil
K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043
Fig. 2. Differences (log10transform) in the available N concentrations in soils with and without lucerne: 2a, effects of amending soils with dewatered sewage
(Fig. 2b), but it was reduced with the sludge from St. Marys. The decrease was the result of signi®cantly lower concen-trations of NO23 with higher amounts of sludge (Fig. 1b).
The differences in concentrations of NH14 between (luc1)
and (luc2) increased signi®cantly from 113 to 143 to 227 mg N kg21
soil between 0, 60 and 240 t DWS ha21
, whereas the respective concentrations in NO23 decreased from 341 to 230 to 105 mg N kg21soil.
The variation in the dAN 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.
4. Discussion
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 kg21soil, greater amounts of available N were found than in the control soils. In addition, as the increases in available N in luc2 treatments involved increased amounts of NO23;net nitri®cation was not
inhib-ited 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 sludge-amended ®eld soils after the ®rst few years following treat-ment. 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 (dAN)
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, thedAN 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 dAN in the Goulburn soil amended with St. Mary's
sludge could not be ascribed to its total heavy metal concen-tration. The metal concentration in this treatment, even at 240 t DWS ha21, was less than with other sludges: for example, the Goulburn soil amended with Malabar sludge at 240 t DWS ha21contained 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 dAN 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 dAN between sludge-amended and
control soils.
4.3. Soil factors affecting available N
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 exchange-able 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 r20:72; 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 miner-alized 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
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 the1luc treatments with the highest sludge level, the concentration of NO23 was
decreased as compared with the control. This decrease was usually associated with a correspondingly higher concentration of NH14 in the sludge-amended soils
suggest-ing 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 relative concentrations of NO23 and NH14 in both1luc and in2luc treatments, so that EC was not an adequate expla-nation for the reduction in concentrations of NO23 referred
to above. In addition, in the two soil treatments in which the
dAN decreased, the lower concentration of NO23 at the high
sludge level could not be explained by a correspondingly greater concentration of NH14:This was most apparent in
the Goulburn soil amended with St. Marys' sludge, where the reduction in the concentration of NO23 in the
sludge-amended soil compared to control soil exceeded the increase in concentration of NH14 between the control and
sludge-amended soil by 82 and 122 mg N kg21 soil for sludge levels of 60 and 240 t DWS ha21, 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. Subse-quently, the concentrations of respirable carbon compounds in soil amended with this sludge were perhaps greater, and soil O2concentrations 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 concentra-tions 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 concen-trations 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
K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043
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 concen-trations of about 370 mg kg21 soil and 860 mg kg21 soil, respectively, resulted in accumulation of organic matter, whereas concentrations of 212 (Cu) mg kg21soil and 465 (Zn) mg kg21soil 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 kg21soil, at least 2.5 times the NSWEPA limit. A threshold concentration for Zn is suggested between 480 and 860 mg kg21soil. 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 kg21 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.
References
Babich, H., Stotzky, G., 1985. Heavy metal toxicity to microbe-mediated ecological processes: a review and potential application to regulatory policies. Environ. Res. 36, 111±137.
Barbarika, A., Sikora, L.J., Colacicco, D., 1985. Factors affecting the mineralisation of nitrogen in sewage sludge applied to soils. Soil Sci. Soc. Am. J. 49, 1403±1406.
Boyle, M., Paul, E.A., 1989. Carbon and nitrogen mineralization kinetics in soil previously amended with sewage sludge. Soil Sci. Soc. Am. J. 53, 99±103.
Bray, R.H., Kurtz, L.T., 1945. Determination of total, organic, and avail-able forms of phosphorus in soils. Soil Sci. 59, 39±45.
Brookes, P.C., McGrath, S.P., 1984. Effects of metal toxicity on the size of the soil microbial biomass. J. Soil Sci. 35, 341±346.
Chander, K., Brookes, P.C., 1991a. Microbial biomass dynamics during decomposition of glucose and maize in metal-contaminated and non-contaminated soils. Soil Biol. Biochem. 23, 917±925.
Chander, K., Brookes, P.C., 1991b. Effects of heavy metals from past applications of sewage sludge on microbial biomass and organic matter accumulation in a sandy loam and silty loam UK soil. Soil Biol. Biochem. 23, 927±932.
Chang, F.H., Broadbent, F.E., 1982. In¯uence of trace metals on some soil nitrogen transformations. J. Environ. Qual. 11, 1±4.
Dhalin, S., Witter, E., 1998. Can the low microbial biomass C-to-organic C ratio in an acid and a metal contaminated soil be explained by differ-ences in the substrate utilization ef®ciency and maintenance require-ments. Soil Biol. Biochem. 30, 633±641.
Douglass, B.F., Magdoff, F.R., 1991. An evaluation of nitrogen mineraliza-tion indices for organic residues. J. Environ. Qual. 20, 368±372. Helmke, P.A., Naidu, R., 1996. Fate of contaminants in the soil
environ-ment. In: Naidu, R., Kookana, R.S., Oliver, D.P., Rogers, S., McLaughlin, M.J. (Eds.). Contaminants and the Soil Environment
K.J. Munn et al. / Soil Biology & Biochemistry 32 (2000) 2031±2043
Table 7
Comparative concentrations of heavy metals in soils (mg kg21soil)
NSW EPAa This studyb Woburnc Berkeleyd Luddingtone Lee Valleye
Zn 200 222±481 635 804 277 857
Cu 100 105±249 239 123 369 66
Ni 60 8±86 42 37 53 95
Cr 100 20±187 258 302 naf na
Pb 150 25±86 209 224 na na
Cd 3 0.8±5.2 19 7.6 0.9 5.9
a Maximal allowable concentrations in soil used for food production.
bRange at maximal sludge amounts.
c Brookes and McGrath (1984).
dEstimated 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 cm23.
e Chander and Brookes (1991b).
in the Australasia±Paci®c Region, Kluwer Academic, Dordrecht, pp. 69±93.
Hobson, P.N., Bous®eld, S., Summers, R., 1974. Anaerobic digestion of organic matter. CRC Critical Review Environmental Control, vol. 4, pp. 131±191 (cited in Lerch, R.N., Barbarich, K.A., Sommers, L.E., West-fall, D.G., 1992. Soil Sci. Soc. Am. J. 56, 1470±1476).
Lerch, R.N., Barbarich, K.A., Sommers, L.E., Westfall, D.G., 1992. Sewage sludge proteins as labile carbon and nitrogen sources. Soil Sci. Soc. Am. J. 56, 1470±1476.
Lindemann, W.C., Cardenas, M., 1984. Nitrogen mineralization potential and nitrogen transformations of sludge-amended soil. Soil Sci. Soc. Am. J. 48, 1072±1077.
Martensson, A.M., Witter, E., 1990. In¯uence of various soil amendments on nitrogen-®xing soil microorganisms in a long-term ®eld experiment, with special reference to sewage sludge. Soil Biol. Biochem. 22, 977±982. McCormick, R.W., Wolf, D.C., 1980. Effect of sodium chloride on CO2
evolution, ammoni®cation, and nitri®cation in a sassafras sandy loam. Soil Biol. Biochem. 12, 153±157.
McGrath, S.P., Brookes, P.C., Giller, K.E., 1988. Effects of potentially toxic metals in soil derived from past applications of sewage sludge on nitrogen ®xation byTrifolium repensL. Soil Biol. Biochem. 20, 415±424.
McIntyre, P., Loveday, J., 1974. Methods of Analysis of Irrigated Soils, Wilke, Victoria.
Munn, K.J., Evans, J., Chalk, P.M., Morris, S.G., Whatmuff, M., 1997. Symbiotic effectiveness of Rhizobium trifolii and mineralisation of legume nitrogen in response to past amendment of a soil with sewage sludge. J. Sustain. Agric. 11, 23±37.
Raymont, G.E., Higginson, F.R., 1992. Australian Laboratory Handbook of Soil and Water Chemical Methods, Inkarta Press, Melbourne (p. 9). Ross, A.D., Lawrie, R.A., Whatmuff, M.S., Keneally, J.P., Awad, A.S.,
1991. Guidelines for the use of sewage sludge on agricultural land, 3rd ed. New South Wales Agriculture, Sydney, Australia.
Serna, M.D., Pomares, F., 1992. Nitrogen mineralisation of sludge-amended soil. Bioresou. Technol. 39, 285±290.
Sommers, L.E., Parker, C.F., Myers, G.J., 1981. Volatilization, plant uptake and mineralisation of nitrogen in soils treated with sewage sludge. Tech. Rep. No. 133, Purdue University Water Research Centre, West Lafayette, IN.
Stace, H.C.T., Hubble, G.D., Brewer, R., Northcote, K.H., Sleeman, J.R., Mulcahy, M.J., Hallsworth, E.G., 1968. A Handbook of Australian Soils. Rellim. Tech. Pubs., Glenside, South Australia.
Tyler, G., 1975. Heavy metal pollution and mineralization of nitrogen in forest soils. Nature (Lond.) 255, 701±702.
Vimpany, I.A., Holford, I.C.R., Milham, P.J., Abbot, T.S., 1987. Soil test-ing service-methods and interpretation. Biological and Chemical Research Institute, Rydalmere, New South Wales Agriculture.
