Extractable soil heavy metals following the cessation of biosolids application to agricultural soil

Extractable soil heavy metals following the cessation of biosolids application to agricultural soil

Environmental Pollution 117 (2002) 315–321 www.elsevier.com/locate/envpol Extractable soil heavy metals following the cessation of biosolids applicat...

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Environmental Pollution 117 (2002) 315–321 www.elsevier.com/locate/envpol

Extractable soil heavy metals following the cessation of biosolids application to agricultural soil Ingrid Walter*, Fernando Martı´nez, Luis Alonso, Jose´ de Gracia, Gabriela Cuevas Sustainable Use of Natural Resources Department, Instituto Nacional de Investigacio´n y Tecnologı´a Agraria y Alimentaria (INIA), Apdo. de correos 8111, 28080 Madrid, Spain Received 23 February 2001; accepted 1 June 2001

‘‘Capsule’’: Different rates of decomposition of anaerobically digested biosolids from wastewater treatment plants appeared to explain differences in heavy metal extractability from treated soil over time. Abstract Changes in soil heavy metal extractability following the cessation of biosolids applications were studied in a long-term field experiment. Two anaerobically digested biosolids from wastewater treatment plants in Madrid (Sur and Viveros) were applied to cropland from 1983 to 1990. Soil samples were collected in the 1st, 5th and 9th year after the last biosolids application. Soil pH did not vary significantly after biosolids applications. Organic matter and total heavy metals (Zn, Pb, Cd, Ni, Cr and Cu) concentrations initially increased but then declined over time, mostly after the first 5 years following biosolids application. Metal extracted with DPTA increased in Sur treatments during the 1st year and diminished thereafter. However, in Viveros treatments, heavy metals extracted increased during the 1st year, declined in 1995, and showed a slight increase in 1999. These changes in heavy metal extractability were widely observed in the percentage of extractable metal recovery (EMR). The differences observed in the pattern of the two sources of biosolids applied could be due to the different rates of decomposition of their organic matter. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Long-term experiment; Biosolids; Organic matter; Total heavy metals; Extractable heavy metals

1. Introduction Repeated applications of heavy metal contaminated biosolids can significantly increase toxic metals concentrations in agricultural soils. Metal transfer from biosolids to soil and subsequently to plants pose potential health risks since they can enter the food chain and the environment. There is concern about the long-term effects of these metals at high concentrations in the environment as they can persist in soil for a long time (McGrath, 1987). Biosolids-applied metals remained in the zone of biosolids incorporation (e.g 0–15 cm depth) as the result of their adsorption on hydrous oxides, clays, and organic matter, the formation of insoluble salts, or the presence of residual biosolids particles (Alloway and Jackson, 1991). Mineralisation of biosolids organic matter may release metals into available * Corresponding author. Tel.: +34-9134-76738; fax: +34-913572293. E-mail address: [email protected] (I. Walter).

forms even in the long term following cessation of application (McBride, 1995). The highest value in availability of biosolids-borne metals to plants is in the period immediately following application of biosolids; but with time, organic decomposition rates diminish, and availability is reduced (Bidwell and Dowdy, 1987). This decrease in metal availability is more evident during the 1st years after the cessation of biosolids application. It is, however, dubious whether this pattern remains the same over a longer period following the cessation biosolids application (McBride, 1995). Heavy metals accumulation in soil can be monitored through HNO3–HCl soil extractions (total heavy metals). Evaluation of total metal levels may be useful as a global index of contamination, but it provides little indication of their specific bioavailability, mobility, and reactivity in biosolid-amended soil. Another monitoring approach consists of the use of various chemical reagents to estimate the fraction of the soil metal that is potentially available to plants. The most frequently employed reagents are chelating agents such as

0269-7491/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(01)00181-6


I. Walter et al. / Environmental Pollution 117 (2002) 315–321

diethylenetriaminepentaacetic acid (DTPA; Lindsay and Norvell, 1978). Metals extracted by DTPA largely exist in exchangeable, organically complexed and carbonate forms. Such extraction provides more information about metal availability and tends to correlate with metal uptake by plants (Bidwell and Dowdy 1987; Sommers et al., 1991; Hooda and Alloway, 1994). However, it has been observed that DTPA-extractable metals from soil decrease with time, especially after biosolids applications have ceased (McGrath, 1984). This result suggests that metals revert to less available forms in soil with time. The absolute quantity of metals extracted by DTPA is a useful means of evaluating plant—available metals in the soil in short-term experiments (Morel and Guckert, 1984). For long-term field experiments, the ratio of the extractable metal-to-metal loading is perhaps a better means for expressing extractable metals in the soil. This method of calculating the relative metal availability (extractable metal recovery) may be a more efficient way to predict metal behaviour with time. The objective of this study was to determine the DTPA-extractable Zn, Pb, Cd, Ni, Cr, and Cu concentrations and the percentage of extractable metal recovery in a soil treated with high rates of biosolids over a period of 8 years. To determine the changes produced in metal extractability over time, soil samples were collected on the 1st, 5th, and 9th years following the cessation of biosolids application.

2. Materials and methods In 1983, a field study was conducted in an agronomy farm (research farm from INIA) in the Central Region of Spain (45 km north-east of Madrid). For 8 years, anaerobically-digested biosolids from two different treatment plants, both containing large amount of heavy metals, were surface-applied and immediately incorporated into the top 15 cm of soil (Typic Haploxeralf Caliorthid) with a moldboard plow. The annual

treatments were: untreated plots (C), mineral fertiliser, similar in NPK to the low rate of biosolids (M), biosolids from the Sur treatment plant at 50 and 100 Mg ha1 year1 rates (S1 and S2, respectively) and biosolids from the Vivero treatment plant at 50 and 100 Mg ha1 year1 rates (V1 and V2, respectively). Treatments were replicated three times in a randomised complete design. Walter et al. (1994) previously described the complete study design. Cumulative biosolids and heavy metal loading are shown in Table 1. After biosolids applications ceased, the plots were maintained in agricultural production. Winter wheat (Triticum aestivum L. ‘‘Ansa’’) was grown in each plot during and after application of biosolids. Composite soil samples were collected in 1991, 1995 and 1999. Five cores per plot were collected after wheat harvest and from the surface (0–20 cm depth) in the middle of each 0.10 ha plot, to minimise the boundary effects of neighbouring treatments. The samples were air-dried, ground, and passed through a 2-mm mesh sieve. Some physicochemical characteristics of the untreated and treated soil are given in Table 2. Soil pH was determined with H2O (soil/water ratio w:v, 1:2.5), organic matter (OM), electric conductivity (EC), carbonates (CaCO3), and texture were determined by standard methods (MAPA, 1994). The textural analyses indicated that the texture was quite uniform (clay loam) in each experimental plot. The total contents of heavy metal in the biosolids applied and in the soil samples were determined by plasma-atomic emission spectroscopy (ICP–AES) after acid digestion of soil with HNO3–HCl (McGrath and Cunliffe, 1985). The extractable metals were extracted with the method of Lindsay and Norvell (1978) (DTPA, w:v 1:2 soil/extract solution and 2 h on a reciprocal shaker). The concentrations of Zn, Pb, Cd, Ni, Cr and Cu in the extracts were analysed by ICP–AES. The recovery percentage of biosolids-applied metals (EMR), extracted from the different treatments was calculated by subtracting the metals extracted from the untreated plots (control) from each treatments extracted metal contents and dividing by the cumulative metal loading.

Table 1 Cumulative loading of biosolids and heavy metals on the experimental plots Treatments

mg ha1 (d.m)

Years applied







500 1000

kg ha1 Sur S1: Low S2: High Viveros V1: Low V2: High Limita a

400 800

1983–1990 1983–1990

750 1500

350 700

25 50

25 50

100 200

400 800

1983–1990 1983–1990

550 1100 30.0

300 600 15.0

15 30 0.15

25 50 3.00

200 400 3.00

Heavy metal loading rate limit established by Spanish guideline (RD 1310/1990, MAPA), kg ha1 year1.

200 400 12.0


I. Walter et al. / Environmental Pollution 117 (2002) 315–321 Table 2 Soil characterisation values for surface samples at the three sampling times (1, 5 and 9 years after biosolids application) Soil parameter


Treatmentsa C







1991 1995 1999

8.30.14 8.30.15 8.80.20

8.2 0.11 8.4 0.10 8.7 0.12

7.60.15 7.90.21 8.30.30

7.40.20 7.80.11 8.30.20

7.6 0.12 7.9 0.06 7.8 0.12

7.30.15 7.60.10 7.90.20

CaCarbonate (g kg1)

1991 1995 1999

12216.0 12518.0 12020.0

128 20.0 130 18.0 135 22.0

10615.0 10411.0 1108.04

12412.0 13215.0 12110.0

99 18.5 105 23.0 97 14

13618.2 1338.45 14616.0

Electrical conductivity (S m1)

1991 1995 1999

1.80.8 2.00.6 1.90.6

2.3 0.2 2.1 0.1 2.0 0.5

3.20.6 4.01.0 2.60.8

5.21.2 4.80.5 2.30.8

3.5 0.5 3.0 0.3 2.8 0.2

6.50.8 5.21.0 2.80.6

Organic matter (g kg1)

1991 1995 1999

10.72.0 10.32.1 8.11.8

11.1 0.8 13.0 0.6 11.8 1.2

32.52.6 25.75.5 20.02.8

44.74.8 38.75.9 27.54.6

36.2 5.3 29.3 5.0 24.2 6.0

51.12.2 35.01.7 31.93.8


Clay loam

a C: control (unamended soil), M: mineral fertiliser, S and V Biosolids treatments.1 and 2 low and high rates respectively. Mean valuesS.D., n=3.

Extractable metal recovery (EMR) EMR ¼ "

# Extracted soil metaltreatment 

kg ha1

Extracted soil metalcontrol Cumulative metal loading ðkg ha1 Þ


The data were statistically analysed for each year using one-way analysis of variance (ANOVA) with a significant level of P40.05 using STATGRAF 3.1 program.

3. Results and discussion The soil pH did not vary more than one unit in the different biosolids treatments (Table 2). Soil pH decreased during the year following the applications and then increased in 1995, reaching values similar to the untreated plots (control) in S1 and S2 treatments and slightly lower values than the control in V1 and V2 treatments in 1999. The increase observed over time could have been due to a progressive reduction in the rate of decomposition of soil organic matter (Logan et al., 1997). These results suggest that soil pH should have no influence on the extractable heavy metals contents. Electrical conductivity increased with the rate of biosolids application. This increase is due to the fact that the biosolids had an EC higher than the background soil value. In 1999, leaching reduced the EC, but the values still remained higher in comparison to the untreated plots values in all biosolids treatments (Table 2).

Significant increases (P < 0.05) in soil organic matter content were observed due to the addition of relatively large amounts of biosolids to soil, but these values decreased with time due to the decomposition of the added organic matter. The biggest decrease was observed during the first 5 years (Table 2). Sloan et al. (1998) suggested that the rate of organic matter decomposition decreases with time if no additional biosolids are applied. Lerch et al. (1992) observed that the highest rate of organic carbon mineralisation in biosolidstreated soils occurs 7–11 days after application. Logan et al. (1997) found that organic matter decomposition was substantial in the first 2 years in a fine, mixed, mesic Typic Hapludalf treated with different rates of biosolids. From 1999 to 1991, 44 and 35% (mean values) of the soil organic matter had decomposed in the S and V treatments, respectively (Table 2). These decreases suggest a possibly different pattern of decomposition of organic matter in these biosolids during the 9 year following the cessation of application; these results agreed with studies in which 30% of the organic matter from sludge remained in the soil after biosolids application had ceased 20 years earlier (McGrath and Cegarra, 1992). Similarly, Terry et al. (1979) found that more than 50% of biosolid organic matter was resistant to decomposition. Whereby it can be deduced, that organic matter could play an important role in controlling the bioavailability of heavy metals. According to climatic conditions (Mediterranean semi-arid) and the soil structure, soil pH, and soil carbonate level, most of the biosolids-applied heavy metals presumably remained near the soil surface (upper 30 cm of soil). The experimental evidence in a typical


I. Walter et al. / Environmental Pollution 117 (2002) 315–321

agricultural landscape suggests that there is relatively little movement of biosolids-applied metals below the surface soil (McBride, 1995; Canet et al., 1997; Barbarick et al., 1998; Sloan et al., 1998). For this reason, in the present study soil sample were taken from 0 to 20 cm depth. Biosolids application increased the total concentrations of heavy metals, with the exception of Ni in the low biosolids treatments, (Table 3). The effect of biosolids on the soil total heavy metal content was statistically significant (P < 0.05) in all cases. In each sampling year, total Zn, Cd, and Cu concentrations from the S1 and S2 treatments were higher than those in the V1 and V2 treatments. Soil total Ni and Pb were similar in both biosolids treatments, whereas total Cr was greater in V1 and V2 than in S1 and S2 treatments. These results agree with the metals contents of the applied biosolids. Marked decreases were found for soil total Zn, Cu and Cd contents between 1991 and 1999, especially for the S1 and S2 treatments, but there was little change in total Ni, Pb and Cr within the same period (Table 3). Similarly, no differences were observed in any of the soil total metals between 1995 and 1999.

Soil total metal concentrations in the mineral treatments (M) were relatively uniform. With the exception of Cu, no significant differences (P>0.05) were found between this treatment and the untreated plot. Metal concentrations in the mineral treatment did not experience changes in the three years of analysis (1991, 1995 and 1999). Metals extracted by DPTA increased with loading rate (Table 4). In 1999, there was a general decrease in extractable metals over time for the S1 and S2 treatments. The biggest change occurred in the first 5 years, but there was little variation in extractable metals concentration from 1995 to 1999. From 1991 to 1995, extractable metals decreased in the V1 and V2 treatments, but then they increased slightly in 1999 (Table 4). This pattern could be attributable to the following factors: different organic matter types and amounts, different rates of C mineralisation, and different amounts of soluble organic matters between the biosolids (Alloway and Jackson, 1991). Soluble organic matter has a high capacity to form soluble metal complexes, and therefore DPTA-extractable metals will increase (McBride, 1994).

Table 3 Total soil heavy metal concentrations in the different treatments for three sampling times (1, 5 and 9 years after biosolids application) Years

Metals (mg kg 1)

Treatmentsa C






225 b 112 b 126 b *

321 a 192 a 180 a *

163 c 95.4 c 113 b *

230 b 118 b 169 a *

1991 1995 1999 1991 vs. 1999


24.1 e 18.6 d 25.1 c NS

41.3 d 25.4 d 37.0 c NS

1991 1995 1999 1991 vs. 1999


0.50 d 0.60 d 0.52 e NS

0.52 d 0.70 d 0.64 e NS

1991 1995 1999 1991 vs. 1999


8.12 ef 7.81 e 8.79 f NS

14.3 e 14.7 d 15.6 ef NS

156 b 107 b 105 b *

203 a 179 a 135 a *

1991 1995 1999 1991 vs. 1999


13.0 c 12.8 c 12.5 c NS

13.3 c 12.9 c 12.2 c NS

16.7 b 15.9 b 15.3 ab NS

1991 1995 1999 1991 vs. 1999


32.7 c 32.4 c 30.5 c NS

33.3 c 34.1 c 34.4 c NS

1991 1995 1999 1991 vs. 1999


25.5 c 25.4 d 29.2 c NS

29.6 c 25.6 d 29.8 c NS

9.93 b 5.64 b 4.56 b *

12.9 a 8.55 a 7.42 a *

3.43 c 3.56 c 2.68 cd *

4.13 c 3.56 cb 3.33 c *

76.9 d 67.1 c 62.1 d *

102 c 71.9 c 98.5 bc NS

19.0 a 18.6 a 16.9 a NS

16.2 b 16.0 b 14.5 b NS

19.6 a 18.3 a 16.8 a NS

65.5 b 50.0 b 62.3 b NS

82.6 a 75.1 a 80.4 a NS

67.7 b 65.2 ab 65.8 b NS

78.6 a 73.5 a 73.9 a NS

57.4 b 53.6 c 55.8 b NS

76.7 ab 78.1 a 73.5 a NS

68.7 b 63.3 b 62.0 b NS

85.5 a 82.6 a 79.5 a NS

a Mean values within the same line and year followed by different letters are significantly different at the 0.05 level (Duncan’s Multiple Ranges Test). C: control (unamended soil), M: mineral fertiliser, S and V: Biosolids treatments. 1 and 2: low and high rates, respectively. *P40.05. NS: not significant.


I. Walter et al. / Environmental Pollution 117 (2002) 315–321 Table 4 DTPA-extractable soil metals in the different treatments for three sampling times (1, 5 and 9 years after the last biosolids application) Years

1991 1995 1999 1991 vs. 1999 1991 1995 1999 1991 vs. 1999 1991 1995 1999 1991 vs. 1999 1991 1995 1999 1991 vs. 1999 1991 1995 1999 1991 vs. 1999 1991 1995 1999 1991 vs. 1999

Metals (mg kg1) Zn






Treatmentsa C






2.71 d 1.12 d 1.01 c NS

2.09 d 1.47 d 2.10 c NS

49.8 b 23.0 b 23.3 ab *

78.8 a 33.8 a 28.8 a *

27.6 c 12.9 c 18.0 b *

42.0 b 19.1 b 24.4 a *

0.13 d 0.06 e 0.06 d NS

0.09 d 0.09 e 0.10 d NS

3.17 b 1.46 b 1.44 b *

2.17 d 1.57 d 1.76 d NS

2.25 d 1.59 d 2.58 d NS

0.32 c 0.21 c 0.23 c NS

0.29 c 0.29 c 0.25 c NS

1.21 ab 0.83 a 0.56 b *

1.65 a 0.81 a 0.83 a *

1.10 b 0.54 b 0.71 ab *

1.42 a 0.73 a 0.85 a NS

2.24 c 1.60 d 2.39 c NS

2.53 c 1.91 d 2.55 c NS

10.0 b 5.16 ab 5.10 b *

14.6 a 6.29 a 6.89 a *

7.71 b 3.52 bc 5.37 b *

9.27 b 4.13 b 6.35 a *

0.06 b 0.04 b 0.04 b NS

0.06 b 0.05 b 0.04 b NS

0.08 a 0.05 ab 0.04 b NS

0.11 a 0.06 ab 0.04 b *

0.11 a 0.06 ab 0.08 a NS

0.14 a 0.08 a 0.10 a NS

40.7 b 25.3 b 25.1 b *

4.79 a 2.13 a 1.85 a * 59.1 a 36.9 a 32.3 a *

1.16 c 0.61 cd 0.89 c * 13.3 c 8.96 c 12.9 c NS

1.63 c 0.97 c 1.09 c * 20.2 c 11.9 c 18.8 bc NS

a Mean values within the same line and year followed by different letters are significantly different at the 0.05 level (Duncan’s Multiple Ranges Test). C: control (unamended soil), M: mineral fertiliser, S and V: Biosolids treatments. 1 and 2: low and high rates respectively. *P40.05. NS: not significant.

The ratio of the extracted metals to total loading metal (EMR) for Cd, Zn and Cu was high in the 1st year after the last applications in all the biosolids treatments (Fig. 1). The high percentage recovery of Cd in all treatments is due to the high mobility and activity of this element (Alloway, 1995; Berti and Jacobs, 1996). Biosolids-applied Cr exhibited a very low EMR (0– 0.06%). The low recovery of Cr was probably due to the incomplete extraction by DTPA perhaps due to its low mobility and activity (Cr+3). Kelling et al. (1977) found that DTPA extracted Cd, Cu, Ni, and Zn, but not Cr from a biosolid-treated soil. The results agree with those obtained by other authors (Sims and Kline, 1991; Canet et al., 1997; Sloan et al., 1997; Walter and Cuevas, 1999) who found that nearly all the soil Cr was in a more resistant fraction (less soluble form). For all of the heavy metals studied, the EMR is high in the 1st year after the last application and then decreases in relation to sampling date for the S1 and S2 treatments (Fig. 1). A marked decrease in the EMR occurred in 1995 (in general more than 50%) which contrasts with results obtained in 1999 in which almost

no changes were observed. The lack of change in these metals between 1995 and 1999 suggests a slight change in organic matter content and composition, and also the possibility that the metals are being transformed to more insoluble forms. In contrast, in the V1 and V2 treatments the EMR shows a large decrease from 1991 to 1995 and then a slight increase in 1999. This increase in EMR could be because this biosolids releases more soluble organics over time. This result follows the hypothesis that the biosolids-derived organic matter contributes to metal adsorption. The slow decomposition of this organic matter could release metals into more soluble forms (McBride, 1995). This reasoning contrasts with the sludge protection hypothesis, which states that the sludge-borne heavy metals are maintained in chemical forms of low availability with time as surface adsorbed metals become occluded (Chaney and Ryan, 1993). The AMR for Zn, Cd, Cu, Ni, and Pb was lower in the V1 and V2 treatments than in S1 and S2 treatments in 1991 (Fig. 1). This result agrees with the amounts of the total metal originally present in the biosolids. How-


I. Walter et al. / Environmental Pollution 117 (2002) 315–321

Fig. 1. Percentage of biosolids-applied heavy metals extracted by DTPA (EMR%) from the different treatments for three sampling times (1, 5 and 9 years after biosolids application). C: control (unamended soil), M: mineral fertiliser, S and V: biosolids treatments, 1 and 2: low and high rates, respectively.

ever, in 1999 the EMR percentages reached similar or higher values in the V than in the S treatments.

4. Conclusion The results of the present study indicate that the applications of biosolids had only minor effects on soil pH probably because of its high Ca contents. Soluble salts declined with time, but were still higher in the high rate plots than the untreated plots (control). Biosolidsapplied organic matter decomposed slowly, and 9 years

after the last application, 56 and 65% remained in the soil in the S and V treatments, respectively. Metals extracted with DTPA increased with total soil metal concentrations and declined with time, although a slight increase was observed in the V1 and V2 treatments in the last year of the study. This trend may be due to the different behaviour of organic matter between the two sources of biosolids. The EMR declined with time in S treatments and was especially high for Cd, whereas this percent experienced an increase in the last sampling time in the V treatments. The EMR found in 1991 and 1995 was lower in the V

I. Walter et al. / Environmental Pollution 117 (2002) 315–321

treatments than in S treatments, but in 1999, EMR reached similar or higher values in V than in S treatments. The results of this study suggest that the pattern of metal distribution in the soil from biosolids S and V differs markedly. It is suggested that the organic matter applied with biosolids could play an important role in metal extractability over time.

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