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Published in J. Environ. Qual. 33:496-504 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORT

Heavy Metals in the Environment

Effect of Chloride in Soil Solution on the Plant Availability of Biosolid-Borne Cadmium

Karin Weggler*,a, Michael J. McLaughlinb,c and Robin D. Grahamc

a State Forests of NSW, Research and Development Division, PO Box 100, Beecroft, NSW 2119, Australia
b CSIRO Division of Land and Water, PMB 2, Glen Osmond, SA 5064, Australia
c University of Adelaide, Department of Plant Science, Waite Road, Glen Osmond, SA 5064, Australia

* Corresponding author (karinweggler{at}yahoo.de).

Received for publication November 7, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Increasing chloride (Cl) concentration in soil solution has been shown to increase cadmium (Cd) concentration in soil solution and Cd uptake by plants, when grown in phosphate fertilizer– or biosolid-amended soils. However, previous experiments did not distinguish between the effect of Cl on biosolid-borne Cd compared with soil-borne Cd inherited from previous fertilizer history. A factorial pot experiment was conducted with biosolid application rates of 0, 20, 40, and 80 g biosolids kg–1 and Cl concentration in soil solution ranging from 1 to 160 mM Cl. The Cd uptake of wheat (Triticum aestivum L. cv. Halberd) was measured and major cations and anions in soil solution were determined. Cadmium speciation in soil solution was calculated using GEOCHEM-PC. The Cd concentration in plant shoots and soil solution increased with biosolid application rates up to 40 g kg–1, but decreased slightly in the 80 g kg–1 biosolid treatment. Across biosolid application rates, the Cd concentration in soil solution and plant shoots was positively correlated with the Cl concentration in soil solution. This suggests that biosolid-borne Cd is also mobilized by chloride ligands in soil solution. The soil solution CdCl+ activity correlated best with the Cd uptake of plants, although little of the variation in plant Cd concentrations was explained by activity of CdCl+ in higher sludge treatments. It was concluded that chloro-complexation of Cd increased the phytoavailability of biosolid-borne Cd to a similar degree as soil (fertilizer) Cd. There was a nonlinear increase in plant uptake and solubility of Cd in biosolid-amended soils, with highest plant Cd found at the 40 g kg–1 rate of biosolid application, and higher rates (80 g kg–1) producing lower plant Cd uptake and lower Cd solubility in soil. This is postulated to be a result of Cd retention by CaCO3 formed as a result of the high alkalinity induced by biosolid application.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
LAND APPLICATION OF BIOSOLIDS is a common practice in many countries that allows the reuse of the increasing amounts of biosolids produced by urban populations. Biosolids are a significant source of nitrogen (N), phosphorus (P), and trace elements, which were shown to have a significant fertilizer potential for a number of crops (DeHaan, 1980; Gerzabek et al., 1998; McLaughlin and Champion, 1987; Parker and Sommers, 1983; Pastene, 1981; Poletschny, 1987; Serna and Pomares, 1992; Weggler-Beaton et al., 2003). The beneficial effects of biosolid aplication on productivity should encourage the practice of recycling the biosolid-borne nutrient pool to agricultural land. However, biosolids also contain heavy metals such as Cd, which can have adverse effects on humans when entering the food chain in elevated amounts. Regulations for highest permissible concentrations in a number of crops and food sources are in place (McLaughlin et al., 2000). Hence, biosolid application is only feasible when the phytoavailability of biosolid-derived heavy metals is low and thus metals are unlikely to enter food plants in significant amounts.

Soil properties such as soil organic matter content, clay type, oxidation and reduction status, and particularly soil pH are considered major factors determining the bioavailability of metals in soil (Sommers et al., 1987). Recent studies have shown that the chloride concentration in soils is another major factor that can determine Cd availability (Li et al., 1994; McLaughlin et al., 1994, 1997b; Norvell et al., 2000).

Chloride anions are known to reduce soil sorption of Cd (Boekhold et al., 1993; Garcia-Miragaya and Page, 1976; Hirsch et al., 1989; O'Connor et al., 1984), probably due to the fact that chloride forms relatively strong complexes with Cd (Smith and Martell, 1981). The resulting increase in concentration of Cd in the liquid phase at higher Cl concentrations can enhance Cd mobility in soils (Doner, 1978). An increase in Cl concentration in the soil or soil solution was shown to increase Cd concentration in plants, whether in laboratory or field experiments (Bingham et al., 1983, 1984; Boukhars and Rada, 2000; Li et al., 1994; McLaughlin et al., 1994, 1997b; Smolders et al., 1997; Smolders and McLaughlin, 1996a, 1996b; Smolders et al., 1997; Weggler-Beaton et al., 2000). Chloro-complexation of Cd and the resulting improved diffusion of Cd through the soil to plant roots and possibly uptake of Cd–chloro complexes are suspected to be the reasons for the Cl effect on Cd uptake (McLaughlin et al., 1994, 1997a; Smolders and McLaughlin, 1996b). The phytoavailability of Cd can vary with the source of Cd applied. The phytoavailability for inorganic Cd (Cd salt, Cd in phosphatic fertilizer) is generally higher compared with biosolid-derived Cd (Chaney and Oliver, 1996, p. 323–360). The influence of Cl ions on Cd complexation and mobility could also differ with the different sources of Cd. Weggler-Beaton et al. (2000) demonstrated that increasing Cl concentration in soil solution also enhances the uptake of Cd by wheat and Swiss chard (Beta vulgaris L.) in soils amended with biosolids. However, they could not separate effects of Cl on biosolid-borne Cd from the effects on soil Cd in the agricultural soil used. The aim of the current experiment was to determine to what extent biosolid-borne Cd, in comparison with soil Cd, is mobilized by increasing Cl concentrations in solution. A factorial experiment with increasing biosolid rates and increasing Cl applications was used to measure the Cd availability for wheat plants.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Biosolids were obtained from Bolivar and Port Adelaide treatment works in South Australia. Preliminary experiments indicated the biosolids had a high salt content injurious to seedling germination and emergence. After air-drying and grinding to <2 mm, the biosolids were leached with deionized water to reduce the NaCl content before the introduction of NaCl treatments in the pot experiment. The biosolid material was placed about 10 cm deep onto a grid in a bucket and soaked in deionized water for 12 h. The material was then leached with deionized water until the leachate reached a stable electrical conductivity of around 3.5 dS cm–1. The material was then dried (60°C) and again sieved to <2 mm. The metal and salt contents of the biosolids are shown in Table 1. The leachate was analyzed for Cd concentration to determine if significant quantities of Cl-soluble Cd were present in the leachate. A maximum of 20% of the total Cd was lost by the prewashing of the biosolid.


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  		Table 1. Element concentration in biosolids, determined by digestion in aqua regia (Zarcinas et al., 1996).

 
The soil used was the 0- to 100-mm stratum of an Alfisol, having a pH of 6.3 and low concentrations of Cd and Zn (Table 2). The soil was amended with biosolids at rates of 0, 20, 40, and 80 g kg–1 (0, 10, 20, and 40 g biosolids in 500, 490, 480, and 460 g soil, respectively), the equivalent of 0, 25, 50, and 100 Mg biosolids ha–1, assuming an application depth of 10 cm. The biosolid-amended soil was incubated at a moisture potential equivalent to 70% of field capacity (20% w/w) at 20 and 15°C (day–night cycle) for 14 d. The control pots received a CaCO3 application of 0.25 g kg–1 to increase soil pH to a level similar to the biosolid treatments. Basal nutrients were supplied before incubation, before seeding, and weekly during growth, as listed in Table 3. Additional N, P, and micronutrients were added to the control soil to compensate for biosolid nutrients, assumed to be released following a biosolid application of 20 g kg–1 (Table 3). For N and P the equivalent of 5 kg nutrient Mg–1 biosolid and for Cu and Zn 10% of the total content in biosolids was assumed to be plant available during the time of the experiment (30 d). The N and P release in the current experiment was calculated from results gained in field experiments using Bolivar biosolids at agronomic rates of 0 to 10 Mg ha–1 (Weggler-Beaton et al., 2003).


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  		Table 2. Selected soil chemical characteristics.

 

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  		Table 3. Fertilizer additions to the soil prior, during, and after planting for control and biosolid-amended soil.

 
Salinity treatments were delivered in irrigation water as five concentrations of NaCl: 0, 6.8, 13.7, 20.5, 27.4, and mM (0, 400, 800, 1200, and 1600 mg L–1 of NaCl). These salt levels are typical of those found in irrigation waters in southern Australia (M.J. McLaughlin, unpublished data). Treatments were replicated fourfold. Five wheat plants were sown in each pot and grown for 30 d in a controlled environment growth chamber at 20 and 15°C (day–night cycle) under artificial lighting (300 µmol quanta m–2 s–1). Shoots were then cut at the soil surface and washed in a solution of 1% Dextran for 2 min to eliminate surface contamination and then rinsed with deionized water three times. Shoots were dried at 70°C for 48 h and whole shoots digested in concentrated nitric acid (Aristar grade; BDH Chemicals, Poole, UK). Element concentrations (except Cd) in digest solutions were determined by inductively coupled plasma atomic emission spectroscopy (ICP–AES). Cadmium concentrations were determined with a graphite furnace atomic absorption spectrophotometry (GFAAS) using deuterium background correction and orthophosphoric acid as the modifier.

After plant harvest, soil solution was displaced from the soil according to the method of Thibault and Sheppard (1992) by centrifugation at 4000 RCF for 30 min. Extracted solutions were centrifuged at 25000 RCF for 60 min and filtered through a 0.2-µm filter. Electrical conductivity, pH, and alkalinity of the solution were determined immediately. Alkalinity was determined by titration with diluted hydrochloric acid to the total alkalinity end point of pH 4.5. Anions in solution (SO2–4, NO–3, Cl–) were determined by ion chromatography (Dionex [Sunnyvale, CA] 4000i using a AS4A column) and cations (Ca, K, Mg, Mn, Zn, Cu, Fe) and P by ICP–AES. Organic carbon in solution was analyzed using a total organic carbon analyzer (Dohrmann DC 180; Teledyne Tekmar, Mason, OH). Cadmium concentrations in solution were determined by GFAAS.

Activities of metal species in solution were predicted using the computer program GEOCHEM-PC (Parker et al., 1995). The composition of biosolids-derived dissolved organic matter (DOM) varies between biosolids and is difficult to determine. The mixture model by Sposito and Bingham (1981), developed with some other biosolids, was used to convert soluble organic carbon concentrations to a form useable by GEOCHEM-PC. The model approximates metal complexation behavior of biosolids-derived DOM by replacing DOM in the model by a suite of low molecular weight organic acids with similar proton titration curves to biosolid-derived DOM (Sposito and Bingham, 1981). This mixture model gives an approximate description of the functional group chemistry of the dissolved organic carbon in solution insofar as it relates to the formation of trace metal complexes.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Plant dry matter production was slightly affected by the NaCl additions and the biosolid treatments (Table 4), but there was no interaction between the two. Plant growth was slightly reduced by high additions of NaCl and enhanced in the highest biosolid treatment. Growth reductions and enhancements were in the range of 25 and 20% of the control, respectively.


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  		Table 4. The effect of Cl concentration in soil solution and biosolid application rate on shoot dry weight, pH, and alkalinity in soil solution.

 
The soil pH remained unchanged by biosolid treatments and slightly decreased by the NaCl treatment (Table 4). The alkalinity in the soil solution was similar in the control and the low biosolid treatment, but increased in the medium and particularly in the highest biosolid treatment.

Chloride concentration in soil solution was positively correlated to total Cd concentration in soil solution for the control and each biosolid treatment (Fig. 1) . Furthermore, Cd concentration in soil solution was increased by application of biosolids rates of up to 40 g kg–1, but there was a reduced effect of Cl in the 80 g kg–1 treatment.



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  		Fig. 1. Relationship between chloride concentration in soil solution and Cd concentration in soil solution of soils amended with 0 ({circ}----), 20 ({triangleup}----), 40 ({blacktriangleup}----), and 80 ({square}----) g biosolids kg–1. The exceptional high value in the 20 g kg–1 biosolid treatment was omitted in the calculation for the fitting line.

 
To avoid the problem of nutrient and heavy metal dilution effects, the Cd uptake values (Cd concentration multiplied by total dry weight) were also used for treatment comparisons. Whether judged by Cd uptake or Cd concentration values, the relative differences due to Cl treatments remained the same in the 0, 20, and 40 g biosolids treatments. The only exceptions arise in the 80 g biosolid treatment, where Cd concentration values showed a lower Cd availability than when Cd uptake values were used. However, Cd concentration as well as uptake values in the 80 g biosolid treatments were still significantly higher than in the control treatment. The Cd uptake of plants was positively correlated with the Cl concentration in soil solution for the control and all biosolid treatments (Fig. 2) . The plant Cd uptake was also significantly enhanced by biosolid treatments but there was little difference in the effect of higher rates of biosolids (Fig. 2 and Table 5).



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  		Fig. 2. Relationship between Cl concentration in soil solution and Cd uptake in wheat with soil amended with 0 ({circ}----), 20 ({triangleup}----), 40 ({blacktriangleup}----), and 80 ({square}----) g biosolids kg–1.

 

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  		Table 5. Linear regression models, describing the effect of Cl concentration in soil solution on total Cd concentration in soil solution and on the Cd uptake by shoots grown in control and biosolid-amended soil.

 
Linear regression models, describing the positive correlation between Cl concentration and Cd concentration in soil solution, were highly significant for the control and each biosolid rate (Table 5). To be able to distinguish the effect of chloride ligands on biosolid-borne Cd and on soil Cd, a comparison of regression models is needed. If biosolid-borne Cd is in fact mobilized by Cl, then with increasing biosolid rates an increase in slope of the regression models, describing the relation between Cl concentration in solution and Cd uptake in plant shoots, would be expected. Additionally, the intercept in the various models should increase with biosolid rate due to the increase in Cd application.

Regression models, describing the effect of Cl concentration in soil solution on Cd concentration in soil solution for biosolid treatments, were significantly different from the control treatment (Table 5). The Cd concentration in soil solution increased with increasing biosolid rates and increasing chloride concentrations. These results suggest that biosolid-borne Cd is mobilized by an increase in Cl concentration in soil solution. In the highest biosolid treatment a reduced Cd concentration compared with the 40 g kg–1 biosolid rate was noticed, which will be discussed later. The regression models describing the effect of Cl concentration in soil solution on plant Cd uptake showed also a significant difference between the control and the biosolid treatments. However, differences between the increasing rates of biosolids were mainly insignificant (Fig. 2, Table 5). Hence, the increase in soil solution Cd due to biosolid application and increasing Cl concentrations in soil solution was not reflected in a similar difference in Cd uptake by plants. This lack of enhanced plant Cd uptake when biosolid rates increased, which included a difference in total Cd application and total Cd in soil solution, may indicate a low phytoavailability of biosolid-borne Cd under the influence of elevated Cl concentrations. This aspect is different to soil-borne Cd (control), where increases in soil solution Cd, under the influence of Cl, were reflected in increased Cd uptake by plants.

Another approach to compare the solubility of biosolid-borne Cd and soil Cd is to compare the percentage of total soil Cd that is actually in solution. Soluble Cd as a percentage of the total Cd is listed in Table 6 (assuming a water content of 20%). For low NaCl concentrations, the percentage of total Cd in solution increases with the increase in biosolid application rate up to medium rates (40 g kg–1), suggesting that biosolid Cd is more soluble than soil Cd under those circumstances. However, when Cl is added to the system, then the percentage of total Cd in solution is similar for all treatments, except the 80 g kg–1 treatment. Hence the relative increase in soluble Cd from low to high NaCl treatments was less marked in the biosolid treatments than in the control soil, suggesting a reduced effect of Cl on biosolid-borne Cd solubility. It should be remembered that a significant amount of Cd was leached from the samples before the experiment. Furthermore, Cd uptake values expressed as a percentage of the total soil Cd are significantly higher in the control than in the biosolid treatments (Table 6), suggesting a lower availability of biosolid versus soil (fertilizer) Cd.


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  		Table 6. Cadmium in soil solution and Cd uptake as percentage of the total Cd in biosolid-amended soils, calculated for the lowest (0 mg L–1 NaCl) and highest (1600 mg L–1 NaCl) Cl treatment.

 
The Cd speciation in soil solution was significantly altered by increasing chloride concentrations in solution, with Cd–chloro complexes becoming the dominant Cd species in solution as salinity increased (Table 7). At low Cl concentrations sulfate and carbonate complexes of Cd were, besides the free Cd2+ ion, the dominant form of Cd. Organically complexed Cd increased with the increase in biosolid rate and increases in dissolved organic C levels. However, those levels were low in absolute terms and further decreased with increasing Cl levels (Table 7).


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  		Table 7. The influence of biosolid rate and NaCl addition on Cd speciation in soil solution of control and biosolid amended soil.

 
The activity of free Cd2+ in solution remained largely unchanged with increases in Cl concentration in soil solution, except for the control treatment (Table 8). However, the activity of CdCl+ and CdCl2 species increased significantly with the increase in Cl concentration in solution. Using stepwise-multiple regression the CdCl+ species accounted for 59, 19, 28, and 9% of the variation in Cd concentration in wheat shoots in the 0, 25, 50, and 80 g kg–1 treatments, respectively, whereas the free Cd2+ activity accounted for 68, 3.8, 0, and 0.1% of the variance in the same treatments, respectively. The transfer coefficient, describing the ratio of Cd taken up by plants to total Cd concentration in soil solution, decreased with the increase in Cl concentration in the control treatment. This suggests a lower plant availability of Cd–chloro complexes compared with free Cd2+, with Cd–chloro complexes being more abundant under high Cl conditions. However, for the biosolid treatments the transfer coefficient was low under low Cl conditions and was relatively less affected by Cl addition compared with control soil. The transfer coefficient of biosolid-borne solution Cd was much less reduced due to Cl addition, and thus chloro-complexation, than for soil (fertilizer) solution Cd. This may have been because a significant proportion of the Cd in solution existed in complexed form (with sulfate or carbonate), even under conditions of low Cl.


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  		Table 8. Activity of Cd chloride complexes and the free Cd2+ ion in soil solution and Cd transfer coefficient (ratio of Cd in shoots [mg kg–1] to Cd in soil solution [mg L–1]) as affected by NaCl addition to biosolid-amended soil.

 
The application of biosolid changed the soil environment in a number of aspects that may have affected Cd uptake. There was an increase in Zn, Cu, sulfate, Ca, dissolved organic C, and, to a lesser degree, phosphate concentration in soil solution (Table 9). Also the Zn, Cu, and S uptake of plants and to a lesser degree the P uptake were enhanced by increasing biosolid application (data not shown). Soil alkalinity increased significantly with an increase in biosolid rate and could have affected Cd uptake (Table 4). Cadmium speciation was calculated using GEOCHEM-PC, disallowing precipitation (as the separated soil solutions were free of solids). However, in subsequent calculations assuming an open system and with solid-phase precipitation allowed, GEOCHEM-PC predicted the following compounds to precipitate: CaCO3, MgCO3, CaHPO4, MnCO3, Fe(OH)3, and Al(OH)3 (Table 10). The amounts of Fe(OH)3 and Al(OH)3 predicted to precipitate were low and did not vary with treatments [Fe(OH)3: 1.2–1.9 µM, Al(OH)3: 0.5–7 µM]. Predicted oversaturation with CaCO3 and MgCO3, and to a lesser degree CaHPO4 and MnCO3, was considerable and depended on biosolid rates, but not on salt rates. The predicted activity of Cd species in soil solutions (Cd2+, CdCl+, CdCl2) was 10 to 20% lower than when precipitation was disallowed (data not shown). The changes in Cd activity most likely occurred because the program was run as an open system in terms of CO2 exchange when precipitation was allowed. However, activity was similarly decreased for the different Cd species in solution. Hence the ratios of the various Cd species in solution remained unchanged.


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  		Table 9. The effect of Cl concentration in soil solution on the Zn, Cu, P, Ca, dissolved organic carbon (DOC), and SO4 concentration in soil solution of biosolid-amended soils.

 

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  		Table 10. Compounds predicted to precipitate in soil solutions according to GEOCHEM-PC.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Cadmium has long been recognized as a potential threat to the human food chain and highest permissible concentrations have been set for a number of food crops, such as wheat (for a review see McLaughlin et al., 2000). Biosolid applications will increase the total concentration of Cd in soils and the bioavailability of this Cd will determine whether plant Cd uptake occurs to a significant degree. Soil salinity was found to be a major factor enhancing soil–plant transfer of fertilizer-borne Cd (McLaughlin et al., 1994, 1997b; Smolders et al., 1997), irrespective of the soil pH.

The current study has shown that increased Cl concentrations also enhance the Cd concentration in soil solution when biosolids are the source of Cd. The significant increase in Cd concentration in soil solution with the application of biosolids suggests a sizeable amount of biosolid-borne Cd is water soluble and potentially phytoavailable. This was despite the easily soluble Cd being removed from the biosolids before experimentation and despite the high soil pH (H2O), 7.75 to 8.19, due to biosolid application. With increasing additions of Cl, there was a distinct increase in soil solution Cd between the control soil and the biosolid-amended soils, and also a distinct increase with biosolid rate up to medium rates.

When soluble Cd is expressed as a percentage of total Cd (Table 6) the percentage of Cd in solution increases with biosolid rates under low Cl conditions, suggesting an enhanced solubility of biosolid-borne Cd compared with soil Cd. However, when Cl is added to the system, the percentage of water-soluble Cd is similar for the control soil and biosolid-amended soil and thus relatively less Cd was mobilized in the biosolid treatment when changing from the low to the high Cl conditions. Smolders et al. (1997) calculated a regression model, describing the relation between Cl and Cd concentrations in soil solution, with a significantly higher slope for agricultural soils than in the current study. This would suggest a more pronounced effect of Cl on fertilizer-borne Cd. However, soil pH was lower in the study of Smolders et al. (1997) and it should be remembered that we removed the most weakly bound biosolid Cd fractions by the leaching before the pot experiment.

The changes in soil solution Cd with treatment application were partly reflected in the Cd uptake by plants. Plant Cd uptake increased after biosolid application compared with the control and was positively correlated with Cl concentration in soil solution. However, plant Cd uptake did not increase proportionally to the total soil Cd across biosolid rates. This was the case both under high and low Cl conditions. Similarly, Gerzabek et al. (1998) found an increase in mobile fractions of Cd after sewage sludge application, but heavy metal uptake by plants did not respond proportionately. Also, Van Erp and Van Lune (1989) found a low Cd availability in soil treated with metal-contaminated compost compared with untreated soil, even though they considered Cd speciation in their assessment.

Furthermore, in the current study the Cd transfer coefficient of biosolid Cd in soil solution was lower than from soil (fertilizer) Cd (Table 8). This information would suggest that the uptake of solution Cd is lower in biosolids-amended soils. Boukhars and Rada (2000) found a similar tendency, with Cd(NO3)2 being taken up more readily than biosolid-borne Cd under low and high saline conditions. However, their study used Cd concentrations an order of magnitude higher than in the current study.

The lower Cd availability of solution Cd from biosolids compared with the control could be due to enhanced cation competition, such as Ca and Zn. However, we feel that the enhanced concentrations of Zn or Cu in biosolid-amended soils are unlikely to explain differences in Cd availability due to competitive uptake, because the ratio of Zn to Cd and Cu to Cd in soil solution was actually higher in the control soil than in the biosolid-amended soils.

It is generally accepted that the Cd uptake of plants is dependent on the chemical speciation of Cd in solution (Checkai et al., 1987). Although initially it was argued that only "uncomplexed" free ion metal species are taken up by the plant, there is increasing evidence that also some metal complexes, such as chloro complexes, can be absorbed by the plant root, although at a lower efficiency (Boukhars et al., 2000; McLaughlin et al., 1997c; Smolders and McLaughlin, 1996a, 1996b).

In the current study the principal Cd species in soil solution were free (ionic) Cd2+, chloro, sulfate, and carbonate complexes, and a small fraction of organically complexed Cd, which is in agreement with other studies of biosolid-amended soils (Hirsch and Banin, 1990; Sposito and Bingham, 1981). With increases in Cl concentrations in soil solution, Cd–chloro complexes became the dominant species in solution. Shoot Cd concentration was most closely correlated with the CdCl+ activity in solution while the activity of free Cd2+ was only weakly correlated. The results suggest that in saline soils CdCl+ species have a crucial importance in enhancing plant Cd uptake, whether as a form taken up by plants or a transport medium for Cd, being less prone to adsorption. This is in agreement with studies using fertilizer-borne Cd, where formation of chloro complexes was shown to enhance Cd in solution (Smolders et al., 1997) and also to enhance transport of Cd across the soil–root interface (Smolders and McLaughlin, 1996a, 1996b). Nevertheless, the effect of chloro-complexation on biosolid-borne Cd seems less pronounced, as less of the variability in Cd uptake is explained by chloro complexes than in the control treatment (59, 19, 28, and 9% for the 0, 20, 40, and 80 g kg–1 biosolid treatments, respectively). In biosolid treatments a higher percentage of Cd was complexed by sulfate than in the control. Although nutrient solution studies have shown that Cd–sulfato complexes can be taken up by plants (McLaughlin et al., 1998a), the effects in soil seem to be more complex and appear to have little effect on plant Cd uptake (McLaughlin et al., 1998b). The availability of Cd–carbonate solution species is unknown, but presumably is also less than that of free Cd2+.

A factor that may have affected Cd availability in biosolid treatments is alkalinity, which increased considerably in the higher biosolid treatment compared with lower rates. This was due to an increase in carbonate and bicarbonate ions and to some degree dissolved organic C, while the OH– concentration (pH) remained unchanged. A reduced phytoavailability of Cd was particularly evident in the 80 g kg–1 treatment where Cd uptake was slightly decreased compared with the intermediate biosolid rate of 40 g kg–1. With increases in carbonate and bicarbonate ions, probably due to microbial activity, precipitation of CaCO3 is possible as the biosolid contained a considerable amount of Ca. Calculations by GEOCHEM-PC allowing precipitation to occur predicted CaCO3, CaHPO4, MgCO3, FeOH3, and MnCO3 to precipitate. Precipitation of CdCO3 was not predicted and is unlikely to occur at low levels of Cd encountered in most soils (McBride, 1980). McBride (1980), Pickering (1983), and Papadopoulos and Rowell (1988) found that Cd can be very effectively retained by calcium carbonate surfaces at very low solution Cd2+ activities and the adsorption reactions of Cd on calcite surfaces seemed to be very selective for Cd when Ca was a competing cation (Papadopoulos and Rowell, 1988). Hirsch et al. (1989) suspected that adsorption of CdHCO+3 complexes to soil surfaces may reduce Cd concentrations in solution. Adsorption of Cd to carbonate surfaces or as coadsorption in the form of the Cd–bicarbonate anion could have reduced Cd availability in the higher biosolid treatments. The enhanced carbonate concentration in biosolid-amended soils is probably due to a flush of microbial activity after biosolid application. However, an effect based on alkalinity and thus soil microbial activity is likely to decrease over time. Field experiments with increasing rates of biosolids generally found that plant Cd uptake increased with biosolid rate, when conducted over at least a season (Logan et al., 1997).

Results of the current study suggest that Cl concentration in soil solution is also an important factor in determining the phytoavailability of biosolid Cd in biosolid-amended soils. Regulations or guidelines concerned with application of biosolids to agricultural land should consider the Cl concentration in soils as one of the risk factors, as is currently the case in South Australia (McLaughlin et al., 2000). Potential sources for increased Cl loading of agricultural soils, particularly in drier areas, are (i) irrigation water, (ii) inherent soil salinity (dryland areas), and (iii) Cl contained in biosolids. The latter could be overcome by preleaching of biosolids or altering biosolid production processes. A lower salt content may also add to the overall value of biosolids, because salt accumulation in agricultural areas is generally undesirable and applied salt can reduce germination and early growth of plants during periods of insufficient rainfall. In drier countries it is also likely that irrigation water quality is low (high salt, Cl levels) and thus could result in enhanced Cl levels in irrigated land. Furthermore, inherent soil salinity is a known problem in some dryland agricultural areas. Regulations for biosolid application to land in drier countries therefore may have to be more cautious than in countries with a wetter climate and should include measures to restrict biosolid-amended soils from being exposed to elevated Cl levels.


   ACKNOWLEDGMENTS
 
Karin Weggler thanks the Australian government for granting a scholarship and also SA Water (formerly Engineering and Water Supply Department, South Australia) for support of this study. The authors thank the staff of CSIRO Land and Water and Department of Plant Science, University of Adelaide, for assistance with chemical analyses.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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N. T. Basta, J. A. Ryan, and R. L. Chaney
Trace Element Chemistry in Residual-Treated Soil: Key Concepts and Metal Bioavailability
J. Environ. Qual., January 1, 2005; 34(1): 49 - 63.
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