Effects of Plant Residue and Salinity on Fractions of Cadmium and Lead in Three Soils
By Abbaspour, Ali Kalbasi, Mahmoud; Hajrasuliha, Shapoor; Golchin, Ahmad
Bioavailability and mobility of heavy metals (HMs) in soils are determined by their partitioning between solution and solid-phase and their further redistribution among solid-phase components. A study was undertaken to determine the effects of organic matter (OM) and salinity on cadmium (Cd) and lead (Pb) distribution among soil fractions. Three agricultural soils were treated with 20 mg Cd/kg as Cd (NO^sub 3^)^sub 2^.4H^sub 2^O, 150 mg Pb/kg as Pb (NO^sub 3^)^sub 2^, 20 g/kg alfalfa powder, and 50 mmol/kg of NaCl, and then incubated at 60% water holding capacity (60% WHC) and constant temperature (25[degrees]C) for 12 weeks. Various fractions of Cd and Pb were extracted from the soils after 2 and 12 w of incubation using a sequential extraction technique. Results showed that in the early stage of incubation (2 w), added Pb were found mainly in the specifically sorbed (SS) and amorphous Fe oxides (AFeO) fractions and added Cd found in SS and Mn oxides (MnO) fractions. Addition of 2% OM decreased the exchangeable (EXC) Pb fraction almost in all soils, whereas it had a different effect on the EXC Cd fraction depending on soil pH. Addition of NaCl increased the EXC Cd fraction in two soils, but it did not alter Pb fractions. At the end of the incubation period, Pb decreased in the EXC and MnO fractions except in the neutral soil and Cd decreased mainly in the SS fraction. Keywords Cadmium, lead, organic matter, salinity, fractionation
Introduction
Elevated levels of heavy metals (HMs) in soils are the results of atmospheric deposition, impurities in commercial fertilizers, mining and smeltering operations and land application of sewage sludge (Kalbasi et al., 1995; Martinez and Motto, 2000; Kumar et al., 2005). A heavy metal contamination of natural soil environment is a major concern for soil quality, human health, and environmental protection (Krishnamurti and Naidu, 2000). Geochemical forms of heavy metals existing in the soils affect their solubility, which may directly influence their phytoavailability (Chen et al., 2000). Studies performed in Central Iran on a set of 255 topsoil samples gathered randomly from an area of 6800 km^sup 2^ showed that total Cd concentrations exceeded the Swiss guide value in more than 80% of the samples whereas Pb concentrations exceeded the respective guide value only in 2% of the samples (Amini et al., 2005). Golchin (2003) found that the total concentrations of Cd and Pb on a set of 10 topsoil samples in the vicinity of Pb-Zn smelters in Zanjan province, Iran, ranged from 10-3233 and 59-15850 mg/kg of soil, respectively. It should be mentioned, however, that total content of a heavy metal (HM) in soil is not necessarily a good measure of its bioavailability. In the last decade, several researchers have attempted to assess phytoavailability of heavy metals in contaminated soils using metal sequential extraction methods (Han and Banin, 1999; Kabala and Singh, 2001; Renella et al., 2004). Most of the HMs are persistent in soil because of then- immobile nature. Cadmium is known as a more soluble and mobile metal as compared to other metals in soils, while lead (Pb) is known to be immobile and unavailable for plant uptake (Chen et al., 2000; Massadeh et al., 2004). Shuman (1998) found that the addition of organic waste amendments (i.e. commercial compost, poultry litter, cotton gin litter, and industrial secondary sewage sludge) to 2 different soils had a profound effect on the distribution of added Cd and Pb among the fractions. Han and Banin (1999) studied the transformation of added soluble HMs in two calcareous soils incubated for 1 year at the field capacity moisture content. They concluded that applied Cd and Pb were transferred from the EXC and carbonate (CARB) fractions into the CARB and MnO fractions, respectively.
Soil salinity constitutes one of the most severe agricultural problems in many parts of the world (Parkpian et al., 2002). Limited information, however, is presently available on the environmental implications of salinity on HMs concentrations in soil solution (Mc Laughlin et al., 1997). Recent studies have shown that the chloride concentration in soils is an important factor that can determine Cd availability (Norvel et al., 2000; Weggler et al., 2004). Weggler et al. (2004) found that the Cd concentration in plant shoots and soil solution was positively correlated with the Cl concentration in soil solution. Khoshgoftar et al. (2004) applied five levels of irrigation water salinity (0, 60, 120 and 180 mM NaCl) on a calcareous soil and found that salinity mobilized soil Cd and increased its phytoavailability in wheat plant.
Soil organic matter (OM) has also been of particular interest in studies of HMs retention in soils due to the tendency of HMs to form stable complexes with organic ligands (Halim et al., 2003; Almas et al., 2000). Contradictory results were found about the effect of organic matter on bioavailability and mobility of HMs in soils. Prior research suggests that soil OM can either enhance or inhibit HMs adsorption, depending on other soil properties such as pH, cation exchange capacity, competing ions, etc. (Barter and Naidu, 1995; Romkens and Dolfing, 1998). Sauve et al. (1998) applied OM (as leaf compost) to a Pb-contaminated soil and found that the solubility of Pb decreased linearly from pH 3 to 6.5 and was independent of soil organic matter in this pH range. From pH 6.5 to 8, increasing pH promoted the formation and dissolution of organo- Pb complexes that increased Pb solubility. Most of the existing literature on the effect of organic matter on the availability and mobility of heavy metals in soil was conducted using sewage sludge or partially humified compost material as the source of organic carbon. Very limited information is available on the effect of fresh organic materials on the bioavailability of heavy metals in soil. In fact, plant materials are annually applied to soil in order to improve soil chemical and physical properties.
The objectives of this study were to: i) investigate the transformation of Cd and Pb in 3 unspiked and spiked soils during a 3-month incubation period; ii)assess the effect of OM and NaCl application on fractionation of Cd and Pb in 3 untreated and treated soils.
Materials and Methods
Soils
Bulk samples of topsoils (0-30 cm) were collected from 3 cultivated areas in Iran, an acid soil from a tobacco field, a neutral soil from a paddy around Anzali wetland area (both in Gilan province, north of Iran) and a calcareous soil from a wheat field in central Iran (Charmahal-Bakhtiari province). Selected properties of the three soils are presented in Table 1. The soil samples were air- dried, gently crushed to pass through a 2 mm sieve and stored for the incubation study.
Table 1
properties of soils studied
Incubation Study
An incubation experiment with 2 heavy metal treatments (20 mg Cd/ kg as Cd (NO^sub 3^)^sub 2^.4H^sub 2^O and 150 mg Pb/kg as Pb (NO^sub 3^)^sub 2^, maximum permitted metal loadings in soil, established by the USEPA-503 regulations (Mc Bride, 1995)), one OM treatment (20 g/kg of alfalfa powder), one salinity treatment (50 mmol/kg of NaCl), combination of OM and salinity treatments and a control (no treatment), was conducted. Triplicate treated samples (75 g) were incubated in separate polyethylene containers at constant moisture (60% water holding capacity) and temperature (25[degrees]C) for 2 incubation periods (2 and 12 weeks). During the incubation period, containers were sealed with a cap, perforated by a pin to diminish evaporation and at the same time permit gas exchange with the atmosphere. The moisture content of the soils was kept constant throughout the experiment by periodically weighing and replenishing evaporated water by de-ionized water. After each incubation period, samples were taken from each container for analysis. At the same time, a subsample was taken to determine the soil moisture content.
A selective sequential dissolution fractionation procedure (Tessier et al., 1979; Amacher, 1996; Han and Banin, 1999) was employed to divide Cd and Pb in the soils into six operationally (Table 2), defined solid-phase fractions (Exchangeable (EXC), specifically sorbed (SS), bounded to Mn oxide (MnO), bounded to amorphous Fe oxide (AFeO), bounded to organic matter (OM) and residual (RES)). The concentration of heavy metals in each fraction was measured by atomic absorption spectroscopy (AAnalyst 200) and very low concentration of HMs measured using graphite furnace atomic absorption spectrometry (SHIMADZU, GFA-4A model). The soil pH and EC were also determined in 1:2.5 soil:water suspension using pH meter (model pH 262) and Metrohm conductometer, respectively.
Table 2
Protocol for sequential extraction procedure
Data Analyses
The following parameters were used to describe the transformation kinetics of metals in soil (Banin et al., 1990; Han and Banin, 1997; Man et al., 2003):
Where i is the extraction step number increasing with the increase in the aggressiveness of extradants used in the sequential steps (EXC = 1 through RES = 6). F^sub i^ is the fractional content of the element in component i out of total extracted, and n is an integer equal to 2 and k equal to 6 (i.e. the number effractions). The parameter IR was introduced to quantitatively describe the relative bonding intensity of metals in soils and enable the comparison of the binding intensity of a given metal among soils, and of different metals in the same soil. Thus, a low value of I^sub R^ (close to the minimum), represents a distribution pattern where much of the metal resides in the EXC fraction, whereas a high value(close to 1 ) results from a situation where a high proportion of the metal is bound in the RES fraction. Intermediate values represent various patterns involving metal partitioning among all the solid-phase components. Data were analyzed by one-way ANOVA procedures using SPSS software and significance were based on p
Results and Discussion
Cadmium and Pb Recoveries
For the discussion of the transformations of metals in the amended soils, it is necessary to assess (a) the recovery of added metals by the total analysis and by the sequential extraction procedure, and (b) the variability of extracted metals from each fraction with time, as compared to the variability of their sum with time.
The estimated recoveries (ER(%)) of spiked Cd and Pb in the soils are shown in Table 3. Slightly lower recoveries were observed for Pb (80-95% of added) as compared to Cd (95-110% of added) in all soils.
In general, the analyses by the sequential extraction procedure gave a reasonable degree of analytical reproducibility and metal recovery (sum). To assess reproducibility, the coefficients of variation (CV(%)= std variation *100/ average) were determined for the whole experimental period. The sum of extracted Cd had higher CV (%) values (15-25%) in all soils as compared to that of Pb (5-10%). This is attributed to the analytical problems related to at the low concentrations of Cd (data not shown). However, the CV(%) of Cd and Pb content in individual fractions were slightly larger than those of their sum, indicating significant Cd and Pb redistribution in three soils during the incubation period.
Table 3
Estimated recovery (RE(%)) ( average and standard deviation, 3 replicates ) of added Cd and Pb in 3 soils after 2 and 12 weeks of incubation
Table 4
Relative errors (RE), I^sub R^ and U^sub ts^ of Cd in the soils after 2 and 12 weeks of incubation
The relative errors (RE(%)) between the sum of metals extracted by the sequential extraction procedure, and their total content mostly ranged from -15 to 30 for Cd (Table 4) and from -15 to 15 for Pb (Table 5). On the other hand, RE (%) values for Cd were higher than those for Pb, likely due to the analytical problems in the AAS analyses, particularly at low concentrations.
Table 5
Relative errors (RE), I^sub R^ and U^sub ts^ of Pb in the soils after 2 and 12 weeks of incubation.
Soil pH and EC
Figure 1 shows the effects of OM and salinity treatments on pH in three soils for different incubation periods. Application of OM (O treatment) increased the pH in the acid soil, decreased it in the calcareous soil and did not change it in the neutral soil. The increase in the pH of the acid soil was due to the neutral pH and high buffering capacity of OM used. The decrease in the pH of calcareous soil was likely due to the increase of CO2 partial pressure as the result of microbial activities. The magnitude of pH changes was greater in the acid soil as compared to the calcareous soil.
Figure 1. Effects of treatments and incubation time on soils pH (S^sub 1^: acid soil, S^sub 2^: neutral soil, and S^sub 3^: calcareous soil).
NaCl treatment decreased pH both in the acid and calcareous soils and to a lower extent in the neutral soil. This is likely due to the electrolyte effect on the diffused double layer and the release of H+ into the solution. Except for the calcareous soil, pH values were slightly lower after 12 w of incubation as compared to 2 w of incubation. The neutral soil is a paddy soil and pH of these soils generally decreases under aerobic condition (Charlatchka and Cambier, 2000) due to the consumption of proton in the oxidation processes.
Application of 50 mmol NaCl / kg increased the EC about 2.3 to 2.7 dS/m as compared to the control (C) in the soils (Figure 2). Organic matter treatment also raised EC slightly in all soils and with the incubation time, probably due to the salt content of the plant residue and mineralization of organic matter during the incubation period.
Figure 2. Effects of treatments and incubation time on soils EC ( S^sub 1^: acid soil, S^sub 2^: neutral soil, and S^sub 3^: calcareous soil).
Cadmium
Distribution of Cd among various fractions as influenced by treatment type in the unspiked soil samples (after 2 and 12 w of incubation) is shown in Table 6. Total concentrations of unspiked Cd in soils 1 to 3, as calculated by summation of all fractions, were 1.72, 2.16 and 2.98 mg/kg, respectively. The overall distribution pattern of the unspiked Cd in the acid and neutral soils was the same (RES [much greater than] EXC > OM>/= AFeO > SS > MnO), and was somewhat different in the calcareous soil (RES > SS > OM > EXC > MnO > AFeO). This may show the positive effect of CaCOs on the specifically sorbed (SS) Cd and negative effect of soil pH on the EXC Cd. Percentage of total Cd in the RES fractions varied from 46% to 59% in the untreated soils. These results were consistent with the observations of Ma and Rao (1997), who found that the RES fraction was the most abundant pool for Cd and some other HMs in nine contaminated soils. Heavy metals in specifically sorbed fraction are generally associated with carbonates and greater percentage of Cd in the SS fraction in the calcareous soil probably reflects the effects of carbonates in specific sorption of Cd in calcareous soils as compared to the other soils (Table 1). In the acid soil, the concentration of unspiked Cd decreased in the EXC fraction and increased in the MnO and OM fractions with incubation time and resulted in IR index increase in the soil. In the neutral soil, a part of the unspiked Cd was transferred from RES fraction to MnO, AFeO and to a lesser extent to the OM fractions after three months. The highest reduction in partition index (I^sub R^) of the unspiked Cd was, therefore, found in the neutral soil and it was the lowest among the soils at the end of incubation period (Table 4). This may be attributed to a relative decrease of pH (Figure 1) and a sharp decrease in the EXC Mn and Fe (Table 7) with time in this soil.
When oxidized soils are submerged they may release HMs trapped by Fe and Mn oxides (Chuan et al., 1996; Charlatchka and Cambier, 2000). Phillips (1999) studied the sorption behavior of HMs in three soils under air-dried and waterlogged conditions. They found that the increased extractability of added Cu, Pb, Cd, and Zn was closely related to the increased solubility of Fe and Mn under reducing conditions. On the contrary, oxidized conditions may enhance formation of Fe and Mn oxides, thereby increasing HMs associated with them. These results suggested that decrease of moisture in a submerged soil may cause redistribution of Cd into fractions that are relatively less labile in soil and may remain in forms that are not readily available for plant uptake.
Salinity (NaCl) treatment increased significantly the unspiked Cd in the EXC fraction of the acid soil and OM application (O) increased exchangeable Cd only in the calcareous soil. Increasing concentrations of chloride (Cl) in soil solution markedly increased Cd concentrations in soil solution, probably due to the complexation of Cd by Cl and desorption of Cd from the soil solid phase (McLaughlin et al., 1997). Increased Cd concentrations in field crops grown in the saline soils have been attributed to the formation of chlorocomplexation of Cd (McLaughlin et al., 1994; Smolders and McLaughlin, 1996). The unspiked Cd concentration in the EXC fraction increased in the acid and calcareous soils and decreased in the neutral soil by applying both salinity and OM treatments (Cl+O).
The applied Cd was mainly associated with the SS, MnO, and to some extent with the EXC and AFeO fractions (Table 6). Han and Banin (1999) also found that the added Cd to arid-zone soils resided mainly in SS fraction after 1 year of incubation at a field capacity moisture content. I^sub R^ of Cd in the treated samples was significantly lower than the untreated soils, indicating a different distribution pattern of applied Cd as compared to the unspiked Cd (Table 4). During the incubation period, small amounts of added Cd were transferred from the SS and MnO fractions to the OM fraction in the acid soil. On the contrary, a decreasing trend in the EXC and SS and an increasing trend in the other fractions with tune were observed in the neutral soil. In addition, a decrease of Cd in the SS fraction and a partial increase of Cd in the MnO, AFeO, RES and OM fractions were observed in the calcareous soil. I^sub R^ of Cd increased slightly in the neutral and calcareous soils with incubation time.
Table 6
Effect of NaCl and OM treatments on the concentration Cd among different fractions of soils after 2 and 12 weeks of incubation
Table 7
Effect of NaCl and OM treatments on the soluble and exchangeable Fe and Mn during the incubation time
In both incubation periods, OM treatment decreased Cd in the EXC fraction and increased it in the SS and AFeO fractions in the acid soil, whereas it increased the EXC Cd and lowered the MnO and AFeO fractions in the calcareous soil. This may be due to the effect of OM treatment on soil pH increase in the acid soil and its decrease in the calcareous soil (Figure 1). The solid phase fractionation study by Bolan et al. (2003) on two slightly acid soils treated with various levels of Cd (0-10 mg/kg) indicated that the addition of biosolid compost decreased the concentration of the soluble and exchangeable Cd fractions but it increased the concentration of the organic bounded Cd fraction in soils. Organic matter application decreased the Cd in EXC and MnO fractions after 2 w of incubation in the neutral soil and increased it in the EXC and SS but not in the MnO fraction after 12 w of incubation. Amendment of soils with OM and subsequent decomposition of OM may change the soil pH and thereby indirectly affect the bioavailability of metals. He and Singh (1993) found that addition of peat to soils increased the DTPA extractable Cd due to a decrease in soil pH caused by peat application. Yuan and Lavkulich (1997) and Arnesen and Singh (1999) also found that the lowering of pH in a peat-amended soil decreased the sorption of Cd, Cu, Zn, and Ni in the soil. Results from fractionation and speciation analysis of soil Cd on two soils by Almas and Singh (2001) also showed that application of 4% OM (pig manure) to the soil could transfer Cd towards more bioavailable fractions. On the contrary, alkaline organic amendments may reduce the concentration of HMs in soil solution by raising soil pH, thereby allowing formation of insoluble metal precipitates, complexes, and secondary minerals (Knox et al., 2001). Basta et al. (2001) applied alkaline organic amendment (limestabilized biosolid) to some slightly acid soils and found that it increased soils pH as well as Cd in (SS) fraction (1 M NaCH^sub 3^COO extractable) and decreased Cd in the (EXC) fraction (Ca(NO^sub 3^)^sub 2^- extractable).
The salinity treatment raised the EXC Cd in both the calcareous and acid soils and lowered the Cd bounded to AFeO fraction in the acid soil. A slight decrease of IR of Cd by salinity treatment indicates a higher mobility of Cd in saline soils as compared to nonsaline ones. Soil treatment with NaCl may affect soil chemical properties in 3 different ways. Added Cl- may complex certain cations, such as Cd and Pb, the added Na+ may compete with some other cations for sorption sites, and increase in ionic strength of soil affects activity coefficient of ions in the solution and the degree of sorption preference of divalent over monovalent cations by the cation exchange complex (Khoshgoftar et al., 2004). The extend of salinity effect on exchangeable Cd diminished in the neutral soil, probably due to the higher initial ionic strength of this soil compared to the other 2 soils before NaCl treatment.
The highest whole-soil attainment of equilibrium (U^sub ts^) of the spiked Cd was observed in both acid and neutral soils and the lowest was found in the calcareous soil (Table 4). The point of interest was that during the incubation time, its values approached gradually the distribution pattern of the Cd in the untreated soils. The early low Uts in the calcareous soil indicated that the distribution of added Cd was nearly similar to that of untreated soil. Low U^sub ts^ and its relative stability during the incubation period indicate a lower mobility and likely bioavailability of Cd in calcareous soil as compared to the neutral and acid soils. Salinity and OM treatments increased the Uts in all soils.
Lead
The results of fractionation experiment for Pb are shown in Table 8. Total concentrations of Pb were 33.27,44.19 and 35.14 mg/kg in the acid, neutral and calcareous soils, respectively. The overall distribution pattern of the unspiked Pb among different fractions in the acid and neutral soils was RES [much greater than] AFeO > OM[asymptotically =]SS > MnO > EXC and in the calcareous soil was RES [much greater than] AFeO > SS > OM > EXC > MnO. The higher SS fraction of Pb in the calcareous soil as compared to the acid and neutral soils may indicate the role of CaCO^sub 3^ on specifically sorbed Pb. It showed that Pb in the untreated samples, similar to Cd, was mostly concentrated in the RES fraction, although the contribution of Pb in the EXC fraction was lower as compared to that of Cd. The percentage of total Pb in the AFeO fraction of soils (17- 31%) was much higher than that of Cd (5-12%). Lead in the untreated calcareous soil had a relatively small I^sub R^ value, likely due to the higher content of the SS fraction as compared to the humid-zone soils where larger fractions of Pb are bounded to OM and Mn-Fe oxides. This is in agreement with the results reported by Han and Banin (1997).
During the incubation period, Pb in the untreated acid soil transferred from the MnO fraction to the AFeO and OM fractions and in the calcareous soil from the EXC, AFeO and OM fractions to the MnO fraction. In the neutral soil, Pb increased in all fractions except in the RES fraction. I^sub R^ index of Pb, therefore, decreased only in the neutral soil, indicating higher mobility of Pb in this soil with incubation time. This may be due to the higher initial ionic strength of this soil as compared to the other 2 soils.
Salinity and OM treatments showed no significant effect on the Pb in untreated soils. A sequential extraction performed by Sloan et al. (1997) in two cultivated soils after cessation of sewage sludge application indicated that in control and biosolid-amended soil, 60 to 70% of Cd was found in the more easily extracted EXC and SS forms, but it had little effect on the EXC and SS fractions of Pb.
Addition of Pb changed the original distribution pattern of Pb in the soils (Table 5). In the acid soil, the spiked Pb was mainly distributed among AFeO, SS and to some extent to the MnO, OM and EXC fractions. But, during the incubation period, Pb decreased in the MnO fraction. In the neutral soil, the distribution pattern of added Pb was AFeO > SS > OM > MnO > EXC and at the end of incubation period, Pb increased in the OM and particularly MnO fractions, being different from the other soils. It seems that transformation of Mn oxides in this soil and the decrease of soluble and EXC Mn (Table 7) increased the amount of Pb bounded to MnO fraction with incubation time. In the calcareous soil, added Pb mainly resided in AFeO and SS fractions and transferred from the first three fractions to the AFeO fraction with time. In any case, Pb concentration in the first two fractions decreased in all soils during the incubation time and the Pb treatment, therefore, reduced the IR of Pb in all soils. The magnitude of the IR reduction was lowest in the calcareous soil and highest in the acid soil (Table 3).
In the calcareous soil, OM treatment raised Pb bounded to OM significantly and lowered Pb in the EXC and AFeO fractions, whereas in the acid soil, it lowered Pb significantly in the EXC, SS, and MnO fraction and raised it in the AFeO and particularly in OM fractions. This was in partial agreement with findings of Basta et al. (2001), who found the application of alkaline organic amendment to slightly acid soils increased soils pH and decreased Pb in the EXC and SS fractions. The EXC fraction did not change with OM application in the neutral soil. This may be due to the higher OM (Table 1) and EC (Figure 2) of this soil as compared to the other 2 soils. In this soil, only a significant increase was observed in the OM-Pb fraction.
Table 8
Effect of NaCl and OM treatments on the concentration Pb among different fractions of soils after 2 and 12 weeks of incubation
Salinity did not alter the EXC Pb significantly in the soils. It seems that Pb is generally unaffected or very little affected by NaCl. In the acid and calcareous soils, the OM treatment increased the I^sub R^ of Pb, indicating a lower bioavailability and mobility of Pb. The U^sub ts^ of Pb was highest in the acid soil and lowest in the calcareous soil, indicating high perturbation of metal partition in the acid soil. On the other hand, distribution of the added Pb was closer to that of Pb in the untreated calcareous and neutral soils as compared to the acid soil. The Uts increased significantly only in the acid soil by OM treatment at 2 w incubation time.
Conclusion
In the selective sequential extraction, all fractions are more likely to be defined operationally rather than chemically. It is recognized that in no case can an extractant remove all of a targeted solid-phase component without any attack on the other components. No selective sequential extraction scheme can be considered completely accurate in distinguishing between different forms of an element. Despite these shortcomings common to any chemical extraction procedure, sequential extraction techniques still furnish more useful information on metal binding, mobility, and availability than can be obtained with a single extraction. The present study showed that Cd and Pb applied to the soils in soluble form were slowly transferred and partitioned among the solid-phase component of the soil and its distribution pattern depended on the chemical nature of the element and soil properties. The applied Cd and Pb resided mainly in the SS and AFeO fractions, respectively. The I^sub R^ index of Cd was less than Pb in all soils, indicating the higher mobility and bioavailability of Cd in the soils. Fresh OM also lowered the lability and mobility of the added Pb. Effectiveness of OM on Cd was highly depended on pH variations of the soils. Salinity raised the lability of the Cd both in treated and untreated soils.
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ALI ABBASPOUR,1 MAHMOUD KALBASI,1 SHAPOOR HAJRASULIHA,1 AND AHMAD GOLCHIN2
1 Isfahan University of Technology, Isfahan, Iran
2 Zanjan University, Zanjan, Iran
Address correspondence to Ali Abbaspour, Department of Soil Science, Faculty of Agriculture, Isfahan University of Technology, Isfahan 84156, Iran. E-mail: abbaspour.a@ag.iut.ac.ir
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