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Published online 2 June 2005
Published in Soil Sci Soc Am J 69:1036-1046 (2005)
DOI: 10.2136/sssaj2004.0148
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Soil Chemistry

Effect of Ammonium, Potassium, and Sodium Cations and Phosphate, Nitrate, and Chloride Anions on Zinc Sorption and Lability in Selected Acid and Calcareous Soils

Jim Jian Wang* and Dustin L. Harrell

Dep. of Agronomy and Environmental Management, 313 Sturgis Hall, Louisiana State Univ. Agricultural Center, Baton Rouge, LA. 70803

* Corresponding author (jjwang{at}agcenter.lsu.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Zinc availability and mobility in soils is controlled by its interaction with the soil matrix and amendments. Contradicting evidence has been reported for factors influencing Zn behavior in soils. This study was conducted to investigate the effect of common cations and anions on Zn sorption and lability characteristics. Zinc sorption isotherms were conducted on three acid and four calcareous soils in NH4+, K+, and Na+ and H2PO4–, NO3– and Cl– backgrounds. Lability of the sorbed Zn was evaluated by DTPA following sorption. Calcareous soils exhibited greater Zn sorption than did acid soils. Predicted Langmuir maxima for Zn sorption differed among the various ionic backgrounds. A majority of the total sorbed Zn (60–96%) was recoverable in the labile fraction. Both NH4+ and K+ equally decreased Zn sorption, as opposed to Na+ in acid and calcareous soils; however NH4+ yielded 4 to 12% more of sorbed Zn into the labile pool than did K+ in acid soils. Zinc sorption was enhanced by H2PO4– as opposed to Cl– or NO3– in acid soils, but it was decreased in three out of four calcareous soils. The effect of H2PO4– on the lability of the sorbed Zn in acid soils was similar to that of Cl– or NO3–, but in calcareous soils the phosphate held 10-25% more of the sorbed Zn in the nonlabile pool. It was concluded that even in calcareous soils, total Zn sorption could be impacted by phosphate–Fe oxide interactions. Furthermore, the effect of background ions on the lability of sorbed Zn varied between acid and calcareous soils. These results have important implications on Zn management in relation to other nutrients.

Abbreviations: b, Langmuir sorption maximum • CEC, cation exchange capacity • DTPA–TEA, diethylenetriaminepentaacetic acid–triethanolamine • ICP–OES, inductively coupled plasma–optical emission spectroscopy • k, Langmuir bonding energy constant • OM, organic matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
APPLICATIONS OF ZINC and other heavy metal-rich biosolids to agricultural lands have caused considerable concern about impacts on environmental soil and water quality (Alloway and Jackson, 1991; Moore et al., 1998). The importance of Zn as an essential micronutrient to plants and animals, as well as its occurrence as a toxic pollutant at high concentrations, has prompted Zn as a subject in various studies. Nonetheless, our current understanding of the process involved in Zn chemical reactions in soils, especially when applied with other fertilizer elements (inorganic sources or biosolids), is limited (Kabata-Pendias and Pendias, 2001).

Zinc mobility and bioavailability in soils is likely controlled by its sorption behavior (McBride, 1989). Zinc sorption capacity correlates with soil contents of aluminosilicate clays, metal oxides, and carbonates (Udo et al., 1970; Reddy and Perkins, 1974; Kalbasi et al., 1978; Brümmer et al., 1983). Aluminosilicate clays affect Zn sorption mainly through their effects on soil cation exchange capacity (CEC) (McBride, 1989). The interactions between Zn and Fe or Al oxides through the formation of a covalent bond with surface aquo and/or hydroxo groups have been thought to be the major mechanism of Zn retention by acid soils (Kalbasi et al., 1978; McBride, 1989), whereas Zn sorption by CaCO3 and precipitation of Zn hydroxides or Zn hydroxylcarbonates are the mechanisms controlling Zn solubility in calcareous soils (Papadopoulos and Rowell, 1989). Soil organic matter (OM) can influence Zn sorption behavior by forming both soluble and insoluble Zn-humic substance complexes, although the role of OM on Zn sorption appears to be less significant in soils relative to other heavy metals such as Cu and Cd (Elliot et al., 1986; Catlett et al., 2002).

Soil pH, solution ionic strength, and solution ionic composition also affect Zn sorption. Increasing soil pH increases the total number of negative sites of clay minerals and OM, and therefore increases the capacity for Zn sorption (Harter, 1983; McBride, 1989). In addition, pH influences Zn speciation in soils. An increase in pH from 4.8 to 7.1 in acid soils decreased exchangeable Zn from 42 to 2%, with an associated increase in inorganically complexed and oxide-bound Zn (Sims and Kline, 1991). Increasing ionic strength decreased Zn retention in acid soils but did not cause any significant effect on Zn sorption in calcareous soils (Elrashidi and O'Connor, 1982; Shuman, 1986). At a similar ionic strength, Ca2+ inhibited Zn sorption more effectively in sandy soils than did Mg2+ and K+ (Zhu and Alva, 1993). However, no difference was observed among Ca2+, K+, and Na+ ions in their ability to desorb soil Zn in Andepts (Pardo and Guadalix, 1996). Sulfate increased Zn sorption capacity in acid soils (Shuman, 1986), but had no effect on Zn sorption in calcareous soils as compared with NO3– and Cl– (Elrashidi and O'Connor, 1982). On the other hand, depending on accompanying cations, SO42– may have different effects on Zn retention (Pulford, 1986). Previous studies focused primarily on total Zn sorption, and little effort was made to reveal the fate of sorbed soil Zn under different chemical environments.

The interaction between Zn and fertilizer components such as NH4+ and PO43– salts or N- and P-rich biosolids is not well understood. Very little literature is available on how NH4+ affects Zn behavior in soil, although application of nitrogen as certain NH4+ salts has been reported to yield higher wheat tissue Zn content (Spratt, 1973). Although different ionic species may be present in soils (Kabata-Pendias and Pendias, 2001), free Zn ion is known to form a complex with NH3 (Evangelou, 1998), which could be generated from application of NH4+ salts or biosolids (Fenn and Hossner, 1985; Sims and Wolf, 1994). An antagonistic relationship between P and Zn was often observed for plant uptake (Burleson et al., 1961; Stukenholtz et al., 1966). While this antagonistic relationship was attributed by some to P toxicity rather than Zn deficiency (Stukenholtz et al., 1966; Loneragan et al., 1979), it is much less clear how P interacts with Zn in soils (Norvell et al., 1987). An early study reported conditions favorable for Zn-phosphate precipitation in aqueous solution, and suggested that similar reactions could also occur in soils (Jurinak and Inouye, 1962). However, this hypothesis was rejected based on a thermodynamic equilibrium evaluation (Lindsay, 1979). Other evidence showed that increasing soil phosphate did not affect soil Zn buffering capacity and only slightly affected Zn intensity (Norvell et al., 1987; Pasricha et al., 1987). Additional studies showed that treating soils with KH2PO4 changed existing soil Zn from a less-available form, such as that coexisting with Fe or Mn oxides, to a more available and exchangeable form (Shuman, 1988). In contrast, phosphate application reduced Zn mobility in agricultural and contaminated soils (Melton et al., 1973; McGowen et al., 2001). Clearly, contradicting evidence exists in the literature and further study is necessary to clarify P x Zn interactions in soils.

Recent studies show that many Louisiana soils are deficient in Zn (Harrell et al., 2004; Wang et al., 2004a). Early work reported that as high as 86.4% of total Zn in certain Louisiana soils was in a residual mineral fraction (Sedberry and Reddy, 1976). Because of the wide distribution of different soils, a clear understanding of Zn behavior, especially its lability, as impacted by other fertilizer elements, is necessary to better manage these soils. Therefore, the objective of this study was to investigate the effect of NH4+, K+, NO3–, H2PO4–, and other background ions on Zn sorption and lability in selected Louisiana acid and calcareous soils.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Samples and Basic Characterization
Surface samples (0–15 cm) of three acid and four calcareous soils collected from Louisiana were used for this study. The three acid soils included Calloway (fine-silty, mixed, active, thermic Aquic Fraglossudalfs), Crowley (fine, smectitic, thermic Typic Albaqualfs), and Dundee (fine-silty, mixed, active, thermic Typic Endoaqualfs). The four calcareous soils included Commerce (fine-silty, mixed, superactive, nonacid, thermic Fluvaquentic Endoaquepts), Jeanerette (fine-silty, mixed, superactive, thermic Typic Argiaquolls), Mer Rouge (fine-silty, mixed, superactive, thermic Typic Argiudolls), and Norwood (fine-silty, mixed, superactive, hyperthermic Fluventic Eutrudepts). The soil samples were air-dried and ground to pass through a 2-mm sieve before use. The chemical and physical characteristics of the soils are presented in Table 1. Soil particle size distribution was determined by the pipette method (Gee and Bauder, 1986), OM content by the Walkley-Black method (Walkley, 1947), and pH was determined at a 1:1 soil/water ratio. Soil exchangeable Ca, Mg, and K were determined by 1 M NH4OAc extraction (Thomas, 1982) and CEC by saturating the soil with 1 M NH4OAc at pH 7 followed by distillation and titration (USDA-NRCS, 1996). Soil P was extracted by 0.03 M NH4F-0.1 M HCl (Bray and Kurtz, 1945) and Fe, Mn, and Zn by DTPA-TEA [diethylenetriaminepentaacetic acid-triethanolamine: N,N-bis(2-(bis-(carboxymethyl)amino)ethyl)-glycine and 2,2',2-Nitrilotriethanol] (Lindsay and Norvell, 1978). Soil calcium carbonate content was determined by the gravimetric method (U.S. Salinity Laboratory Staff, 1954) and amorphous Fe by ammonium oxalate extraction in the dark (Loeppert and Inskeep, 1996).


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  		Table 1. Basic properties of surface soils for this study{dagger}.

 
Zinc Sorption Isotherms
The sorption experiment was conducted by equilibrating 2-g soil samples with 20 mL of a series of different electrolyte solutions containing 0, 0.5, 1.0, 2.0, 5.0, and 10.0 mM of ZnSO4 in 30-mL centrifuge tubes (preweighed) for 24 h on a reciprocating shaker. A relatively large initial Zn concentration range was used in this study so that the results would be also analogous to the situation of high water-soluble Zn levels resulting from certain biosolid applications (Epstein, 2003). A preliminary experiment indicated that the large initial Zn concentration range was also necessary to evaluate soils with high Zn sorption capacities. Five 50-mM electrolyte background solutions were used in the study, and they were KCl, KNO3, KH2PO4, NH4NO3, and NaNO3. The cation effect was evaluated by comparing Zn sorption in nitrate salts of Na+, K+, and NH4+, whereas the anion effect was evaluated in K+ salts of Cl–, NO3–, and H2PO4– backgrounds. Use of 50-mM background solutions was to ensure that effects of cations and anions were evaluated at a dominant and constant ionic strength. After equilibrium, samples were centrifuged and the supernatant was filtered through a Whitman No. 42 filter paper. Two replicates were used for collecting each data point. Zinc concentration in the supernatant was analyzed by ICP–OES (inductively coupled plasma–optical emission spectroscopy) at a wavelength of 213.8 nm. The amount of Zn sorbed was calculated by difference between the initial concentration and the final concentration after equilibration.

Zinc sorption parameters were obtained by fitting experimental data to the Langmuir model using the Proc nonlinear procedure with SAS statistical software package version 8.02 (SAS Institute, 1999):

[1]
where q is the sorbed Zn amount in mg kg–1, C is the equilibrium solution concentration in mg L–1, b is the sorption maximum, and k is the bonding energy constant in L kg–1 (Langmuir, 1918). A higher k indicates that a larger sorption of a solute by the solid surface exists at very low equilibrium solute concentrations (Olsen and Watanabe, 1957). Use of the Langmuir model offers a simple advantage in managing sorption data (Harter, 1984), even though reaction mechanism inferred from Langmuir fitting alone for a specific set of experimental data may need further independent confirmation (Veith and Sposito, 1977).

All pHs of initial experimental solutions, as well as soil-sorption solution mixtures after equilibration, were measured and presented in Table 2. A chemical equilibrium simulation using GEOCHEM-PC (Parker et al., 1995), based on published stability constants, was performed and the results indicated that there was no precipitation in the initial pure experimental solutions between Zn and background electrolytes at all Zn concentrations and pH ranges.


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  		Table 2. Measured range of pH values for initial experimental solution and for each soil-sorption mixture after 24 h equilibrium in different electrolyte solutions.

 
Lability of Sorbed Zinc
Availability status of sorbed Zn was evaluated at the end of sorption experiment by conducting a DTPA-TEA extraction (Lindsay and Norvell, 1978). A 20-mL DTPA-TEA solution was added to each centrifuge tube containing Zn-sorbed samples from the sorption experiment. The tubes were then shaken for 2 h on the reciprocating shaker to extract Zn. After a 2-hr extraction, the samples were centrifuged, and the supernatant solutions were filtered and analyzed for Zn content by ICP–OES. The amount of Zn extracted by DTPA-TEA at the end of Zn sorption experiment was corrected for Zn concentration in the interstitial solution based on preweighed tubes, soil weight, and added solution weight. The corrected DTPA-TEA extractable Zn was designated as the portion of sorbed Zn retained in the labile pool, whereas Zn unextractable by DTPA-TEA was attributed to soil Zn in the nonlabile pool. All sorption isotherm and DTPA-TEA extraction experiments were conducted at 295.2 ± 0.2°K.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Effect on Total Zinc Sorption
Data in Fig. 1 and 2 showed that both cations and anions had large effects on Zn sorption, especially at high Zn concentrations. The total amount of Zn sorbed within the experimental concentration range was larger in calcareous soils than in acid soils. Zinc sorption in calcareous soils was more dominated by specific site sorption, as suggested by generally larger k values than those in acid soils (Tables 3 and 4). Of the three cations, the greatest Zn sorption was in the Na+ background, followed by K+ and NH4+ backgrounds. There was little difference in total Zn sorption behavior in K+ or NH4+ background solutions. This trend held true for both acid and calcareous soils.



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  		Fig. 1. Zinc sorption on selected acid soils as affected by differing cationic and anionic background solutions. C = equilibrium solution concentration; q = sorbed zinc amount.

 


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  		Fig. 2. Zinc sorption on selected calcareous soils as affected by differing cationic and anionic cationic background solutions. C = equilibrium solution concentration; q = sorbed zinc amount.

 

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  		Table 3. Langmuir sorption isotherm parameters and correlation coefficient for total Zn sorption by selected acid and calcareous soils in differing cationic solutions of nitrate salts.

 

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  		Table 4. Langmuir sorption isotherm parameters and correlation coefficient for total Zn sorption by selected acid and calcareous soils in differing anionic solutions of potassium salts.

 
The differences in total Zn sorption caused by different cations were generally reflected in Langmuir sorption maxima for all soils except for Jeanerette (Table 3). The strong Zn sorption nature of the Jeanerette soil made it difficult to fit the Langmuir model and calculate b and k values for the Na+ background within the Zn experimental concentration range, although it is clear that Na+ background yielded the largest Zn sorption by the soil (Fig. 1). The large total Zn sorption in the Na+ background, as opposed to that in either K+ or NH4+ backgrounds, could be due to the less-competitive nature of Na+ with Zn for sorption sites as compared with K+ and NH4+. The Na+, because of its large hydrated ionic size, forms only outer-sphere complexes with clay mineral surfaces, whereas K+ and possibly NH4+ are able to form inner-sphere complexes with mineral surfaces (Sposito, 1984). The inhibitory effect of K+ on Zn sorption as compared with Na+ has been previously reported for acid sandy soils and Andepts (Zhu and Alva, 1993; Pardo and Guadalix, 1996). Our results (Fig. 2) clearly showed that K+ also reduced Zn sorption in calcareous soils, although the extent of this inhibitory effect appeared to be relatively small, possibly due to generally large Zn sorption capacity in calcareous soils. It is interesting to note that K+ and NH4+ greatly decreased the bonding energy parameter of Zn sorption in two out of four calcareous soils and one out of three acid soils (Table 3). This reduction in the Langmuir bonding energy constant could suggest that Zn sorption by specific site was somewhat restricted by the presence of K+ and NH4+ as opposed to the presence of Na+.

The NH4+ was thought to resemble K+ in cation exchange reactions in soils due to their similar ionic size and charge (Rich and Black, 1964; Sposito, 1984). Recent evidence, however, suggests that NH4+ and K+ have different impacts on the interlayer surfaces of certain clay minerals, such as vermiculites (Lumbanraja and Evangelou, 1990). The ammonium ion induced expansion, rather than collapse, of the interlayer of vermiculite as opposed to K+ (Lumbanraja and Evangelou, 1990). The expansion of the clay mineral interlayer by NH4+ would suggest higher Zn sorption in the presence of NH4+. Further, it is well known that application of NH4+ fertilizer may cause the evolution of NH3 even in slightly acid soils (Fenn and Hossner, 1985), and NH3 could complex with Zn as Zn(NH3)2+ (Evangelou, 1998). We did not characterize the clay mineralogy for the soils used in this study, but previous studies with similar soils showed that the acid soils contained appreciable quantities of vermiculites (Tassin, 1966; Wang et al., 2004b). The results shown in Fig. 1 and 2 indicated that the difference, if any, in NH4+ and K+ interaction with clay mineral interlayers or in complex with Zn2+ yielded very small difference in total Zn sorption, except for MerRouge soil where the Zn sorption was slightly larger in the presence of K+ than NH4+ at high Zn solution concentrations (Fig. 2). The latter was similar to that observed for Cd sorption in the presence of K+ as opposed to NH4+ by illite and bentonite (Evangelou, 1998). Nonetheless, the small or negligible difference in total Zn sorption as influenced by K+ and NH4+ appears to be well reflected in generally similar Langmuir sorption maxima and bonding energy parameters (Table 3).

The effects of anions on total amounts of Zn sorbed were different between acid and calcareous soils (Fig. 1 and 2). In acid soils, the largest Zn sorption was observed in the H2PO4– background followed by that in Cl– and NO3–, whereas in calcareous soils, the presence of H2PO4– decreased Zn sorption with the exception of the Commerce soil. The overwhelming increase in total Zn sorption in the H2PO42– background shown in Fig. 1 for acid soils is consistent with results reported for Hawaiian acid soils (Saeed and Fox, 1979). It has been shown that oxyanions such as PO43– and AsO43– can form inner-sphere surface complexes with iron and aluminum oxides that increase negative surface charges (Bolland et al., 1977; Bolan et al., 1999). The predominance of Fe and Al oxides in acid soils (Table 1) may explain our observation of higher Zn sorption caused by the presence of H2PO4– in acid soils.

It is interesting to note that for calcareous soils, the presence of H2PO4– decreased total Zn sorption in the MerRouge, Jeanerette, and Norwood soils, but not in the Commerce soil (Fig. 2). This result of H2PO4– lowering total Zn sorption differed from previous work which found that adding P reduced both the soluble Zn concentration and the Zn concentration ratio ([Zn2+]/[Ca2+ + Mg2+]) in alkaline and calcareous soils (Norvell et al., 1987). In the present study, the result of lowering Zn sorption by H2PO4– in calcareous soils may be explained by the pH difference between H2PO4– and the other electrolyte solutions. The pH ranges of soil-sorption solution mixtures were generally similar for a soil among NH4NO3, NaNO3, KNO3, and KCl, except for KH2PO4 (Table 2). This was especially true in calcareous soils. Strong buffering by KH2PO4 maintained relative lower pH ranges in its soil–solution mixtures than the other electrolytes. We propose that the strong acidity of H2PO4– could dissolve a certain quantity of CaCO3 in these soils, which could not only decrease the total carbonate surface for Zn sorption but also release more Ca ions to compete with Zn for surface sites of aluminosilicate clays (McBride, 1989; Zhu and Alva, 1993). In addition, increased H+ may also compete with Zn for negatively charged clay surface sites. All these could constitute the cause for low total Zn sorption observed for Jeaneratte, MerRouge, and Norwood soils, which contained 3.8, 0.9, and 3.8% CaCO3, respectively (Table 1).

The Commerce soil, which contained 1.2% CaCO3, behaved differently from other calcareous soils; however, it contained eight to 10 times more amorphous oxide-Fe than the other three calcareous soils (Table 1). Note also the relationship between the amount of total Zn sorption (at the highest initial Zn concentration) and the amorphous Fe as shown in Fig. 3 for soils used in this study. The regression equation (Y = 914.3X + 3306, R2 = 0.988) obtained for the three acid soils indicated that approximately 98.8% of the variation in total sorbed Zn could be explained by the amorphous Fe oxide content. The calcareous Commerce soil apparently followed this relationship for the acid soils whereas other calcareous soils, Jeanerette, MerRouge, and Norwood, did not. Clearly, the integrated effect of very high amorphous Fe oxide content and low CaCO3 content caused an increased, rather than decreased, Zn sorption by the presence of H2PO4– in Commerce soil. This result has an important implication. It means that even in calcareous soil, the interaction between P and Fe oxides can still significantly influence Zn sorption characteristics. This interaction could be a major mechanism in controlling Zn mobility in calcareous soils with high Fe oxide contents. It may also explain some of contradictory behavior observed for P x Zn interactions for calcareous soils reported in previous studies (Adams, 1980; Norvell et al., 1987).



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  		Fig. 3. Relationship between the total Zn sorption (at the highest initial Zn concentration in KH2PO4 background) and the amorphous Fe content for acid and calcareous soils. q = sorbed zinc amount.

 
Langmuir sorption parameters showed that in acid soils, the presence of H2PO4– increased Zn sorption maxima as well as the bonding energy value (Table 4). The latter indicated that H2PO4– enhanced specific Zn sorption as opposed to Cl– or NO3– background. In calcareous soils, H2PO4– decreased specific Zn sorption, as seen by the large drop in k values except for amorphous-Fe-oxide-rich Commerce soil. It is interesting to note, however, that Langmuir sorption maxima were actually increased by H2PO4– as opposed to Cl– and NO3–, although within the experimental Zn concentration range, Zn sorption was clearly decreased by H2PO4– in Jeanerette, MerRouge, and Norwood soils (Table 4 vs. Fig. 2). It should be pointed out that the Langmuir model was employed to predict the overall sorption capacity beyond experimental concentration range. Limitations of using the Langmuir model have been discussed elsewhere (Veith and Sposito, 1977; Harter, 1984). Nonetheless, a small k bonding energy constant generally yields a less-sharp initial slope of the sorption curve, which is consistent with a low-affinity (nonspecific) sorption (Langmuir, 1918; Harter, 1984). The results (Fig. 2 and Table 4) clearly demonstrate the impact of H2PO4– (as opposed to Cl– and NO3–) on the shape and description of Zn sorption isotherms using the Langmuir model. Although the intrinsic sorption mechanisms involved may require further examination, the Langmuir parameters indicated that the presence of H2PO4– may increase the nonspecific sorption capacity of Zn in calcareous soils.

There was no difference in Zn sorption caused by the presence of Cl– and NO3– (Fig. 1 and 2). This result was true for both acid and calcareous soils. The similar effect of Cl– and NO3– on total Zn sorption was consistent with those reported by others for an alkaline sandy soil (Elrashidi and O'Connor, 1982) and for two acid coastal plain soils (Shuman, 1986). On the other hand, Zn sorption on goethite was enhanced by Cl– and not by NO3– at low pH conditions, possibly because of specific sorption of Zn-Cl complex ions (Padmanabham, 1983; Shuman, 1986). In the present study for acid soils, there was no difference between the Cl– and NO3– effect in the specifity of sorption as seen by equal Langmuir bonding energy parameters for acid soils (Table 4). For calcareous soils, Cl– did increase k values as compared with NO3– for the Jeanerette, MerRouge, and Norwood soils, but not for the Commerce soil. Predicted Zn sorption maxima by the best-fit Langmuir model were similar in general for all acid and calcareous soils in the presence of Cl– and NO3–, although some slight differences existed (Table 4).

Effect on Lability of Sorbed Zinc
The sorbed Zn was partitioned into labile and nonlabile pools by extracting with DTPA-TEA at the end of sorption experiment. The DTPA-TEA extraction was originally developed for extracting plant-available Zn in neutral and alkaline soils (Lindsay and Norvell, 1978). Analysis using isotopic exchange and dilution techniques showed that this extraction was able to characterize the labile pool of Zn in both acid and calcareous soils (Sinaj et al., 1999). For this reason, we chose DTPA-TEA extraction to fractionate the total sorbed Zn into labile and nonlabile pools. The results of this fractionation in the presence of different background electrolytes for the acid Dundee and calcareous Norwood soils are shown in Fig. 4 and 5 , respectively. Similar results were also observed for other acid and calcareous soils (data not shown).



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  		Fig. 4. Sorption isotherms for total, labile, and nonlabile Zn sorption for an acidic Dundee soil as affected by different background cationic and anionic species. C = equilibrium solution concentration; q = sorbed zinc amount.

 


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  		Fig. 5. Sorption isotherms for total, labile, and nonlabile Zn sorption for a calcareous Norwood soil as affected by different background cationic and ionic species. C = equilibrium solution concentration; q = sorbed zinc amount.

 
The majority of total sorbed Zn was labile (as defined by DTPA-TEA extraction for 2 h of equilibration; Fig. 4 and 5), suggesting that much of the sorbed Zn was readily bioavailable. This was true for all soils regardless of what background ions were present, although the amount of labile Zn differed among certain ionic backgrounds. On average, about 60 to 96% of the total sorbed Zn by all soils was labile (Table 5). The effect of the three cations on the lability of the total sorbed Zn was very similar in calcareous soils, whereas in acid soils, NH4+ appeared to yield a slightly higher percentage of labile Zn (by 4 to 12% higher) than K+ (Fig. 4 and Table 5). The latter result was unlikely due to the pH effect since NH4+ and K+ salts yielded generally similar pH values in soil-solution mixtures (Table 2). It is known, however, that these acid soils contain predominately fully and partially expandable clay minerals such as montmorillonite and vermiculite (Tassin, 1966; Wang et al., 2004b). Since the hydrated ionic size of Zn is smaller than those of K+ and NH4+, the difference between NH4+ and K+ in influencing interlayer expansion or collapse of these minerals, as suggested by Lumbanraja and Evangelou (1990), could affect the accessibility of sorbed Zn for DTPA-TEA extraction. Incorporation of Zn into hydroxyl-Al interlayered phyllosilicates was recently confirmed in an acid subsoil of a contaminated site by x-ray absorption spectroscopy (Scheinost et al., 2002). The results in Table 5, along with those shown in Table 3 and Fig. 1 and 2, indicate that although K+ and NH4+ did not differ in competing with Zn for total sorption sites, they may differ in their affect on the lability of sorbed Zn in acid soils.


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  		Table 5. Average percentage (mean ± SE) of labile pool of the total sorbed Zn in the presence of different cations and anions.{dagger}

 
The difference among the three anions in influencing lability of total sorbed Zn was small (<8%) in acid soils but relatively large (>10%) in calcareous soils (Table 5). In general, the presence of H2PO4– caused 10 to 25% more nonlabile Zn than Cl– or NO3–, depending on the individual calcareous soil. This result is very interesting, considering that within the experimental concentration range, the presence of H2PO4– decreased initial Zn sorption in three out of four calcareous soils (Fig. 2). This suggests that although total Zn sorption was decreased due to lower pH effect of H2PO4–, the sorbed Zn was less labile and possibly less available for plant uptake. Nonlabilty of a metal ion in soil has been generally attributed to the entrapment of the metal into small pores of particle aggregates or clay mineral structures, the strong surface adsorption by oxides, and/or the formation of insoluble metal precipitates (McBride, 1989; Hamon et al., 2002). All of these reactions (especially the latter) may account for the loss of lability in certain fractions of sorbed Zn in the present study.

It should be pointed that, in this study, we did not measure carbonate ions in soil-solution mixtures for both acid and calcareous soils. It is possible that carbonate ions in calcareous soils with low P concentrations could react with Zn to form ZnCO3 precipitates (Papadopoulos and Rowell, 1989). However, it is unlikely that ZnCO3 would be present as a stable compound even in calcareous soils since it is generally more soluble than soil-Zn, an undefined Zn mineral (Lindsay, 1979). Precipitation as Zn hydroxylcarbonate, Zn5(OH)6(CO3)2, was proposed in calcareous soils (Papadopoulos and Rowell, 1989). Other research suggests that the formation of Zn-Fe precipitates such as ZnFe2O4 (franklinite) may occur in both acid and calcareous soils (Pulford, 1986; Catlett et al., 2002; Scheinost et al., 2002). In the presence of H2PO4–, we speculate that the increase in the nonlabile fraction of sorbed Zn in calcareous soils may be largely due to the formation of Zn-phosphate precipitates such as Zn3(PO4)2 · 4H2O (hopeite). This was indirectly supported by the results of thermodynamic equilibrium modeling using GEOCHEM-PC based on initial Zn and H2PO4– concentrations and the observed pH ranges of sorption soil-solution mixtures for the calcareous soils. However, it may not be valid to assume equilibrium without further defining solid surfaces in the system (McBride, 1989; Parker et al., 1995). Earlier, Lindsay (1979) pointed out that the formation of hopeite was not likely because H2PO4– could be fixed by strengite-soil-Fe at low pH and by CaHPO42H2O and other Ca-phosphates or carbonates at higher pHs. However, recent evidence suggests that in Zn-contaminated soils the addition of KH2PO4 may indeed lead to the formation of hopeite (McGowen et al., 2001) and an increase in the nonlabile pool (irreversible sorption) of soil Zn as confirmed by isotopic exchange and dilution techniques (Hamon et al., 2002). The results of the present study using Zn and P concentrations commonly found in localized soil environments with or without applications of inorganic P and/or Zn fertilizers or P- and Zn-rich biosolids are consistent with such studies.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The present study clearly demonstrates various effects of background K+, Na+, NH4+ cations and H2PO4–, Cl–, NO3– anions on Zn sorption characteristics and lability in acid and calcareous soils. The labile fraction of total sorbed Zn does not necessarily correspond to the amount of total Zn sorption observed in the presence of these ions, with important implications for managing nutrient interactions. The presence of NH4+ or K+ similarly reduced total Zn sorption as compared with Na+. Ammonium, however, was able to maintain slightly more sorbed Zn in the labile pool as opposed K+ in acid soils. The latter suggests that, in acid soils, a more efficient application of Zn would occur with co-application of NH4, not K, salt fertilizers. The presence of H2PO4– enhanced Zn sorption in acid soils but decreased Zn sorption in three out of four calcareous soils when compared with Cl– or NO3– backgrounds. Higher total Zn sorption caused by H2PO4– in one calcareous soil, Commerce, was attributed to its high amorphous Fe oxide content. This result indicates that even in calcareous soils, Zn x P interaction could be greatly influenced by Fe oxides. Furthermore, although in acid soils total Zn sorption was enhanced by the presence of H2PO4–, the sorbed Zn was generally labile, implying a greater mobility and availability. On the other hand, although in three out of four calcareous soils total Zn sorption was decreased by H2PO4–, a larger percentage of the sorbed Zn was associated with the nonlabile pool. Overall, the presence of these various cations and anions changed both the capacity and specificity characteristics of Zn sorption in acid and calcareous soils.


   ACKNOWLEDGMENTS
 
This work was supported in part by the Louisiana Soybean and Grain Research and Promotion Board and the American Sugar Cane League.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution of Louisiana Agric. Exp. Stn. Journal No. 04-14-0207 and is published with the approval of the Director.

Received for publication April 26, 2004.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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