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Cation influence on sulfate leaching in allophanic soils.

Introduction

Sulfur deficiencies in soils have been increasingly reported in many countries worldwide including New Zealand (Nguyen and Goh 1994; Schnug et al. 1995; Scherer 2001; Edmeades et al. 2005), especially since the reduction of air pollution enforced by 'clean air' legislation (Henderson 1996; Greenstone 2004). The use of non sulfur-containing nitrogen (N) or phosphorus (P) fertilisers, such as urea and di-ammonium phosphate, and the introduction of new high yielding and sulfur-demanding crop cultivars tend to intensify this problem. Sulfur deficiency in soils leads to yield losses and can influence crop quality, as well as increase plant susceptibility to some diseases (Zhao et al. 1999; Bloem et al. 2005; Klikocka et al. 2005; Rausch and Wachter 2005). In many cases, specific sulfur amendment to these soils is required.

Sulfate (S[O.sub.4.sup.2-]) is the most important inorganic form of sulfur in soils and is readily available to plants. It is applied to the soil through different available sulfur fertilisers, with different accompanying cations. Ammonium sulfate, potassium sulfate, and gypsum (dihydrated calcium sulfate), alone or as part of single superphosphate, are some of the important sulfur sources used in agriculture. Gypsum is used as a source of calcium and sulfate, and also as an amendment to improve soil structure (Bolan et al. 1993). Substantial sulfate losses via leaching can occur in soils with low retention capacity, and this has been observed in New Zealand (Edmeades et al. 2005). Pumice soils are an example of these low retentive soils, and they present the highest yield responses for sulfur fertilisation in New Zealand, in contrast, for example, with the variable-charge Egmont soils, which are more effective in the retention of sulfate (Edmeades et al. 2005).

The sulfate content in soils is in a dynamic equilibrium with other forms of sulfur, especially organic forms. This equilibrium may vary over the year (Tan et al. 1994). Nevertheless, because sulfate is the mobile form of sulfur, leaching is, in general, related to fertiliser input. It has been shown that sulfate adsorption can be influenced by the presence of other ions in the soil solution. The influence may be competitive for ions with the same charge, such as phosphate, or it can be cooperative for ions with opposite charge, such as calcium. The interaction also depends on the properties of the soil. Specific adsorption of anions may increase the soil net negative charge and thus allow further cation adsorption. This has been reported in many studies, especially involving phosphate (Ryden and Syers 1976; Bolan et al. 1988; Agbenin 1996; Bolan et al. 1999a, 1999b), or sulfate (Alva et al. 1990; Curtin and Syers 1990). Cations can also be specifically adsorbed and induce the retention of anions in soils (Bolan et al. 1993; Cajuste et al. 1998). Cooperative adsorption that occurs without changing the soil net charge has also been observed, especially in variable-charge soils (Marcano-Martinez and McBride 1989; Alva et al. 1990; Ajwa and Tabatabai 1995; Qafoku et al. 2000; Qafoku and Sumner 2002). In these cases, cation adsorption increases due to the presence of an anion, and vice versa. Because the amount of additional adsorption resulting from the co-adsorption process is equivalent to the ratio of the molar masses of the anion and cation, this process has also been called salt adsorption, or ion-pair adsorption (IPA) (Marcano-Martinez and McBride 1989; Qafoku and Sumner 2002).

IPA seems to be a phenomenon related to variable-charge soils, although not all studies with variable-charge soils indicate IPA. Even though it can occur with various ion combinations, IPA is more likely to happen with multivalent ions (Ajwa and Tabatabai 1995; Pearce and Sumner 1997). IPA has been identified, in particular, for calcium and sulfate (Marcano-Martinez and McBride 1989; Alva et al. 1990; Bolan et al. 1993; Davis and Burgoa 1995; Mora et al. 2005). Despite many studies describing the IPA phenomenon in soils, its influence on sulfate leaching has not been well explored yet. If the adsorption of both calcium and sulfate is influenced by each other, then their movement through the soil is also likely to be affected by these interactions.

To determine these interactions, we conducted a series of experiments in order to unravel the influence of cations on the movement of sulfate in 2 contrasting New Zealand soils. These soils are contrasting in relation to their variable-charge components and ion adsorption capacity. In this paper, we describe experimental observations from several miscible displacement experiments. Evidence of IPA in the variable-charge soil is shown, and its effect on sulfate leaching is determined and contrasted with the results from the low-adsorptive soil.

Material and methods

Soils

Two New Zealand soil types were used. One soil was Egmont loam (Typic Dystrandept), an allophanic soil from South Taranaki which is dominated by variable-charge components; and the other was Taupo soil (Typic Vitrandept), a low allophane-containing pumice soil from the Taupo region. The Egmont soil is a weathered soil of volcanic origin, containing allophane as the main clay component (Molloy 1988). It has a well-developed structure and the bulk density is usually <900 kg/[m.sup.3]. The weakly weathered and well-drained Taupo soil has almost no structure, and a bulk density ranging from 600 to 800 kg/[m.sup.3]. Soil samples were collected between the depths of 0.05-0.20 m. Some basic properties of the soils are presented in Table 1. To illustrate the influence of soil mineralogy on the IPA process, one additional experimental run was carried out with a Taupo soil with higher allophane content than the soil previously described.

Miscible displacement experiments

Breakthrough curves (BTCs) for calcium, potassium, chloride, and sulfate were measured in the leachate of 4 columns each, for Egmont (identified by letter E) and Taupo (identified by letter T) soils. The additional allophanic Taupo soil column was identified by [T.sub.a]. Each experiment was divided into 2 phases: a pre-leaching event, which aimed to minimise indigenous ions and attain a known soil solution composition; and a main leaching event for applying the appropriate sulfate solutions. Details of the solutions used in the experiment and characteristics of the soils columns are presented in Table 2. For Taupo soil, and 2 columns of Egmont soil, 2 different concentrations of the leaching solution were used, in order to investigate any influence of concentration on sulfate movement. For each concentration an intact soil and a repacked soil column were used with Taupo soil. [T.sub.a] was an intact sample. In the case of the Egmont soil, only repacked samples were available.

As single adsorption, and possibly IPA, were expected to occur in the Egmont soil, in addition to the leaching with CaS[O.sub.4] solutions, we used 2 other columns set up to determine the BTCs of sulfate and calcium when leached without (or minimising) the influence of each other. In these columns, [K.sub.2]S[O.sub.4] and Ca[Cl.sub.2] were used in the leaching solutions.

All leaching experiments were conducted under unsaturated conditions, at a pressure potential of -0.10 kPa, except for the allophanic Taupo soil ([T.sub.a]), which was at -0.05 kPa. The columns were 0.15 m high and had diameter of 0.10 m. The experiments were carried out following a procedure similar to that described by Magesan et al. (2003). The solution was applied using a disk permeameter placed in the top of the soil column (Fig. 1). The leachate solution was collected in small aliquots varying from 50 to 100 mL, the collecting containers were in a pressure controlled chamber. The pre-leaching solution (Table 2) was applied until at least 5 pore volumes (PV) of leachate was collected; after this the input solution was immediately replaced by the main leaching solution, and applied until another 5 PV of leachate was collected.

[FIGURE 1 OMITTED]

Soil and leachate analyses

The leachate aliquots were analysed for pH and the concentrations of sulfate, chloride, calcium, potassium, and magnesium. Similar analyses were carried for the soils before and after leaching. Sulfate was extracted using K[H.sub.2] P[O.sub.4] (0.01 M), and N[H.sub.4]OAc (1.0 M) was used to extract cations. Soil pH was measured in water. Calcium, potassium, and magnesium were determined using atomic absorption spectroscopy, sulfate was measured by the methylene blue method (Johnson and Nishita 1952), and chloride by the mercury thiocyanate-iron method (Florence and Farrar 1971).

To assess the performance of the analyses, since we used only 1 replication, the ion mass balance ([beta], %) was evaluated. [beta] was defined as the ratio between the sum of the final ion amount in the soil ([Q.sub.F]) plus the amount collected in the leachate ([Q.sub.L]), and the sum of the initial amount of ion in the soil ([Q.sub.I]) plus the amount added via leaching solution ([Q.sub.A]):

[beta] = 100 [Q.sub.F] + [Q.sub.L]/[Q.sub.I] + [Q.sub.A] (1)

Results and discussion

The mass balance for chloride, calcium, and sulfate for each leaching column is presented in Table 3. The recovery for all solutes was close to 100%, indicating good performance of the experimental procedure.

The BTCs for sulfate highlight the differences between the soils with respect to cation-induced sulfate leaching (Fig. 2). The BTCs for Taupo soil indicate negligible sulfate sorption for this soil, even though it was applied simultaneously with calcium. There was no effect of the concentration of the CaSO4 leaching solution on the shape of BTC. However, the spread of the solution, or its dispersivity, was greater for intact soil columns than for repacked ones. For the columns with Egmont soil where sulfate was applied with calcium (e.g. [E.sub.2]), the BTCs were noticeably delayed, indicating that sulfate adsorption has occurred (Fig. 2b). On the other hand, when using potassium as the accompanying cation ([E.sub.3]), sulfate was only slightly retarded (adsorbed) in the Egmont soil, and the BTC was almost identical to those obtained with the non-allophanic Taupo soil. Sulfate retention was also high in the allophanic Taupo soil (column [T.sub.a]), being even larger than in the Egmont soil (Fig. 2a). Differences in soil pH might also have influenced these variations in sulfate adsorption. It is known that adsorption capacity of variable charge components such as allophane is dependent on soil pH. However retention of sulfate in column [E.sub.3], leached with [K.sub.2]S[O.sub.4], presents only slightly higher retention compared with the Taupo columns, despite the differences in pH which would favour greater sulfate adsorption in Egmont soil. Therefore, the presence of calcium was the main contributing factor for the increase in sulfate retention.

[FIGURE 2 OMITTED]

Similarly, calcium behaviour differed between the two contrasting soils. No significant calcium adsorption occurred in the non-allophanic Taupo soil, and its BTCs followed predictable patterns, with the concentration dropping from the pre-leaching value to the final leaching level (Fig. 3a). Again column [T.sub.a] (allophanic Taupo soil) exhibited a different BTC, closer to those of Egmont soil. For the Egmont soil, only column [E.sub.4], which was leached without sulfate, exhibited a calcium BTC of similar shape to those of Taupo soil, although presenting a delay caused by the higher calcium adsorption in this soil (Fig. 3b). Those columns leached with CaS[O.sub.4] solution (e.g. [E.sub.2]) showed an additional calcium adsorption which caused a significant drop in the concentration of the leachate solution. This drop coincided with the decrease of chloride, and subsequent rise of sulfate concentrations. Measurements of magnesium and potassium confirmed that, at this stage, calcium was the only major cation present. Although column [E.sub.3] was pre-leached with more than 6 PV of potassium solution, a small concentration of calcium could still be measured in the output solution when the leaching (with sulfate) started. This concentration also dropped to almost zero before sulfate could reach the end of the column, indicating again interaction between calcium and sulfate.

[FIGURE 3 OMITTED]

These results suggest that there are cooperative interactions between sulfate and calcium in the Egmont soil that enhance their adsorption. Some authors (Bolan et al. 1993; Cajuste et al. 1998), suggested that specific adsorption of calcium could lead to an increase in positive charge, and therefore promote additional amount of anions, such as sulfate and chloride, to be adsorbed. However, the chloride BTC obtained from the column leached without calcium ([E.sub.3]), was nearly identical to that with calcium ([E.sub.2]) (Fig. 4). This indicates that there was no response by chloride to this possible charge enhancement, as would be expected in this case. Furthermore, the shapes of the calcium BTCs (Fig. 3) show that the adsorption of calcium was also noticeably influenced by the presence of sulfate.

[FIGURE 4 OMITTED]

In a similar way, sulfate-specific adsorption can be ruled out as the sole explanation for the observed increase in the adsorption of calcium. Specific adsorption of anions has been reported to occur in variable-charge soils, especially with phosphate (Ryden and Syers 1976; Bolan et al. 1988, 1999b; Agbenin 1996; Mora et al. 2005), but also occur with sulfate (Alva et al. 1990; Curtin and Syers 1990). However for column [E.sub.3], where the leaching solution was [K.sub.2]S[O.sub.4], the adsorption of sulfate was very small (Fig. 2a), and similar to those from Taupo soil.

Some specific adsorption of sulfate might also have occurred, however. The amount of adsorbed sulfate in column [E.sub.3], although very small, was enough to alter the pH of the soil solution (Fig. 5). A pH increase, probably caused by the release of OH- due to ligand-exchange sulfate adsorption, was observed when the chloride concentration was dropping and sulfate was being adsorbed. This would have caused an increase in negative charges, and subsequent adsorption of potassium, and its BTC showed a drop at the same time (Fig. 6). Columns [E.sub.1] (not shown) and [E.sub.2] also exhibited a peak in the pH measurements (Fig. 2), which coincided with the drop in the calcium concentration in the leachate. Column [E.sub.4] did not exhibit any significant alteration in the pH. This increase in negative charge does not seem to have been enough to explain the shape of the calcium BTCs, although it probably accentuated their shapes.

[FIGURES 5-6 OMITTED]

The mutual adsorption of sulfate and calcium is clearly not negligible, and needs to be considered when modelling sulfur movement in allophanic soils. Assuming a linear Freundlich adsorption isotherm, the BTCs of sulfate reveal a distribution coefficient, [k.sub.d], of 0.35 L/kg for column [E.sub.3] and about 1.20 L/kg for columns [E.sub.1] and [E.sub.2]. The leaching of sulfate could thus be largely over, or underestimated, if co-adsorption, or IPA, with the accompanying cation is not considered. Similarly, the leaching of calcium would also be poorly predicted.

The likelihood of IPA is also strongly supported by comparing the amounts of additional adsorption of calcium and sulfate in the columns. The difference between the final sulfate concentration of columns [E.sub.2] and [E.sub.3] represented an additional retention of 8.96 mmol/kg in column [E.sub.2], which had calcium as the accompanying cation (the same sulfate concentration was applied to both columns). This value is approximately the same as the difference of calcium concentrations between columns [E.sub.2] and [E.sub.4], namely 8.44 mmol/kg. Additional adsorption of equivalent amounts of an anion and a cation is one basic tenet of IPA theory (Marcano-Martinez and McBride 1989; Qafoku and Sumner 2002).

The fact that the pH of leachate presented almost no differences before and after simultaneously applying calcium and sulfate, and the equivalent amounts of additional retention of both ions provide strong evidence for IPA in the Egmont soil. The presence of allophane gives this soil its variable-charge characteristic, and this has been commonly related to IPA (Qafoku and Sumner 2002; Mora et al. 2005). In Taupo soil, almost without allophane, no additional adsorption was observed, whereas column [T.sub.a], containing this soil with significant allophane content, exhibited quite similar behaviour to the Egmont soil columns.

Studies of ionic interactions which result in IPA are still incipient and the mechanisms involved have yet to be clarified. Soil mineralogy, pH and soil ionic composition are the main factors with influence on sulfate adsorption and movement in allophanic and other variable charge soils. The role of these factors on IPA needs to be addressed. The influence of IPA on ion movement especially in variable charge soils can be large enough not to be ignored, and certainly still demands further studies.

Conclusions

Results from several miscible displacement experiments with 2 New Zealand soils, Egmont and Taupo, have revealed differences in the adsorption processes. In the case of the low allophane-containing Taupo soil, the adsorption process and the movement of sulfate were not affected by the solution composition. For this soil, the use of intact or repacked columns only changed slightly the dispersive spread of the solute.

In the Egmont soil, with higher allophane content, evidence of ion-pair adsorption (IPA) was found. Both sulfate and calcium adsorption increased significantly when applied simultaneously. Also the molar mass of the extra adsorption due to IPA was similar for both ions. For the Taupo soil, only the column with a significant allophane content showed similar behaviour. Therefore allophane seems to be a crucial component determining the ion retention capacity of these New Zealand soils. Our results also show that the IPA is likely to occur in allophane-containing soils. The extent of IPA can be large and should not be ignored when describing sulfur or calcium movement in such soils.

Acknowledgments

We wish to thank the Brazilian Government (through CAPES) for financially supporting this research (Process 1516-027), and Dr David Scotter for his comments on the preparation of this research.

Manuscript received 29 May 2006, accepted 30 November 2006

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R. Cichota (A,B,C), I. Vogeler(B), N. S. Bolan(A), and B. E. Clothier(B)

(A) Institute of Natural Resources, Massey University, PO Box 11222, Palmerston North, New Zealand.

(B) HortResearch, PO Box 11030, Palmerston North, New Zealand.

(C) Corresponding author. Email: rcichota@hortresearch.co.nz
Table 1. Selected properties of the experimental soils

Particle size measured by the pipette method; Allophane determined by
Al:Si ratio (Parfitt and Wilson 1985); organic matter (OM) by loss on
ignition method; CEC and calcium using N[H.sub.4] 0Ac (1.0 M)
extraction method (pH 7); and sulfate by K[H.sub.2] P[0.sub.4]
extraction (0.01 M)

Soil pH Sand Silt Clay Allophane
 ([H.sub.2]0) (g/kg)

Egmont 4.6 390 460 150 92
Taupo 6.3 820 110 70 24
Taupo 5.6 700 150 150 54
([T.sub.a])

Soil OM CEC Calcium Sulfate
 ([mmol.sub.c],/kg) (mmol/kg)

Egmont 110 343 36.8 1.75
Taupo 1 17 2.4 0.21
Taupo 16 59 5.4 1.80
([T.sub.a])

Table 2. Concentration of the leaching solutes and some physical
characteristics of the Egmont (E) and Taupo (T) soil columns used in
the miscible displacement experiments (columns [T.sub.a], [T.sub.1],
and [T.sub.3] were intact samples)

[rho], Soil bulk density; [theta], soil water content; WFPV, water
filled pore volume; [q.sub.w], water flux density in the leaching
stage

 Solution concentration (mol/L)

Column Pre-leaching Leaching

[E.sub.1] 0.0067, Ca[Cl.sub.2] 0.005, CaS[0.sub.4]
[E.sub.2] 0.0133, Ca[Cl.sub.2] 0.01, CaS[0.sub.4]
[E.sub.3] 0.03, KCl 0.01, [K.sub.2] S[O.sub.4]
[E.sub.4] 0.0133, Ca[Cl.sub.2] 0.01, Ca[Cl.sub.2
[T.sub.a] 0.0133, Ca[Cl.sub.2] 0.010, CaS[0.sub.4]
[T.sub.1] 0.0133, Ca[Cl.sub.2] 0.010, CaS[0.sub.4]
[T.sub.2] 0.0133, Ca[Cl.sub.2] 0.010, CaS[0.sub.4]
[T.sub.3] 0.0067, Ca[Cl.sub.2] 0.005, CaS[0.sub.4]
[T.sub.4] 0.0067, Ca[Cl.sub.2] 0.005, CaS[0.sub.4]

 [theta]
 [rho] ([m.sup.3]/ WFPV [q.sub.w]
Column (kg/[m.sup.m3]) [m.sup.3]) [cm.sup.3]) (mm/h)

[E.sub.1] 754.8 0.643 819.2 44.11
[E.sub.2] 754.5 0.607 763.2 29.86
[E.sub.3] 769.7 0.627 789.1 20.48
[E.sub.4] 752.4 0.638 807.9 34.30
[T.sub.a] 892.8 0.550 700.5 9.25
[T.sub.1] 859.6 0.465 580.9 41.87
[T.sub.2] 884.6 0.468 580.7 46.64
[T.sub.3] 839.8 0.454 578.6 33.23
[T.sub.4] 683.0 0.409 511.1 72.91

Table 3. Mass balance (%) for chloride ([Cl.sup.-]), calcium
([Ca.sup.2+]), and sulfate (S[0.sub.4.sup.2-]) for all soil columns of
the miscible displacement experiments on Egmont (E) and Taupo (T) soils

 [E.sub.1] [E.sub.2] [E.sub.3] [E.sub.4]

[Cl.sup.-] 101.6 103.4 106.7 n.a.
[Ca.sup.2+] 94.3 92.4 107.9 98.5
S[O.sub.4.sup.2-] 97.6 102.4 93.5 102.4

 [T.sub.a] [T.sub.1] [T.sub.2]

[Cl.sup.-] 102.6 102.1 104.5
[Ca.sup.2+] 92.8 100.8 100.5
S[O.sub.4.sup.2-] 95.4 91.7 92.9

 [T.sub.3] [T.sub.4]

[Cl.sup.-] 95.7 104.8
[Ca.sup.2+] 102.4 98.5
S[O.sub.4.sup.2-] 94.0 90.6

n.a., Not available.
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Author:Cichota, R.; Vogeler, I.; Bolan, N.S.; Clothier, B.E.
Publication:Australian Journal of Soil Research
Geographic Code:8NEWZ
Date:Feb 1, 2007
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