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Mechanisms of potassium release from calcareous soils to different salt, organic acid and inorganic acid solutions.

Introduction

Potassium (K) is an important and necessary nutrient for plant growth and development and its behaviour in soil is of great interest to soil researchers. Soil K exists as soluble, exchangeable, non-exchangeable (NEK) and mineral forms (Sparks 2000). The mean contents of these K forms in calcareous soils of Iran are 20, 244, 763 and 5300mg [kg.sup.-1] respectively, with a total content of 6300 mg [kg.sup.-1] (Najafi Ghiri et al. 2011c). Although soil available K in calcareous soils of Iran is sufficient, some researchers recently showed that its content has decreased due to extensive cropping with no K fertiliser application (Balali and Malakouti 1998).

Potassium release from K-bearing minerals and its fixation are two important processes of soil K in calcareous soils. The release of NEK from illite, vermiculite and smectite may increase the availability of K, while fixation of K ions by smectite, vermiculite and illite decreases its availability (Najafi Ghiri et al. 2011 a; Najafi Ghiri and Abtahi 2012). Many researchers have studied the release and fixation of K and factors affecting these processes in calcareous soils (Jalali 2006; Najafi Ghiri et al. 2011ft; Hosseinpur et al. 2012). Potassium released to different solutions including salt solutions (e.g. dilute Ca[Cl.sub.2]), inorganic acids (e.g. HCl, [H.sub.2]S[O.sub.4] and HN[O.sub.3]), organic acids (e.g. oxalic, citric, acetic and malic) and sodium tetraphenyl boron may vary widely and depends on different factors such as content and type of clay minerals, mineral size, calcium carbonate (CaC[O.sub.3]) and quartz contents, soil development and weathering status (Gil-Sotres and Rubio 1992; Cox and Joern 1997; Rao et al. 1999; Tu et al. 2007; Najafi-Ghiri and Jaberi 2013).

Different solutions extract K from soils via exchange reaction and/or mineral dissolution (Kong et al. 2014; Ramos et al. 2014). Some extractants like salt solutions and sodium tetraphenyl boron may release K via interlayer exchange, but inorganic acids may have different mechanisms for K extraction. Some researchers concluded that inorganic acids extract K from K-bearing minerals via exchange reaction of H+ with [K.sup.+], while others indicated that minerals may be dissolved by attack of [H.sup.+] on the mineral structure and its dissolution (Gil-Sotres and Rubio 1992; Voinot et al. 2013; Li et al. 2015). However, mechanisms of K release in calcareous soils may differ because of the occurrence of Ca and magnesium (Mg) carbonates that affect the extracting solutions in the short term. However, neutralisation of carbonates over longer periods may change the K release mechanisms.

Different organic acids are produced in soils by microorganisms, plant roots and organic material decomposition, including oxalic, citric, malic, succinic, malonic and maleic acids (Adeleke et al. 2017). Oxalic and citric acids are important organic acids in soils, and affect soil mineral dissolution and K release. These acids have different ability to extract K from soils due to their differences in molecule size, negative charge density, functional group composition, stability and reaction with soluble ions (like Ca) and thereby have different mechanisms for K extraction including exchange reaction or mineral dissolution (Strobel 2001; Strom et al. 2002; Gang et al. 2012; Andrade et al. 2013; Ramos et al. 2014).

Factors affecting K fixation by calcareous soils include content and type of minerals, mineral size, K depletion intensity and wetting-drying and freezing-thawing cycles (Dhaliwal et al. 2006; Murashkina et al. 2007; Najafi Ghiri and Abtahi 2012, 2013). Calcareous soils may fix more than 500 mg [kg.sup.-1] of the added K (Najafi Ghiri and Abtahi 2012). However, depletion of K in soils with intensive agriculture may affect K-fixation capacity of soils. Awareness of K-fixation capacity in soils treated with different acidic solutions may be important for soil K-fertility management because acidic solutions may dissolve carbonates and K-bearing minerals in calcareous soils.

Little or no information is available about the effect of different inorganic and organic acids on K release from calcareous soils in short- and long-term experiments. The K release from mineral edges occurs in short-term soil extraction (3-5 extraction periods equals 50-100 min of extraction), while interlayer and structural K are released in longer extraction periods (Najafi Ghiri et al. 2011a). We hypothesise that interaction between soil carbonates and extracting solutions in short-term experiments is considerable and the role of acidic solutions in minerals dissolution is negligible; but with long-term treatment of soil with acidic solutions and decrease of carbonate content (due to neutralisation with H+), dissolution of K-bearing minerals becomes more important. These different mechanisms of action of acidic solutions with carbonates and minerals may affect K release and fixation status of calcareous soils.

The main objectives of this investigation were: (1) comparison of four solutions including Ca[Cl.sub.2], HC1 and oxalic and citric acids in extraction of K from four calcareous soils with divergent mineralogy and carbonate contents, (2) study of the mechanism of K release to these extractants in short- and long-term experiments and (3) comparison of K-fixation capacity of soils treated with these extractants after shortand long-term experiments.

Materials and methods

Soil collection and experiments

After a detailed soil survey, four soil series with different physicochemical and mineralogical properties were collected from different regions of Fars Province, Iran. These soils belong to Alfisols, Vertisols, Aridisols and Entisols. For each soil series, a composite soil sample (~50 kg) was collected randomly from eight points by an auger at depth of 0-30 cm (Ap horizon). The soil samples were air-dried, crushed and sieved (<2 mm) for routine analysis. Particle size distribution of soil samples was determined with the hydrometer method (Gee and Or 2002). Organic carbon was determined by wet oxidation with chromic acid and back titrated with ferrous ammonium sulfate (Nelson and Sommers 1996). Calcium carbonate equivalent (CCE) was determined by titration with HC1 (Loppert and Suarez 1996). Soil pH was determined in saturated soil paste (Thomas 1996). Electrical conductivity of soil samples was determined in saturated extract (Rhoades 1996). Cation exchange capacity was determined with use of sodium acetate at pH 8.2 (Sumner and Miller 1996).

Different forms of K were determined according to the methods of Helmke et al. (1996). Water-soluble K was determined in the saturated extract. Distilled water was added to the soil sample until the saturated extract was obtained and the suspension was kept for 2h to reach equilibrium. Then a soil specimen was used for soil moisture determination after oven drying at 110[degrees]C for 24 h. Soil solution was extracted with No. 42 Whatman paper and a vacuum pump. The K concentration was determined in the clear solution and actual content of soluble K (w/w) was calculated according to the soil moisture content. The content of exchangeable K was determined after four extractions of soil sample with 1 mol[L.sup.-1] ammonium acetate (N[H.sub.4]OAc) (pH = 7.0). The NEK was determined by extraction of the soil sample with boiling 1 mol[L.sup.-1] HN[O.sub.3]. Total K was determined following digestion (at 110[degrees]C) of the soil sample with HF and aqua regia. The K was measured in all filtrated extracts by flame photometer (Coming 405, ELE, UK).

The K-fixation capacity of soil samples was determined using the methods of Najafi Ghiri and Abtahi (2012). For this, 30 mL of 100 mg[L.sup.-1] K (as KC1) was added to 3 g of untreated soil sample (equivalent to 1000mg K [kg.sup.-1] soil) and shaken for 24 h. Then soil samples were extracted for K three times with 1mol[L.sup.-1] N[H.sub.4]OAc. This method was performed with another set of the soil samples after addition of 20 mL of distilled water (control set). The K concentration was determined by flame photometer (Corning 405). The K-fixation capacity was calculated as follows:

K fixed = added K + N[H.sub.4]OAc--extractable K of control sample--N[H.sub.4]OAc - extractable K after K addition (1)

For mineralogical analysis, soil samples were treated with 1 N sodium acetate (pH 5), 30% [H.sub.2][O.sub.2] and citrate dithionate in a water bath at 80[degrees]C for removal of carbonates, organic matter and free Fe oxides respectively (Mehra and Jackson 1960). The sand, silt and clay fractions were separated by wet sieving and repeated sedimentation and decantation. The soil fractions were saturated with KC1 and Mg[Cl.sub.2]. The Mg saturated samples were glycolated. The K saturated samples were heated to 550[degrees]C. The samples were X-rayed with a Philips D500 diffractometer using Ni-filtered CuKa radiation (40 kV, 30 mA). The content of minerals in soil fractions was determined semiquantitatively according to Johns et al. (1954).

Potassium release experiments

For removal of soluble and exchangeable K, soil samples were equilibrated with 1 mol[L.sup.-1] Ca[Cl.sub.2] for 24 h. Then soil samples were extracted with deionised water and ethanol 95% for removal of excess Ca[Cl.sub.2]. The soil samples were oven-dried at 60[degrees]C for 10 min (for removal of excess water) and then airdried. This fast drying can inhibit the short-term release of K before initiating the experiment (Najafi Ghiri et al. 2011a).

Three grams of the Ca-saturated soil sample was equilibrated in triplicate with 30 mL of 0.025 mol[L.sup.-1] Ca[Cl.sub.2], HCl, oxalic acid or citric acid solutions and shaken on a reciprocating shaker for 15 min and centrifuged. The K concentration was determined in the clear solution by flame photometer. This method was repeated three times (short-term experiment equal to 45 min of extraction) after addition of the new portion of extractant solutions. The concentrations of Ca and Mg were also determined in the clear solutions (Suarez 1996). Aluminium (Al) concentration in the solutions was determined with atomic absorption spectrophotometer (PG 990, PG Instruments Ltd, UK).

Another set of soil samples was also extracted in triplicate in the long-term experiment (ten stages equal to 150 min of extraction) with the used solutions and K, Ca, Mg and Al concentrations in each extraction stage were determined in the clear solutions by the mentioned methods.

Potassium fixation experiment

The K-fixation capacity of the soils was determined immediately after the short- and long-term experiments. For this, 30 mL of 100mg[L.sup.-1] K as KC1 (equivalent to 1000mg [kg.sup.-1]) was added to the treated soil samples and shaken for 24 h. Then soil samples were extracted for K three times with 1 mol [L.sup.-1] N[H.sub.4]OAc. Another set of soil samples was similarly extracted with 0.025 mol [L.sup.-1] Ca[Cl.sub.2], HCl, oxalic acid or citric acid for three or 10 times and then shaken for 24 h after addition of 30 mL of distilled water (control set) and extracted with 1 mol [L.sup.-1] N[H.sub.4]OAc. The extractant was measured for K by flame photometer (Corning 405). The K-fixation capacity was calculated according to Eqn (1).

Statistical analysis

Statistical analysis of data was carried out using SPSS 20.0 software (SPSS Inc., Chicago, IL, USA), and all statistical tests were considered significant at P<0.05. The normality test of the data was performed using kurtosis and skewness determinations. The data were transformed when necessary. To determine the significant differences, paired-sample t-tests were performed on cumulative K release and K-fixation capacity between two groups (short/long-term experiments). For multiple comparisons of extractant treatments, analysis of variance and Tukey's HSD post hoc test were used to determine the significance of variables between different groups. The Pvalue adjustments for multiple comparisons were performed using the false discovery rate correction (Benjamini and Hochberg 1995).

Results and discussion

Some properties of the studied soils are shown in Table 1. The selected soils had different characteristics. Clay content ranged within 11-51%. All soils were calcareous, slightly alkaline and CCE ranged within 3-56%. Different forms of K are also indicated in Table 1. Generally, total K contents in the soils were 5000-8800 mg [kg.sup.-1] and soluble, exchangeable, nonexchangeable and mineral K constituted 0.4-0.27, 3-6, 15-20 and 73-82% of total K respectively. The K-fixation capacity of the soils ranged within 127-368 mg [kg.sup.-1], and the highest value was in the Alfisol with high clay and smectite content. According to the Soil Moisture and Temperature Regime Map of Iran (Banaei 1998), the moisture and temperature regimes of Alfisols, Vertisols, Aridisols and Entisols were xeric-mesic, xeric-mesic, aridic-thermic and ustic-hyperthermic respectively. The studied soils were under wheat cultivation. Mineralogical investigation (Table 2) indicated that all soils had similar minerals in the clay and silt fractions. Smectite, illite, chlorite and palygorskite were the main minerals in the clay fraction; and the silt fraction was dominated with quartz and minor contents of smectite, illite, chlorite and feldspar. Smectite constituted more than 50% of the minerals in the clay fraction of the Alfisol and Vertisol. The minerals found in the studied soils were similar to those in previous investigations for calcareous soils of Iran (Khormali and Abtahi 2003; Owliaie et al. 2006; Najafi Ghiri et al. 2011a; Abbaslou et al. 2013).

To study the effect of different extractants on clay mineral and soil carbonate dissolution (due to the importance of carbonates in the studied soils), the contents of Ca, Mg, Al and K extracted by Ca[Cl.sub.2], HC1, citric acid and oxalic acid solutions were determined during the short- and long-term experiments (Table 3). For all soils, there was no significant difference between Ca and Al concentrations in Ca[Cl.sub.2] and HC1 solutions in the short-term experiment. The lack of difference between Ca concentration in Ca[Cl.sub.2] and HC1 solutions may be due to the effect of HC1 on soil carbonate dissolution and increase in soluble Ca concentration. No difference in Al concentration indicates that HC1 was not able to dissolve soil minerals in the short-term experiment and carbonates were more reactive than clay minerals in HC1 solution. This is in agreement with findings of Arvidson et al. (2003) and Kampman et al. (2009). The concentration of Ca in oxalic acid was lower than that in Ca[Cl.sub.2] and HC1, possibly due to precipitation of Ca oxalate (Strom et al. 2005). The concentration of Ca in citric acid was also lower than in oxalic acid, possibly due to the lower capacity of citric acid for dissolution of carbonates resulting from a higher dissociation constant (pK) (Strobel 2001) - a low pK value indicates a higher ability of an acid to release [H.sup.+]. Concentration of Mg in Ca[Cl.sub.2] was 7.4-10.4 mg [L.sup.-1] and increased in HC1 due to carbonate dissolution and release of Mg to the solution. Although some researchers (Sanjay and Sugunan 2008; Li et al. 2015) concluded that HCl is able to extract Mg from some clay minerals such as smectites, this process appeared to not be effective for calcareous soils (as evidenced by no change in Al concentration). In fact, carbonates in soil are dissolved much faster than K-bearing minerals (Arvidson et al. 2003; Kampman et al. 2009). However, Mg concentrations in oxalic and citric acids were higher than in Ca[Cl.sub.2] and HCl, possibly due to the low concentration of Ca in solutions and also dissolution of Mg carbonates by [H.sup.+] and complexation of Mg of minerals by citrate and oxalate (Golubev et al. 2006). Concentration of Al in Ca[Cl.sub.2] was 1.6-2.6mg[L.sup.-1] and showed no change in HCl. For Alfisol, Al concentration in oxalic acid was higher than in other solutions - the other soils showed no change - due to the low content of carbonates in Alfisol and oxalate adsorption on clay surfaces and their dissolution. The Al concentration in citric acid increased because of the adsorption of some citrate ions on clay surfaces and their dissolution (Bray et al. 2015; Li et al. 2015) and release of Al to solution. The K concentrations in the extractants used showed no significant differences in the short-term experiment. The slight decrease in K concentration in oxalic acid for the Vertisol, Aridisol and Entisol may be due to the trapping of K ions or K-bearing minerals by Ca oxalate precipitation (McBride et al. 2017).

Concentrations of Ca, Mg, Al and K in extractants in the long-term experiment differed among the studied soils. The mean concentrations of Ca in Ca[Cl.sub.2] and HCl were 265 and 242 mg[L.sup.-1] respectively (Table 3). The Ca concentration in oxalic and citric solutions decreased significantly to 2 and 125 mg [L.sup.-1] respectively. The decrease in Ca concentration in HCl was due to the decrease in CaC[O.sub.3] during successive extractions of soils. The decrease of Ca in oxalic acid may be due to precipitation of Ca oxalate and low buffering of soluble Ca by soil CaC[O.sub.3] after partial dissolution (Dinkelaker et al. 1989; Burford et al. 2003). The concentration of Ca in citric acid also decreased and this decrease for the Alfisol and Vertisol was significantly more than for the Aridisol and Entisol. This may be due to the lower content of CaC[O.sub.3] in the Alfisol and Vertisol for buffering of soluble Ca. However, low solubility of Ca citrate may also be a reason for precipitation of Ca. Generally, Ca concentration in solutions was lower in the long-term than the short-term experiments and the differences were greater for organic acids.

Concentration of Mg was also affected by extractant. Concentrations of Mg in Ca[Cl.sub.2] for the studied soils were 4.0-7.6 mgL 1 and increased in HCl, oxalic acid and citric acid especially for the Aridisol and Entisol. This may be attributed to the dissolution of Mg carbonate and some Mgbearing minerals like palygorskite (Xavier et al. 2014). The Mg concentration in Ca[Cl.sub.2] was lower in the long-term than the short-term experiment but showed no change in HCl and citric acid. The Mg concentration in oxalic acid depended on the soil and decreased for the Alfisol and Vertisol due to the low Mg carbonate content and increased in the Aridisol and Entisol with their high carbonate contents.

The mean concentration of Al in Ca[Cl.sub.2] was 1.9 mg[L.sup.-1] and showed no change in HCl. It increased for the Alfisol and may be due to the low content of carbonate and thereby partial dissolution of clay minerals by HCl (Sanjay and Sugunan 2008; Kong et al. 2014). The Al concentration in oxalic acid increased significantly due to the low Ca concentration in the solution and thereby more oxalate solubility and adsorption on clay minerals and their dissolution. The Al concentration in citric acid depended on soil type and increased in the Alfisol and Vertisol with low carbonate content but showed no change in the Aridisol and Entisol with high carbonate content. Dissolution of carbonates by citric acid may have occurred in the longer period due to its higher pK value compared with oxalic acid.

The concentrations of K in Ca[Cl.sub.2] were 0.9-1.4mg[L.sup.-1] and increased significantly in HCl as a consequence of NEK exchange with soluble Ca derived from CaC03 dissolution by HCl. The K concentration in oxalic and citric acids was significantly more than in Ca[Cl.sub.2] because of the dissolution of K-bearing minerals by citrate- and oxalate-complexing agents (Burford et al. 2003; Kong et al. 2014). Generally, the K concentration in extractants was lower in the long-term than the short-term experiment and this was more obvious for Ca[Cl.sub.2] solution.

Cumulative K released in the short- and long-term experiments on the basis of extractant and soil are shown in Fig. 1. For the Alfisol (Fig. 1a), there was no significant difference in K release among Ca[Cl.sub.2], HCl and oxalic acid solutions in the short-term experiment but citric acid extracted more K. The highest content of K released in the long-term experiment was for oxalic acid, and Ca[Cl.sub.2] released the lowest content of K. There was no difference between HCl and citric acid in K extraction. The K extracted by HCl may be due to the exchange of [H.sup.+] or Ca and Mg (originated from carbonate dissolution by acid) by interlayer K (Schneider 1997; Najafi Ghiri et al. 201 la; Li et al. 2015). Li et al. (2015) showed that hot HCl could extract more than 30% of NEK from biotite via exchange of [H.sup.+] with interlayer K with no serious change in the mineral structure in the short term. For the Vertisol (Fig. 1b), cumulative K released to Ca[Cl.sub.2], HCl and citric acid in the short-term experiment was similar and significantly more than that to oxalic acid. The ability of solutions to extract K in the short- and long-term experiments was in the order of oxalic acid > citric acid > HCl >Ca[Cl.sub.2]. For the Aridisol (Fig. 1c), the cumulative K released to Ca[Cl.sub.2], HCl and citric acid in the short-term experiment was similar and significantly more than to oxalic acid. Oxalic acid extracted more K than other solutions in the long-term experiment. For the Entisol (Fig. 1d), the cumulative K released to Ca[Cl.sub.2] and HCl was significantly more than to oxalic and citric acids in the short-term experiment. Oxalic acid extracted more K than other solutions in the longterm experiment.

Generally, 55, 48, 37 and 48% of total K was extracted by Ca[Cl.sub.2], HCl, oxalic acid and citric acid respectively in the short-term experiment - thus Ca[Cl.sub.2] was more able than other solutions to extract K from calcareous soils. However, in the long-term experiment, oxalic acid was more effective. Generally, in the absence of carbonates, organic acids may dissolve K-bearing minerals especially on mineral edges after adsorption on clay surfaces (Bray et al. 2015; Li et al. 2015). Other researchers concluded that organic acids had more ability than Ca[Cl.sub.2] to release K from K-bearing minerals in calcareous soils (Touhan et al. 2010; Mousavi et al. 2014). However, organic acids may be important in K release from soils with low carbonates (Alfisol) in short- and long-term experiments, and citric and oxalic acids extracted more K in the short- and long-term experiments respectively. The K release from the studied soils may occur via cation exchange or dissolution of K-bearing minerals. The oxalic acid may have dissolved K-bearing minerals and released K in slightly calcareous soils in the short-term experiment and in highly calcareous soils in the long-term experiment. Citric acid was able to dissolve minerals in the short- and long-term experiments, but was more effective in slightly calcareous soils. The tendency of citric acid for reversible adsorption on clay surfaces is higher than for oxalic acid, possibly due to more carboxylic groups that participate in adsorption; and oxalate tends to precipitate as Ca oxalate especially in calcareous soils (Strom et al. 2005; Gang et al. 2012; Andrade et al. 2013). Van Hees et al. (2003) concluded that the concentration of citric acid on surfaces of clay minerals may be 3000 times its concentration in soil solution. The dissolution of clay minerals with organic acids may be due to the complexation of Al and Mg with oxalate and citrate. The concentration of oxalate in soil solution of calcareous soils remains low due to its reaction with Ca and precipitation of Ca oxalate (Strom et al. 2005). Ramos et al. (2014) believed that the minimum concentration of oxalate for clay mineral dissolution was 0. 1mmol[L.sup.-1]. However, in low carbonate soils, oxalic acid is more effective in mineral dissolution due to its low dissociation constant and more release of compared with citric acid (Strobel 2001). The citric acid adsorption on K-bearing minerals like illite is very low (Kubicki et al. 1999; Lackovic et al. 2003) and oxalate is a strong complexing agent for structural Al (Ramos et al. 2014). The HCl may have dissolved K-bearing minerals of slightly calcareous soils in the longterm experiment. Voinot et al. (2013) indicated that HCl released K via cation exchange reaction but citric acid released it via exchange and dissolution processes. Our results concerning dissolution of clay minerals by HCl and organic acids were not in agreement with those of Kong et al. (2014), who concluded that silicon released from clay minerals (montmorillonite, illite and kaolinite) to HCl was more than to oxalic and citric acids. This difference may be due to the use of pure minerals (with no carbonate) and also treatment of minerals with concentrated HCl (0.5 mol [L.sup.-1]) for 15 days. Additionally, exchange of Ca or H ions with interlayer K of clay minerals is an important process of K release. In calcareous soils, H dissociated from HCl or organic acids may dissolve Ca and Mg carbonates and buffer soluble Ca for exchange with interlayer K. Hydronium ions may also directly exchange with interlayer K. Li et al. (2015) indicated that more than 80% of K released from K-bearing minerals to organic acids may be via cation exchange of [H.sup.+] with interlayer K.

The mean capacity of the soils to fix K after K extraction in the short- and long-term experiments are shown in Table 4. The effect of extractants and extraction duration on K fixation was similar for all soils. The content of K fixed by soils treated with Ca[Cl.sub.2] after the short-term treatment was 231-535 mg [kg.sup.-1] and did not significantly differ from soils treated with HCl (223-506 mg [kg.sup.-1]). It decreased in soils treated with oxalic and citric acids. This trend was also observed for soils in the longterm experiment, but the decrease was more marked compared with the short-term experiment. The main clay minerals for K fixation in calcareous soils of Iran are smectites (Najafi Ghiri and Abtahi 2012). It seems that HCl was not able to dissolve smectites in the calcareous soils. Kong et al. (2014) observed no change in X-ray diffraction peaks of smectites treated with inorganic acids. Decrease in K fixation in soils treated with organic acids may be due to the coverage of K.-fixing sites by Ca oxalate or dissolution of the edge of K-fixing minerals, especially smectites (Golubev et al. 2006; Kong et al. 2014). The lowest K fixation was in soils treated with oxalic acid, which has more ability to dissolve smectites than citric acid due to its low pK (Tu et al. 2007; Kong et al. 2014). However, as a result of its small size and high charge density, oxalic acid may be adsorbed on clay edges (e.g. smectites) and accelerate clay dissolution and limit K fixation (Burford et al. 2003; Ramos et al. 2014).

The content of K fixation in soils of the long-term experiment showed significant decrease regardless of soil and extractant. This decrease in soils treated with Ca[Cl.sub.2] and HCl may be due to the saturation of clay surfaces with Ca (originating from Ca[Cl.sub.2] or dissolution of CaC03) and its fiocculation and thereby slower diffusion of K ions to mineral interlayer. This decrease in soils treated with organic acids may be due to severe dissolution of the clay edge responsible for K fixation due to the longer extraction period.

Conclusions

Mechanisms of K release from four calcareous soils to different extractants (Ca[Cl.sub.2], HCl and oxalic and citric acids) were studied in short- and long-term experiments. The HCl extracted K from different soils due to soil carbonate dissolution, increase in soluble Ca and no dissolution of soil minerals in both short- and long-term experiments. This means that HCl extracted K from calcareous soils via exchange reaction of Ca with interlayer K. However, organic acids had no effect on mineral dissolution in the short-term experiment and may have dissolved soil carbonate and increased soluble Ca for exchange reaction of Ca with interlayer K; and oxalic acid may have extracted less K than other solutions due to the precipitation of some Ca ions as Ca oxalate. Organic acids behaved differently in the long-term experiment and may have dissolved clay minerals as well as soil carbonates to release K. Due to its smaller size, higher charge density and lower dissociation constant, oxalic acid was generally more effective than citric acid. Organic acids may have also dissolved minerals of soils with low carbonate content in the shortterm experiment. The K-fixation experiments indicated that soils treated with organic acids fixed less K than others due to the effect of oxalate and citrate on the dissolution of mineral edges (e.g. smectite and illite) responsible for K fixation. The K-fixation capacity of soils after long-term treatment with different solutions decreased, possibly due to greater dissolution of K-fixing minerals or fiocculation of minerals by Ca and slower K diffusion to the interlayer. Concerning the highly calcareous nature of the studied soils, it seems that soluble cations in calcareous soils (predominantly Ca and Mg) may be exchanged with NEK of clay minerals and buffer soluble K as well as organic acids produced by plant roots and microorganisms.

Conflicts of interest

The authors declare no conflicts of interest. Acknowledgements

This research did not receive any specific funding.

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Handling Editor: Siobhan Staunton

https://doi.org/10.1071/SR18301

M. Najafi-Chiri (ID) (A,D), M. Niazi (B), M. Khodabakhshi (B), H. R. Boostani (A), and H. R. Owliaie (C)

(A) Department of Soil Science, College of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Islamic Republic of Iran,

(B) Department of Agroecology, College of Agriculture and Natural Resources of Darab, Shiraz University, Shiraz, Islamic Republic of Iran.

(C) College of Agriculture, Yasouj University, Yasouj, Islamic Republic of Iran.

(D) Corresponding author. Email: mnajafighiri@yahoo.com

Caption: Fig. 1. Cumulative K released from soil samples (a) Alfisol, (b) Vertisol, (c) Aridisol and (d) Entisol to different extractants in the short- and long-term experiments (n = 3). Means followed by different letters for each short/long-term experiment significantly differ at P<0.05 by Tukey's HSD test. Means followed by different letters for each extractant (in parentheses) significantly differ at P < 0.05 by paired-sample t-test.
Table 1. Some physicochemical properties and K status of the
studied soils

EC, electrical conductivity; CCE, calcium carbonate equivalent; CEC,
cation exchange capacity; OM, organic matter; SK, soluble K; EK,
exchangeable K; NEK, non-exchangeable K; TK, total K

Soil order     Region    Sand   Silt   Clay    PH
                         (%)    (%)    (%)

Alfisols      Sepidan     13     36     51    7.31
Vertisols     Sepidan     25     32     43    7.39
Aridisols      Neyriz     55     34     11    7.28
Entisols      Kazeroon    17     54     29    7.55

Soil order      EC (dS      CCE   CEC (cmol (+)   OM
              [m.sup.-1])   (%)   [kg.sup.-1])    (%)

Alfisols          0.4        3         34         2.0
Vertisols         0.7       27         24         2.9
Aridisols         3.7       45         13         1.4
Entisols          0.9       56         12         2.7

                  (mg [kg.sup.-1])

Soil order     SK    EK    NEK    TK    K fixation

Alfisols      3.2    410   903   8752      368
Vertisols     2.8    320   865   6946      199
Aridisols     7.8    152   581   4972      127
Entisols      15.4   339   780   5660      134

Table 2. USDA classification and clay and sand mineralogy of the
studied soils C, chlorite; F, feldspar; I, illite; Q, quartz; S,
smectite; P, palygorskite

Soil order    USDA classification            Clay         Silt
                                          mineralogy   mineralogy

Alfisols      Fine, smectitic, mesic,       S >> I       Q > S >
              Typic Haploxeralfs                        F = I = C

Vertisols     Fine, smectitic, mesic,      S >> C >     Q >> S >
              Typic Haploxererts            I = Q       I = C = F

Aridisols     Coarse-loamy, carbonatic,   S > C = I     Q >> S =
              thermic, Typic               > P > Q      I = C > F
              Haplocalcids

Entisols      Coarse-loamy, carbonatic,    S = P >     Q > S = F =
              hyperthermic, Aridic          I > C         I = C
              Ustorthents

Table 3. Mean concentrations (mg [L.sup.-1]) of Ca, Mg, Al and K in
the four different extractants in the short- and long-term
experiments (n = 3)

Means followed by different letters in each row for each element
significantly differ at P < 0.05 by Tukey's HSD test. Means
followed by different letters in each column for each soil (in
parentheses) significantly differ at P < 0.05 by paired-sample t-test

                            Ca[Cl.sub.2]

Soil order       Ca         Mg        Al         K

Short-term experiment

Alfisols       282a(a)   7.9b(a)    1.7b(b)   2.4a(a)
Vertisols      284a(a)   7.4b(a)    1.6b(b)   3.8a(a)
Aridisols      273a(a)   8.0c(a)    1.6b(a)   2.3a(a)
Entisols       291a(a)   10.4b(a)   2.6a(a)   3.9a(a)

Long-term experiment

Alfisols       270a(b)   7.6b(a)    2.9c(a)   0.9c(b)
Vertisols      266a(b)   4.0c(b)    2.5b(a)   1.1c(b)
Aridisols      261a(b)   5.7c(b)    13b(a)    0.9b(b)
Entisols       265a(b)   5.8c(b)    1.0b(b)   1.4b(b)

                                 HC1

Soil order       Ca         Mg        Al         K

Short-term experiment

Alfisols       286a(a)   10.3a(a)   1.3b(b)   2.3a(a)
Vertisols      285a(a)   9.4a(a)    1.4b(a)   3.5a(a)
Aridisols      277a(a)   9.9b(a)    2.0b(a)   2.3a(a)
Entisols       271a(a)   9.5b(a)    1.7b(a)   3.7a(a)

Long-term experiment

Alfisols       242b(b)   9.9a(a)    6.2b(a)   1.8ab(a)
Vertisols      240b(b)   9.6a(a)    1.8b(a)   1.3b(b)
Aridisols      242b(b)   10.3b(a)   2.4b(a)   1.0b(b)
Entisols       245b(b)   8.8b(a)    1,2b(a)   1.5b(b)

                            Oxalic acid

Soil order       Ca         Mg        Al         K

Short-term experiment

Alfisols       272b(a)   9.4a(a)    4.8a(b)   2.3a(a)
Vertisols      270b(a)   10.4a(a)   1.9b(b)   3.1a(a)
Aridisols      276a(a)   11,8ab(a)  2.6b(b)   1.9a(a)
Entisols       276a(a)   11.8ab(a)  1.8b(b)   3.2a(a)

Long-term experiment

Alfisols        2d(b)    1.4c(b)    7.1b(a)   2.0a(a)
Vertisols       2d(b)    1.0b(b)    9.8a(a)   1.9a(b)
Aridisols       ld(b)    12.6a(a)   11.8ab(a) 1.5a(b)
Entisols        2d(b)    11.8a(a)   7.3b(a)   2.2a(b)

                            Citric acid

Soil order       Ca         Mg        Al         K

Short-term experiment

Alfisols       254c(a)   10.0a(a)   4.6a(b)   2.7a(a)
Vertisols      254c(a)   9.7a(a)    3.6a(b)   3.6a(a)
Aridisols      261e(a)   12.6a(a)   3.4a(b)   2.1a(a)
Entisols       269b(a)   12.5a(a)   2.5a(b)   3.8a(a)

Long-term experiment

Alfisols       51c(b)    7.6b(b)    14.2a(a)  1.6b(b)
Vertisols      51c(b)    6.5b(b)    13.1a(a)  1.6ab(b)
Aridisols      199c(b)   13.0a(a)   1.8b(a)   1.0b(b)
Entisols       201c(b)   12.8a(a)   3.7b(a)   1.3b(b)

Table 4. Potassium fixation capacity of soil samples after the
short-and long-term treatments with four different solutions (n = 3)

Means followed by different letters in each row for each short/long
experiment significantly differ at P < 0.05 by Tukey's HSD test.
Means followed by different letters in each row for each extractant
(in parentheses) significantly differ at P < 0.05
by paired-sample t-test

                Ca[Cl.sub.2]             HCI

Soil order     Short     Long      Short     Long

Alfisols      535a(a)   448a(b)   506a(a)   123b(b)
Vertisols     231a(a)   135a(b)   223a(a)   95b(b)
Aridisols     278a(a)   178a(b)   282a(a)   140b(b)
Entisols      268a(a)   156a(b)   247a(a)   135b(b)

                Oxalic acid         Citric acid

Soil order     Short     Long      Short     Long

Alfisols      308b(a)   96c(b)    330b(a)   130b(b)
Vertisols     168b(a)   57c(b)    175b(a)   83b(b)
Aridisols     217b(a)   94c(b)    215b(a)   124bc(b)
Entisols      173b(a)   69d(b)    197b(a)   120c(b)
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Author:Najafi-Chiri, M.; Niazi, M.; Khodabakhshi, M.; Boostani, H.R.; Owliaie, H.R.
Publication:Soil Research
Article Type:Report
Geographic Code:1U9CA
Date:May 1, 2019
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