Competitive adsorption between phosphate and dissolved organic carbon in iron-rich soils

The competitive adsorption between phosphate and dissolved organic carbon (DOC) has been reported in Andosols and Podzols. However, the published results on the competitive adsorption between P and DOC are unclear and sometimes contradictory. The competitive behaviour between phosphate and DOC may be quite different in the surface and subsurface soils. In this study, we used surface and subsurface soils with substantial Fe oxide contents from Wagga Wagga (Chromosol) and Tumbarumba (Ferrosol) in New South Wales (Australia) to evaluate the adsorption behaviour of phosphate and DOC. Adsorption data were fitted into linear initial mass (IM) isotherm. The results showed that both surface and subsurface soils from Tumbarumba had a greater phosphate adsorption capacity than the soils from Wagga Wagga. Phosphate adsorption was greater for the subsurface soil (m = 0.72) than the surface soil (m = 0.82) from Tumbarumba, while this trend for was opposite for Wagga Wagga soils, where phosphate adsorption capacity was greater for the surface (m = 0.55) soil than the subsurface (m = 0.37) soil. The DOC adsorption was greater in the subsurface soils than the surface soils from both sites. In the mixed solution of P and DOC, phosphate adsorption promoted DOC desorption in the surface and subsurface soils from Wagga Wagga and Tumbarumba. The results of this study have crucial implications on the sustainability of Fe-rich soils. The adsorption of phosphate promoted DOC desorption in these soils, which may lead to the destabilisation of OC and impair OC sequestration and therefore enhance the microbial decomposition of OC in these soils.

Introduction

Phosphorus (P) is an essential plant nutrient that is indispensable for several physiological and biochemical processes including photosynthesis, respiration, enzyme regulation, and biosynthesis of structures such as cell membranes, DNA, and RNA (1). Despite the presence of considerable total P content in most soils, the crop productivity of agricultural soils is often limited by P availability, because a very small portion (<1%) of the total P is present in soluble forms (2, 3). Phosphorus is particularly the most commonly limiting nutrient in highly weathered soils of tropical and subtropical regions because of their high P adsorption capacity and low levels of P availability (4–6).

Plant roots absorb P as H₂PO₄⁻ and HPO₄²⁻ ions, which are the predominant orthophosphate species in soil solution (5). Phosphate concentration in soil solution is controlled by several reactions that have been considered under three categories: (i) adsorption–desorption, (ii) dissolution–precipitation, and (iii) mineralisation–immobilisation (7–9). The availability of phosphate in soils is affected by root characteristics, soil properties [such as pH, clay and organic matter (OM) contents, iron (Fe) and aluminium (Al) hydroxides, oxyhydroxides, and oxides], and environmental conditions (4, 10–12). Soluble P can undergo different chemical reactions in soils; however, the solution P concentration is primarily controlled by adsorption–desorption reactions with soil minerals, particularly Fe/Al oxides and edge sites of phyllosilicates (13, 14). Variable charge soils (e.g., Andisols) and highly weathered soils (e.g., Oxisols and Ultisols) often have greater P adsorption capacity than younger and less weathered soils such as Inceptisols (15, 16). Some studies have reported that P adsorption is mostly influenced by Fe oxides in tropical and subtropical soils (17–20). Contrary to this, other studies have observed that Al oxides play a more dominant role than Fe oxides in the P adsorption capacity of both temperate (10, 21) and tropical soils (6, 22). Phosphate adsorption capacity has been closely related to crystalline Fe oxides (dithionite citrate bicarbonate extractable Fe) and amorphous Fe and Al oxides (ammonium oxalate extractable Fe and Al) in soils from different regions (4, 17, 18, 20, 23–27).

Both Fe and Al oxides have a large specific surface area (SSA) and positive charge at field pH, which are important characteristics for P adsorption in soils (18, 22, 28). The process of phosphate adsorption onto variable charge surfaces primarily occurs through replacement of surface hydroxyls by phosphate, the so-called ligand exchange reaction, which results in the formation of inner-sphere surface complexes (29, 30). The inner-sphere surface complexes can be either binuclear or mononuclear surface complexes (31–33). In the binuclear surface complex formation, single-coordinated hydroxyl (OH) groups on the surfaces of Fe oxides is exchanged or replaced by phosphate (19, 34, 35). In the mononuclear surface complex formation, phosphate is coordinated in a monodentate mode to the OH group on the surfaces of Fe oxides (36).

Dissolved organic carbon (DOC) is abundant in terrestrial and aquatic environments. Although DOC forms only a small part of the total organic carbon in soils, it is highly reactive and labile and it tends to be less bioavailable when adsorbed (37, 38). It is operationally defined as the fraction of organic C in solution that pass through a 0.45-µm pore size filter (37, 39, 40). DOC actively influences many biogeochemical processes such as mineral weathering, leaching, nutrient translocation, microbial activity, soil formation, and transport of metals and pollutants (38, 40, 41). Adsorption of DOC onto Fe oxides has been recognised as an important process in the accumulation and preservation of organic carbon in soils (42). Thus, similar to phosphate, DOC concentration in soil solution is controlled mainly by adsorption processes onto mineral surfaces (43). DOC adsorption on minerals can occur via ligand exchange surface complexation, cation bridging, anion exchange, hydrogen bonding, and hydrophobic interactions (44, 45). However, Fe and Al oxides have been reported to adsorb DOC preferentially through ligand exchange reactions (42, 44, 46, 47).

Considering the similar nature of their adsorption reactions, phosphate and DOC may compete for adsorption sites in soils (48, 49). Different observations have been made on the competitive adsorption of DOC and phosphate in soils. Many studies have observed that DOC including high-molecular-weight organic compounds (HOCs), such as humic acid (HA) and fulvic acid (FA), competed strongly with phosphate for adsorption sites and caused a significant decrease in phosphate adsorption (48, 50–55). In contrast, several studies have found that organic compounds and organic matter do not compete with phosphate for adsorption sites (19, 56–58). Spohn et al. (49) observed that adsorption and desorption of organic compounds were greatly influenced by changes in concentration of P in the soil solution. Phosphate adsorption promoted OM desorption, thereby increasing DOC concentration; thus, phosphate ions competed more effectively for reactive sites than DOC (49, 59).

Most earlier studies have considered competitive adsorption between LOAs, HA, FA, DOC, and phosphate in Andosols and Podzols. However, the published results on the competitive adsorption between P and DOC are unclear and sometimes contradictory. Although most agricultural soils in Australia are naturally P-deficient, P accumulation in these soils is due to P fertilisation even to the level of excess application (60–62). With growing focus on increasing OC in soils, research is needed to understand the interaction between DOC and P in Australian soils, many of which have high Fe (and Al) oxide contents. Furthermore, little or no attention has been paid to understand the competitive adsorption behaviour of DOC and phosphate in subsurface soils that have much less organic matter and greater number of reactive sites such as in Luvisols. Thus, phosphate and DOC competition for adsorption sites on minerals may be quite different in the surface and subsurface soils. Therefore, the aim of this study was to evaluate the adsorption of phosphate and DOC behaviour in surface and subsurface soils with substantial Fe oxide contents. We hypothesised that phosphate or DOC adsorption will be greater in subsurface soils than in surface soils. Furthermore, we hypothesised that DOC would inhibit phosphate adsorption in both surface and subsurface soils. In the phosphate adsorption experiment, 2.0 g of soil samples were weighed and transferred into 50 mL centrifuge tubes and equilibrated with 40 mL 0.01 M CaCl2 solution containing 0, 0.16, 0.32, 0.65, 0.97, 1.29, 1.62, 1.94, 2.26 and 2.58 mmol P/L for Wagga Wagga and 0, 1.62, 2.42, 3.23, 4.04, 4.85, 5.65, 6.46, 7.27, 8.08 mmol P/L for Tumbarumba. In the DOC adsorption experiment, 3 g of soil was equilibrated with 30 mL 0.01 M CaCl2 solution containing 0, 0.42, 0.83, 1.67, 3.33, 5.00, 6.66, 8.33, 10.00, 11.66 mmol C/L in 50 mL centrifuge tubes. In the phosphate-DOC adsorption experiment, 2.0 g of soil samples were weighed into 50 mL centrifuge tubes and equilibrated with 40 mL of the mixture solutions containing 0, 0.08, 0.12, 0.32, 0.65, 0.97, 1.29, 1.62, 1.94 mmol P/L and 0, 0.20, 0.42, 0.83, 1.67, 2.50, 3.33, 4.17, 5.00 mmol C/L for Wagg Wagga, and 0, 0.81, 1.21, 1.62, 2.42, 2.83, 3.23, 3.63, 4.04 mmol P/L and 0, 0.20, 0.42, 0.83, 1.67, 2.50, 3.33, 4.17, 5.00 mmol C/L for Tumbarumba. Adsorption experiments were carried out in duplicates and laid out in a complete randomized design.

Materials and methods

Soil sample selection

Surface (0–20 cm) and subsurface (20–40 cm) samples were collected from Wagga Wagga (35.03°S, 147.33°E) and Tumbarumba (35.74°S, 147.98°E) in New South Wales, Australia. The climate for Wagga Wagga and Tumbarumba is humid subtropical to temperate. Mean minimum annual temperature is 9.1°C and mean maximum temperature is 22.3°C and mean annual rainfall is 573.4 mm for Wagga Wagga, while Tumbarumba’s average minimum annual temperature is 5.7°C and the mean maximum annual temperature is 19.8°C and average rainfall is 977.9 mm. Wagga Wagga site was bare fallow that had earlier been cropped with wheat, while Tumbarumba site had native pasture. The soils collected from Wagga Wagga and Tumbarumba are classified as Luvisol and Ferralsol, respectively, according to the IUSS Working Group WRB (63). Soil samples were air-dried, crushed, and passed through a 2-mm sieve and stored at room temperature for laboratory analysis and adsorption experiments.

Soil analyses

Soil chemical and physical properties are shown in Table 1. Soil pH was measured in 1:5 of soil and 0.01 M calcium chloride (CaCl₂) using a glass electrode pH meter (64). Total C (TC) in soils was analysed using a vario MACRO cube Elementar CHN analyser, and considering the acidic pH of both soils, the total carbon was assumed to be all organic C. Particle size analysis (PSA) was determined by the hydrometer method (65). Cation exchange capacity (CEC) was determined by the silver thiourea method (64). The total free Fe oxide content was determined using the dithionite-citrate-bicarbonate (DCB) extraction procedure described by Mehra and Jackson (66). The content of short-range ordered and poorly crystalline Fe oxides was measured by extracting with ammonium oxalate (OX) at pH 3 in the dark for 4 h (67). Organic matter complexed with Fe was determined using the sodium pyrophosphate (PP) extraction procedure described by McKeague (68).

Table 1

Table 1. Selected chemical and physical properties of soils used in the study.

Oriented clay x-ray diffraction (XRD) patterns were obtained using a PANalytical X’Pert PRO instrument (40 kV and 40 mA) with CuKα radiation for the identification of phyllosilicate minerals in the clay fraction of soils. Random powder XRD patterns were obtained using a STOE Stadi P instrument (50 kV and 40 mA) with MoKα radiation for the identification of all minerals in the clay fraction of soils.

Phosphate adsorption experiment

Two grams of surface and subsurface soil samples were weighed in duplicates and transferred into 50-mL centrifuge tubes. The soil samples were equilibrated with 40 mL of different P concentrations (up to 10 concentrations for each soil), ranging from 0 to 2.58 mmol/L for Wagga Wagga and 0 to 8.1 mmol/L for Tumbarumba using potassium dihydrogen phosphate (KH₂PO₄) in a 0.01 M calcium chloride (CaCl₂) matrix. Before adding P solutions to soil, the solution pH was adjusted to 6 using 0.1 M sodium hydroxide (NaOH) because the minimum solubility of phosphate occurs between 5.5 and 7 in non-calcareous soils. Three drops of chloroform were added into each centrifuge tube to inhibit microbial activity. The centrifuge tubes were shaken on a rotary laboratory shaker at 19 ± 1°C for 17 h, centrifuged at 1,800 ×g for 15 min, and supernatants were filtered through a 0.45-µm PTFE syringe membrane filter. Supernatant pH was measured and P concentration in solution was analysed colorimetrically using the ascorbic acid method (69).

DOC adsorption experiment

DOC used for the adsorption experiment was extracted from decomposed residues of pine (Pinus radiata), which is commonly used in timber plantations in Australia. For DOC extraction, 100 g of decomposed residues was soaked in 1,000 mL of deionised water, and the suspension was shaken on a rotary shaker for 1 h. Then, the mixture was allowed to stand for 24 h at 19 ± 1°C. The mixture was then centrifuged at 3,280 ×g for 20 min and the supernatant was filtered through a 0.45-µm Millipore white gridded membrane filter using a vacuum suction unit. The DOC concentration in the solution was ~200 mg/L and the pH was 5.8.

Similar to P adsorption, DOC adsorption experiments were carried out in duplicates for both surface and subsurface soil samples. Briefly, 3 g of soil was mixed with 30 mL of 0.01 M CaCl₂ solution (up to 10 solutions for each soil) containing a range of DOC concentration (0 to 11.65 mmol C/L) in 50-mL centrifuge tubes. The solution pH was adjusted to 6 before adding to soil samples and no chloroform was added. The suspensions were then placed on a rotary shaker for 17 h at 19 ± 1°C. The suspensions were then allowed to settle for 30 min before filtering through a 0.45-µm PTFE syringe membrane filter. After filtration, the pH of the supernatants was measured. The DOC concentration in the initial and equilibrium solutions (supernatants) was analysed using a Shimadzu TOC-L total C analyser.

Phosphate-DOC competitive adsorption experiment

Batch adsorption experiments were conducted using 2.0 g of air-dried surface and subsurface soil samples (in duplicates) into 50-mL centrifuge tubes. Solutions containing different P and OC concentrations (up to nine concentrations of each P and DOC for each soil), with P concentration ranging from 0 to 4 mmol/L using KH₂PO₄ and DOC concentration ranging from 0 to 5 mmol/L in a 0.01 M CaCl₂ solution. The soil samples were equilibrated with 40 mL of the mixture solutions, with the solution pH adjusted to 6 before adding to soil samples. The suspensions were placed on a rotary shaker for 17 h at 19 ± 1°C. The suspensions were then allowed to settle for 30 min and filtered through a 0.45-µm PTFE syringe membrane filter. After filtration, the pH of the supernatants was measured. The initial and final P and DOC concentrations in solution were analysed as described earlier. In addition, control samples without chloroform were used to determine DOC released from soils and it was corrected in the adsorption data.

Fitting of adsorption data

The Langmuir and Freundlich equations did not fit the adsorption data very well as indicated by the small values of coefficient of determination (Langmuir R² = 0.35 and Freundlich R² = 0.74 for Wagga Wagga, and Langmuir R² = 0.76 and Freundlich R² = 0.72 for Tumbarumba). In systems where the solute (or adsorbate) to be adsorbed is already present in the adsorbent (such as for the adsorption of phosphate and DOC on soils), the use of initial mass (IM) isotherm has been recommended to describe the adsorption data (70). In essence, IM isotherm is a linear adsorption isotherm that accounts for a solute initially present within the adsorbent. Hence, the adsorption data were fitted into linear IM isotherm as follows:

$$RE=m{X}_i-b$$

where RE is the amount of P or DOC removed from or released into the solution normalised for soil mass (mmol/kg), X_i is the initial P and DOC concentration normalised for soil mass, m (slope of the linear regression) is the partition coefficient of the IM isotherm, and −b (intercept) is the desorption term.

The m of the linear IM isotherm was used to determine the distribution coefficient (K_d).

$$K_{\text{d}}=\left(\frac{m}{1-m}\right)\times \frac{V}{M}$$

where V is the volume of solution and M is the mass of soil.

The slope (m) and intercept (−b) were used to determine reactive soil pool (RSP) of solute.

$$\text{Reactive soil pool}= \frac{b}{(1-m)}$$

Results

Phosphate adsorption

The linear IM model described the phosphate adsorption data very well (R² = 0.98 and R² = 0.99). The isotherm plots for surface and subsurface samples of the two soils are shown in Figure 1, and the model parameters (m and b) and their derivatives (K_d and RSP) are presented in Table 2. The surface soils from Wagga Wagga did not adsorb P from solutions until the initial P loadings of up to 32 mmol/kg; rather, P was desorbed at loadings lower than this. The amount of P desorbed decreased with increasing initial P addition, and the P desorption estimated from the intercept was −18.3 ± 0.53 mmol/kg (Figure 1a and Table 2). Tumbarumba soil showed greater affinity for P adsorption than Wagga Wagga soil, indicated by the steeper slope (m) value (Table 2). The trend for surface and subsurface was opposite for the two soils, with the Wagga Wagga surface soil showing greater P affinity than the corresponding subsurface soil, whereas the P affinity was greater for the subsurface soil than for the surface soil from Tumbarumba (Table 2). The equilibrium solution pH in all samples decreased from the initial adjusted pH value (6.0). Wagga Wagga surface and subsurface soils had a mean equilibrium pH value of 4.8 and 5.4, respectively, and Tumbarumba surface and subsurface soils had a mean equilibrium pH value of 4.7 and 5.2, respectively. However, a small increase (0.1–0.4) in the equilibrium pH was observed with P adsorption in the subsurface soil from Wagga Wagga and both surface and subsurface soils from Tumbarumba. Phosphate adsorption resulted in DOC desorption in all samples, varying between 6 and 8 mmol/kg in Wagga Wagga soils, and between 24 and 36 mmol/kg in Tumbarumba soils. The desorption of DOC was greater in subsurface soil as compared to surface soil for both sites (Supplementary Figure 1). The plots of the phosphate adsorption or desorption data in relation to the equilibrium P concentration for the surface and subsurface soils from Wagga Wagga and Tumbarumba are shown in Supplementary Figure 2.

Figure 1

Figure 1. Linear initial mass isotherms for phosphate adsorption or desorption on surface and subsurface soils from (a) Wagga Wagga and (b) Tumbarumba.

Table 2

Table 2. Regression parameters derived from the linear initial mass isotherms of phosphate adsorption onto surface and subsurface soils from Wagga Wagga and Tumbarumba.

The P adsorption coefficient showed a tendency to increase with increasing contents of clay, OC, Fe_OX, Fe_DCB, Al_OX, and Al_DCB and decrease with increasing soil pH (Supplementary Figure 3).

DOC adsorption

The linear IM isotherms for DOC adsorption are shown in Figure 2. The surface and subsurface soils from Wagga Wagga and Tumbarumba did not adsorb DOC from the initial DOC additions ranging between 1 and 15 mmol/kg; rather, DOC was desorbed. The amount of DOC desorbed decreased with increasing initial DOC addition. The linear IM isotherm parameters (m and b) and their derivatives (K_d and RSP) are shown in Table 3. The partition coefficient (slope) showed that soils from Tumbarumba had a greater affinity for DOC than the Wagga Wagga soils. Also, the subsurface soils from both sites had greater affinity than their corresponding surface soils.

Figure 2

Figure 2. Linear initial mass isotherms for dissolved organic carbon (DOC) adsorption or desorption on surface and subsurface soils from (a) Wagga Wagga and (b) Tumbarumba.

Table 3

Table 3. Regression parameters derived from the linear initial mass isotherms of DOC adsorption onto surface and subsurface soils from Wagga Wagga and Tumbarumba.

The equilibrium solution pH in all samples decreased from the initial adjusted pH value (6.0). Wagga Wagga surface and subsurface soils had a mean equilibrium pH value of 4.6 and 5.0, while Tumbarumba surface and subsurface soils had a mean equilibrium pH value of 4.4 and 5.4, respectively. A small increase of 0.1 in the equilibrium solution pH was observed with DOC adsorption in the surface soil from Wagga Wagga and both surface and subsurface soils from Tumbarumba. The adsorption of DOC resulted in little desorption of P in both soils, with approximately 0.7 mmol/kg in Wagga Wagga soils and <0.05 mmol/kg in Tumbarumba soils. The desorption of P was greater in surface soil as compared to subsurface soil for both sites (Supplementary Figure 4). The plots of the DOC adsorption or desorption data in relation to the equilibrium DOC concentration for the surface and subsurface soils from Wagga Wagga and Tumbarumba are shown in Supplementary Figure 5.

The DOC adsorption coefficient showed a tendency to increase with increasing contents of clay, OC, Fe_OX, Fe_DCB, Al_OX, and Al_DCB and decrease with increasing soil pH (Supplementary Figure 6).

Competitive adsorption of phosphate and DOC

The linear IM isotherm for phosphate adsorption from the mixed solution of P and DOC is presented in Figure 3, and the isotherm parameters (m and b) and their derivatives (K_d and RSP) are shown in Table 4. Tumbarumba soil showed greater affinity for P adsorption than Wagga Wagga soil, indicated by the steeper slope value. The trend for surface and subsurface was opposite for the two soils, with the Wagga Wagga surface soil showing greater affinity than the corresponding subsurface soil, whereas the P affinity was greater for the subsurface soil than the surface soil from Tumbarumba (Table 4). This was similar to the trend observed from the solution of only P as mentioned earlier. However, the slope value obtained from the mixed solution of P and DOC was higher than the value obtained from the solution of only P. The plot of the phosphate adsorption or desorption data in relation to the equilibrium P concentration from the mixed solution of P and DOC on the surface and subsurface soils from Wagga Wagga and Tumbarumba is shown in Supplementary Figure 7.

Figure 3

Figure 3. Linear initial mass isotherms for phosphate adsorption or desorption from mixed solutions of P and DOC on surface and subsurface soils from (a) Wagga Wagga and (b) Tumbarumba.

Table 4

Table 4. Regression parameters derived from the linear initial mass isotherms of phosphate and DOC adsorption from the mixed solution of P and DOC onto soils.

The linear IM isotherm for DOC adsorption from the mixed solution of P and DOC is presented in Figure 4, and the isotherm parameters (m and b) and their derivatives (K_d and RSP) are shown in Table 4. The surface and subsurface soils from Wagga Wagga did not adsorb DOC from addition of mixed P and DOC solutions; rather, DOC was desorbed, however adsorption occurred on the subsurface soil at 32 mmol/kg DOC additions. Similarly, the surface and subsurface soils from Tumbarumba did not adsorb DOC from the mixed addition of mixed P and DOC solutions; however, adsorption occurred on the subsurface soil at 22 mmol/kg DOC additions. The partition coefficient showed that Tumbarumba soils had a greater affinity for DOC than Wagga Wagga soils. Also, the subsurface soils from both sites had greater affinity than their corresponding surface soils (Table 4). This was similar to the trend observed from the DOC solutions alone (i.e., without P). Nevertheless, the isotherm parameters obtained from the DOC solutions alone were higher than the isotherm parameters obtained from the solutions containing P and DOC in soils from both sites.

Figure 4

Figure 4. Linear initial mass isotherms for dissolved organic carbon (DOC) adsorption or desorption from mixed solutions of P and DOC on surface and subsurface soils from (a) Wagga Wagga and (b) Tumbarumba.

The equilibrium solution pH in all samples decreased from the initial pH value (6.0), with varying degrees depending on the initial soil pH. Also, the equilibrium solution pH values of both soils increased with increasing depth. The mean equilibrium solution pH value for the surface and subsurface soils from Wagga Wagga was 4.8 and 5.1, respectively, whereas the equilibrium solution pH of the surface and subsurface soils from Tumbarumba was 4.6 and 5.0, respectively. Moreover, the adsorption of phosphate or DOC caused no substantial change in the equilibrium solution pH values. The plots of the DOC adsorption or desorption data in relation to the equilibrium DOC concentration from the mixed solution of P and DOC on the surface and subsurface soils from Wagga Wagga and Tumbarumba are shown in Supplementary Figure 8.

Discussion

Adsorption studies

The linear IM isotherm parameters revealed that phosphate adsorption was greater in the subsurface soil than in the surface soil from Tumbarumba. This supports our hypothesis that phosphate adsorption will be greater in subsurface soils than in surface soils. However, the isotherm parameters obtained from Wagga Wagga soils did not support our hypothesis; where, phosphate adsorption was greater in the surface soil than in the subsurface soil from Wagga Wagga.

The soils from Tumbarumba had the greater affinity for phosphate than soils from Wagga Wagga. This result was expected considering that Tumbarumba soils had higher contents of Fe_OX, Fe_DCB, Fe_PP, Al_OX, Al_DCB, and Al_PP than Wagga Wagga soils. The positive linear relationships between the P adsorption partition coefficient and extractable forms of Fe and Al indicated the possible influence of these soil properties on phosphate adsorption. Fe_OX showed a stronger relationship (R² = 0.81) with P adsorption partition coefficient than Fe_DCB (R² = 0.68). This implied that poorly crystalline Fe (Fe_OX) phases contributed more to phosphate adsorption than the crystalline Fe (Fe_DCB) phases, possibly due to the greater SSA of poorly crystalline Fe phases than well crystalline phases. Several studies have reported significant and positive relationships between phosphate adsorption and Fe_OX and Fe_DCB (4, 18, 20, 24, 71–75). Furthermore, Al_OX and Al_DCB also showed strong relationships (R² = 0.81 and 0.80) with the P adsorption partition coefficient, which suggested their significant role in influencing P adsorption in these soils. Some studies have reported three to five times greater contribution of Al_DCB in P adsorption than Fe_DCB in acid soil from Australia (4, 76). Aluminium substitution for Fe is common in soils, with about one-third in goethite and about one-sixth in hematite (77). The crystal size of Fe oxides is known to decrease with increasing Al substitution and, in turn, increases SSA and phosphate adsorption capacity (17, 78). Therefore, Al_DCB indirectly contributed to more reactive surfaces for phosphate adsorption than Fe_DCB (79). Several studies have reported significant relationships between phosphate adsorption and extractable forms of Al (4, 20, 24, 73, 76). The clay content of Tumbarumba soils also positively influenced P adsorption; however, Fe/Al oxides formed a significant portion of the clay fraction.

The desorption of a large amount of P that occurred in the surface soil from Wagga Wagga was expected owing to the long history of P fertiliser application including the historic excess addition of P fertilisers to Australian soils (60–62). Hence, P was highly accumulated in the surface soil from Wagga Wagga. The equilibrium solution pH decreased in all samples towards the original soil pH values, reflecting the buffer capacity of these acidic soils against the input of alkalinity. However, the small increase in equilibrium solution pH with P adsorption was expected as during ligand exchange process, single-coordinated OH groups at the surfaces of Fe/Al oxides become replaced by phosphate (19, 29, 30). The pH of all soil samples was acidic and well below the point of zero charge (PZC) of Fe/Al oxides; thus, no pH effect was observed in the studied soils.

The isotherm parameters showed that DOC adsorption was greater in the subsurface soil than in the surface soil from Wagga Wagga and Tumbarumba. This supports our hypothesis that DOC adsorption will be greater in subsurface soils than in surface soils. The Tumbarumba soils favoured DOC adsorption as compared to Wagga Wagga soils. This was possible owing to the higher content of Fe/Al oxides present in the Tumbarumba soils than Wagga Wagga soils. Similar to that of P adsorption, the adsorption of DOC and their relative affinity for surfaces of mineral are closely linked to Fe and Al oxides in soils (43). The strong positive relationships between the DOC adsorption partition coefficient and different fractions of extractable Fe and Al indicated that both extractable Fe and Al are important soil properties influencing DOC adsorption. A significant positive relationship between Fe_DCB and Al_DCB with the DOC adsorption partition coefficient suggested that the adsorption of DOC is influenced by pedogenic Fe and Al oxides. Other studies have observed such a positive correlation between DOC adsorption and Fe_DCB (80–82). The positive relationship observed between Fe_OX and Al_OX with the DOC adsorption partition coefficient indicated that poorly crystalline Fe and Al oxides also contributed to DOC adsorption. These results are consistent with previous studies (83, 84). The small increase in equilibrium DOC solution pH with DOC adsorption was expected as during ligand exchange reaction, coordinative OH groups at the surfaces of Fe/Al oxides become replaced by organic anion (42, 44, 46, 47). The pH of all soil samples was acidic and below the PZC of Fe/Al oxides, thus supporting the sorption of anionic species, including dissociated organic acids. Tipping (47) reported that the greatest sorption of humic substances by iron oxides occurred at pH 5.

Our results showed that DOC did not have any influence on P adsorption from the mixed solution of P and DOC. The adsorption of P promoted DOC desorption in the surface and subsurface soils from Wagga Wagga and Tumbarumba. The results suggested that P competed more effectively than DOC for sorption sites on mineral surfaces. It also indicated that DOC was adsorbed by a weaker interaction on Fe/Al oxides. This result did not support our hypothesis that DOC would inhibit phosphate adsorption in both surface and subsurface soils. Phosphate addition in the studied soils may affect the adsorption and desorption of DOC, favouring destabilisation and mineralisation of OC (49, 85). Furthermore, continuous phosphate addition can lead to increased release of DOC and thus making OC available for microbial decomposition in the studied soils (59). Hur and Schlautman (86) reported a significant reduction in sorption of DOC onto hematite in the presence of P. Kahle et al. (80) observed that the blocking of OH groups on mineral surfaces with phosphate significantly reduced DOC adsorption. Our finding that increased phosphate concentration promotes DOC desorption is consistent with findings from previous studies on soils (49, 59, 87–90).

Implications

The results of this study have crucial implications on the sustainability of Fe-rich soils. The adsorption of phosphate promoted DOC desorption in these soils, which may lead to the destabilisation of OC and impair OC sequestration and enhance microbial decomposition in these soils. However, the effect of phosphate addition on DOC desorption may depend on different soil environmental conditions, which may vary from one soil type to another.

Conclusions

This study shows that the Tumbarumba soils had the greater affinity for phosphate adsorption than the soils from Wagga Wagga, which is due to the higher contents of Fe and Al oxides in Tumbarumba soils. Phosphate adsorption was greater in the subsurface than in the surface soil from Tumbarumba. This supports our first hypothesis, which states that phosphate adsorption will be greater in subsurface than in surface soils. However, soils from Wagga Wagga did not support the hypothesis since phosphate adsorption was greater in surface than in subsurface soils. The affinity for DOC was greater in the subsurface soils than the surface soils from both sites. This accords with the first hypothesis that DOC adsorption will be greater in subsurface than in surface soils. In contrast to our hypothesis, we found that in the mixed solution of P and DOC, phosphate adsorption promoted DOC desorption in the surface and subsurface soils from Wagga Wagga and Tumbarumba. This suggest that phosphate outcompeted DOC for the adsorption sites on soil mineral surfaces.

Extractable Fe and Al explained well the phosphate and DOC adsorption behaviour in the studied soils and may serve as suitable routine predictors of P and DOC adsorption capacity in surface and subsurface soils.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

BA: Data curation, Investigation, Conceptualization, Writing – review & editing, Methodology, Writing – original draft, Software, Formal Analysis, Visualization. FD: Funding acquisition, Supervision, Writing – review & editing. BS: Supervision, Methodology, Writing – review & editing, Conceptualization, Data curation, Funding acquisition.

Funding

The author(s) declared financial support was received for this work and/or its publication. This work was supported by the Soil Science Challenge Grants Program funded by the Australian Government Department of Agriculture, Fisheries and Forestry, and this article contributes towards the National Soil Strategy and the implementation of the National Soil Action Plan.

Acknowledgments

We acknowledge the Soil Science Challenge Grants Program funded by the Australian Government Department of Agriculture, Fisheries and Forestry, and this article contributes towards the National Soil Strategy and the implementation of the National Soil Action Plan. Bright is thankful to the Tertiary Education Trust Fund (TETFund), Nigeria for the financial support in tuition fees and living expenses.

Conflict of interest

The authors declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

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