Phosphate sorption by natural hematites

Iron (hydr)oxides are active phosphate sorbents in soils and sediments, but no information exists on phosphate (P) sorption by natural hematites. In this study, we characterized the chemical, mineralogical and P-sorption properties of 14 hematite-rich natural materials of different origins. Phosphate sorption was described by a modified Freundlich equation including a time term. Phosphate sorbed at 1d at an equilibrium concentration of 1 mg P dm^?3 ranged widely (0.2–1.7 μmol m^?2). After 1 d, hematites showed a marked slow sorption. At 75 d, and an equilibrium concentration of 6mg P dm^?3, the total amount of P sorbed ranged between 0.8 and 4.1 μmol mp^?2. Phosphate-sorption capacity was influenced by the morphology of the coherently scattering domains: the shorter the domains in the crystallographic c relative to the a direction, the lower the P-sorption capacity. This has been observed by other authors in synthetic hematites and agrees with the idea that the prismatic faces, which have singly co-ordinated Fe-OH groups, are more active in P-sorption than other faces. The average P sorption capacity of hematites was similar to that of natural goethites (2.6 μmol P m^?2) but, in contrast to these, variability among samples was high. In comparison with goethites, hematites show slower sorption and lower affinity for phosphate.     

Phosphate sorption characteristics of major soils in Okinawa,Japan

Abstract

Phosphate sorption isotherms were determined for 16 representative major soils developed from different parent materials on Okinawa. Phosphate sorption characteristics were satisfactorily described by the Langmuir equation, which was used to determine phosphorus (P) sorption maxima of the soils. Phosphate sorption maxima ranged from 630 to 2208 mg P kg^‐1 soil (mean 1,362 mg P kg^‐1). The standard P requirement (i.e., the amount of P required to attain 0.2 mg P L^‐1 equilibrium solution) followed the same trend as sorption maximum (r =0.94***), with values ranging from 132 to 1,020 mg P kg^‐1 soil (mean 615 mg P kg^‐1). This mean value corresponds to fertilizer addition of 923 kg P ha^‐1 indicating that the soils have high P fertilizer requirements. Results of simple linear regression analysis indicated that sorption maximum was significantly correlated with clay content, organic matter, oxalate iron (Fe), pyrophosphate Fe, DCB aluminum (Al), oxalate Al, and pyrophosphate Al, but not with DCB Fe, pH, or available P content. The best regression model for predicting sorption maximum was the combination of clay, organic matter, pyrophosphate Fe, and DCB Al which altogether explained 79% of the variance in sorption maximum. The equation obtained could offer a rapid estimation of P sorption in Okinawan soils.     

THE HIGH- AND LOW-ENERGY PHOSPHATE ADSORBING SURFACES IN CALCAREOUS SOILS

The two-surface Langmuir equation was used to study P adsorption by 24 calcareous soils (pH 7.2-7.6; 0.8-24.2 per cent CaCO₃) from the Sherborne soil series, which are derived from Jurassic limestone. High-energy P adsorption capacities (x″_m) ranged from 140–345 μg P/g and were most closely correlated with dithionite-soluble Fe. Hydrous oxides therefore appear to provide the principal sites, even in calcareous soils, on which P is strongly adsorbed (x″_m 6–51 ml/μg P). The low-energy adsorption capacities (x″_m) ranged from 400–663 μg P/g and were correlated with organic matter contents and the total surface areas of CaCO₃ but not with per cent CaCO₃, pH, or dithionite-soluble Fe. Total surface areas of CaCO₃ in the soils ranged from 4.0 to 8.5 m²/g soil. Low-energy P adsorption capacities agree reasonably with values (100 pg P/m²) for the sorption of phosphate on Jurassic limestones but phosphate was bonded much less strongly by soil carbonates (k″= 0.08–0.45 ml/μg P) than by limestones (k～10.0 ml/μg P). Low-energy P adsorption in these soils is tentatively ascribed to adsorption on sites already occupied by organic anions (and probably also by bicarbonate and silicate ions) which lessen the bonding energy of co-adsorbed P.     

Inorganic and organic ligands induced changes in Pb sorption of arid ‎clay and calcareous loamy sand soils

The objective of this study was to investigate sorption, desorption, and immobilization of Pb in the clay and calcareous loamy sand soils treated with inorganic ligands (NO₃^?, Cl^? and H₂PO₄^?). Pb sorption was also determined in the presence of oxalate and citrate. The maximum Pb sorption capacities (q) ranged from 42.2 to 47.1 mmol kg^?1 for the clay soil, and from 45.2 to 47.0 mmol kg^?1 for loamy sand soil. It was observed that the binding energy constant (k) for Pb sorbed onto loamy sand soil (528–1061) is higher than that for clay soil (24.38–55.29). The loamy sand soil-sorbed greater quantities of Pb compared to the clay soil when initial pH was ≥ 3. However, it had lower sorption capacity at the lowest initial pH of 2. Additionally, the greatest Pb sorption and immobilization occurred in the soil treated with H₂PO₄. In the clay soil, the sorption of Pb was depressed at 0.1 mol kg^?1 of Cl^?, as compared with other ligands. Concerning organic acids, citrate ligand showed the highest decrease in Pb sorption. It could be concluded that the nature of Pb sorption can depend on the type and quantity of ligands present, as well as the soil type.     

Influence of extractable iron,aluminium, and manganese on P-sorption in flooded acid sulfate soils

We evaluated the effect of 1 N NH₄OAc and sodium-citrate dithionite extractable forms of soil Fe, Al, and Mn on P-sorption of a flooded acid sulfate soil (Sulfic Tropaquepts) and a non-acid sulfate soil (Typic Tropaquepts) under different soil oxidation-reduction and pH conditions. We used Maha-Phot soil (Sulfic Tropaquepts) and Bangkok soil (Typic Tropaquepts) from the Bangkok Plain, Thailand, and incubated them with 0.2% rice straw under aerobic (O₂ atmosphere) and anaerobic (N₂ atmosphere) conditions at three different levels of pH (4.0, 5.0, and 6.0) for 6 weeks in stirred soil suspensions with a soil to 0.01 M CaCl₂ solution ratio of 1:7. After the incubation period, the soil suspensions in the first treatment (control) were not washed or pretreated with any extractants. For the second treatment (II), the soil suspensions were treated with 1 N NH₄OAc (buffered to pH 4.0) to remove Fe, Al, and Mn in exchangeable form. In the third treatment (III), the soils suspensions were treated with sodium citrate dithionite solution (20%) to remove Fe, Al, and Mn in the form of free oxides. The soil residues were then equilibrated with KH₂PO₄ ranging from 0 to 500 mg P kg^-1 soil. Sorption isotherms were described by the classical Langmuir equation. The P-sorption parameters under study were standard P requirement (SPR), Langmuir maximum sorption capacity (X _m), Langmuir sorption constant (k), and buffering index (BI). Treating soils with 1 N NH₄OAc reduced X _m by 32–55%, SPR by 68–84%, and also decreased the differences in P-sorption due to the effects of pH and oxidation-reduction conditions. Significant correlations between the P-sorption parameters and the amount of free iron oxides indicated the primary role of iron oxides in P-sorption of acid sulfate soils. Aluminium oxides seemed to play a secondary role in P-sorption of these soils. Manganese also showed an important effect on P-sorption, but the mechanism is ambiguous.This is a contribution from the Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, LA 70803-7511     

Phosphate sorption by red mediterranean soils from Greece

Abstract

In nineteen surface horizons of red Mediterranean soils from various locations of Greece, phosphorus (P) sorption experiments were conducted and the sorption characteristics were studied in relation to soil properties. Phosphate sorption data were fitted both to the Langmuir and Freundlich equations. From these equations, the following P sorption parameters were determined from the Freundlich equation, X = ACⁿ, the parameters A (the phosphate sorbed at C = 1 mg P/L), n (the P sorption intensity), the P sorption index (PS = X/log C) and maximum P sorption (Xmfr). From the Langmuir equation, C/X = 1/KXm + C/Xm, the parameters K (showing the bonding energy), maximum P sorption (Xmla), the quantity of P adsorbed at a standard concentration of 0.2 mg P/L (P0.2), and P maximum buffering capacity (PMBC). The Freundlich parameter A was strongly correlated to the clay and sesquioxides ("free”; iron and aluminum oxides and amorphous iron oxides) content. Seventy‐four percent of the variance of this parameter was explained by clay and “free”; iron (Fe) content. The Freundlich parameter n was significantly correlated with pH and amorphous iron oxides content, while 52% of its variance was explained by amorphous Fe and dithionite extrac‐table aluminum (Al). The P sorption maxima calculated from the Freundlich equation were in general lower than those calculated by the Langmuir equation. Both these parameters were strongly correlated with clay and more slightly with sesquioxides content. About 50% of their variance was explained by clay content of the soils. The P sorption index was strongly correlated with the clay content and less strongly with dithionite‐extractable Fe and Al. The P‐buffering capacity calculated from the data of Langmuir equation was also strongly correlated with these two parameters. In addition, clay content and dithionite‐extractable Fe and Al were well correlated to the amounts of P required to obtain an equilibrium concentration of 0.2 mg P/L while 61% of the variation of this parameter was explained by the clay and the dithionite‐extractable Fe content. From these findings, it seems that for the red Mediterranean soils from Greece, P sorption is affected by clay content and iron and aluminum oxide contents.     

The effect of soil type on phosphorus sorption capacity and desorption dynamics in Irish grassland soils

Abstract. The phosphorus (P) sorption and desorption dynamics of eleven major agricultural grassland soil types in Ireland were examined using laboratory techniques, so that soils vulnerable to P loss might be identified. Desorption of P from soil using the iron-oxide paper strip test (Pfeo), water extractable P (Pw) and calcium chloride extractable P (Pcacl₂) depended on soil P status in all soils. However, soil types with high organic matter levels (OM), namely peat soils (%OM >30), had lower Pfeo and Pw but higher Pcacl₂ values compared to mineral soils at similar soil test P levels. Phosphorus sorption capacity remaining (PSCr) was measured using a single addition of P to soils and used to calculate total P sorption capacities (PSCt) and degree of P saturation (DPS). Phosphorus sorption capacities correlated negatively with % OM in soils indicating that OM may inhibit P sorption from solution to soil. High organic matter soils exhibited low P sorption capacities and poor P reserves (total P, oxalate extractable P) compared to mineral soils. Low P sorption capacities (PSCt) in peat soils were attributed to OM, which blocked or eliminated sorption sites with organic acids, therefore, P remained in the soil solution phase (Pcacl₂). In this work, peat and high organic matter soils exhibited P sorption and desorption characteristics which suggest that these soils may not be suitable for heavy applications of manure or fertilizer P owing to their low capacities for P sorption and storage.     

水溶性有机碳在各种粘土底土中的吸附：土壤性质的影响

Clay-rich subsoils are added to sandy soils to improve crop yield and increase organic carbon (C) sequestration; however, little is known about the influence of clay subsoil properties on organic C sorption and desorption. Batch sorption experiments were conducted with nine clay subsoils with a range of properties. The clay subsoils were shaken for 16 h at 4 ^oC with water-extractable organic C (WEOC, 1 224 g C L^-1) from mature wheat residue at a soil to extract ratio of 1:10. After removal of the supernatant, the residual pellet was shaken with deionised water to determine organic C desorption. The WEOC sorption was positively correlated with smectite and illite contents, cation exchange capacity (CEC) and total organic C, but negatively correlated with kaolinite content. Desorption of WEOC expressed as a percentage of WEOC sorbed was negatively correlated with smectite and illite contents, CEC, total and exchangeable calcium (Ca) concentrations and clay content, but positively correlated with kaolinite content. The relative importance of these properties varied among soil types. The soils with a high WEOC sorption capacity had medium CEC and their dominant clay minerals were smectite and illite. In contrast, kaolinite was the dominant clay mineral in the soils with a low WEOC sorption capacity and low-to-medium CEC. However, most soils had properties which could increase WEOC sorption as well as those that could decrease WEOC sorption. The relative importance of properties increasing or decreasing WEOC sorption varied with soils. The soils with high desorption had a low total Ca concentration, low-to-medium CEC and low clay content, whereas the soils with low desorption were characterised by medium-to-high CEC and smectite and illite were the dominant clay minerals. We conclude that WEOC sorption and desorption depend not on a single property but rather a combination of several properties of the subsoils in this study.     

Zinc sorption by acid tropical soils as affected by cultivation

The sorption of zinc (Zn) by two acid tropical soils, Mazowe clay loam (kaolinitic, coarse, Rhodic Kandiustalf) and Bulawayo clay loam (coarse, kaolinitic, Lithic Rodustalf), was studied over a wide range of Zn solution concentrations. Samples of the two soils used in the experiments were collected at both uncleared, uncultivated (virgin) sites and cultivated sites. The two virgin soils showed similar abilities to bind Zn. Mazowe soil (40 g organic matter kg^?1) presented the highest affinity for Zn. Yet, Bulawayo soil (23.5 g organic matter kg^?1) sorbed almost the same amount. Bulawayo soil had higher pH and Fe and Mn-oxide content than Mazowe soil. Once cultivated, the two soils behaved quite differently. After 50 years, Mazowe soil had lost 60% of its organic matter and effective cation exchange capacity (ECEC). In this soil, Zn sorption capacity had also been decreased by 60%. Clearing and 10 years under cultivation had affected neither the organic matter content nor the ECEC of Bulawayo soil. For this soil, Zn sorption was even higher in the cultivated soil, presumably due to an increase in the amount of Fe and Mn oxide from subsoiling. Zinc sorption was dependent upon pH, with retention dramatically increasing in the pH range 6–7. Sorption occurred at pH values below the point of zero charge (PZC), indicating that the sorption reaction can proceed even in the presence of electrostatic repulsion between the positively charged soil surface and the cation. In the two soils, the reversibility of the sorption reaction was very low. More than 90% of the sorbed Zn was apparently strongly bonded.     

Sorption of dimethylselenide by soils

Air-dry and moist soils were shown to possess the capacity to sorb substantial amounts of (⁷⁵Se)dimethylselenide produced by the yeast Candida humicola in culture, or by soil supplied with (⁷⁵Se)selenite, depending largely upon the organic matter content and selenium concentration of the soils. The sorption capacities of individual soil constituents followed the order; organic matter > clay minerals > manganese oxides > iron oxides > acid-washed sand.A chemical fractionation procedure applied to soils fumigated with (⁷⁵Se)dimethylselenide revealed that the majority of the selenium sorbed was converted after 1 month to other forms, extractable mainly with strong acid solutions. Experiments with sterilized (autoclaved and γ-irradiated) soils indicated that soil microorganisms played little, if any, part in the sorption process.The work reported here indicates that soil is an important natural “sink” for atmospheric dimethylselenide.     

土壤组分对广东省酸性水稻土磷吸附参数的影响

Soil components affecting phosphate sorption parameters were studied using acid paddy soils derived from basalt, granite, sand-shale and the Pearl River Delta sediments, respectively, in Guangdong Province.For each soil, seven 2.50 g subsamples were equilibrated with 50 mL 0.02 mol L-1 (pH=7.0) of KCl containing 0, 5, 10, 15, 25, 50 and 100 ng P kg-1, respectively, in order to derive P sorption parameters (P sorption maximum, P sorption intensity factor and maximum buffer capacity) by Langmuir isotherm equation. It was shown that the main soil components influencing phosphate sorption maximum (Xm) included soil clay, pH,amorphous iron oxide (Feo) and amorphous aluminum oxide (Alo), with their effects in the order of Alo ＞Feo ＞ pH ＞ clay. Among these components, pH had a negative effect, and the others had a positive effect.Organic matter (OM) was the only soil component influencing P sorption intensity factor (K). The main components influencing maximum phosphate buffer capacity (MBC) consisted of soil clay, OM, pH, Feo and Alo, with their effects in the order of Alo ＞ OM ＞ pH ＞ Feo ＞ clay. Path analysis indicated that among the components with positive effects on maximum phosphate buffer capacity (MBC), the effect was in the order of Alo ＞ Feo ＞ Clay, while among the components with negative effects, OM ＞ pH. OM played an important role in mobilizing phosphate in acid paddy soils mainly through decreasing the sorption intensity of phosphate by soil particles.     

The sorption of Cd and Zn by different soils in the presence of dissolved organic matter from sludge

Dissolved organic matter (DOM) is one of the important factors affecting metal mobility and phytotoxicity in the soils receiving sewage sludge. The aim of this study was to investigate the effects of DOM from anaerobically digested dewatered sludge on Cd and Zn sorption by three different soil types (calcareous clay loam, calcareous sandy loam and acidic sandy loam) of different physico-chemical properties through batch studies. The addition of DOM significantly reduced the Cd and Zn sorption capacity by a factor of 2.1–5.7 for Cd and 2.3–13.7 for Zn for these three soils as seen by their K values in the Freundlich equation compared to the control receiving no DOM, suggesting that DOM had a stronger inhibitory effect on Zn sorption than that of Cd. The reduction in metal sorption caused by DOM was very apparent in the pH range of 5 to 8, with a maximum inhibition on metal sorption occurring at pH 7–7.5 especially for Zn but the effect was minimal at lower pH. At a DOM concentration of < 200 mg C l^− 1, Cd and Zn sorption by all the three soils decreased with an increase in DOM concentration. At each given DOM concentration, the inhibition of metal sorption of the different soil types increased in the following order: acidic sandy loam < calcareous sandy loam < calcareous clay loam. DOM derived from sludge would significantly reduce metal sorption and increase its mobility through the formation of soluble DOM–metal complexes and poses risk of metal leaching and phytotoxicty in near-neutral and alkaline soils.     

Clay,Organic Carbon,Available P and Calcium Carbonate Effects on Phosphorus Release and Sorption–Desorption Kinetics in Alluvial Soils

Information on phosphorus (P) release kinetics and sorption–desorption in soils is important for understanding how quickly reaction approaches equilibrium and replenishes the depleted soil solution. Laboratory experiments were conducted to study the P release and sorption–desorption kinetics in soils differing in clay, soil organic carbon (SOC), available P, and calcium carbonate (CaCO₃) contents. Phosphorus release from soils proceeded in two phases: initially faster phase followed by a slower phase as equilibration progressed. Elovich equation (R² ≥ 0.97**) described well the P release versus time data. P release coefficients for power function were significantly correlated with available P and SOC. Freundlich sorption constants increased with increase in clay and CaCO₃ content. With increase in SOC and available P concentration in soils, substantial reduction in sorption constants was observed. It was concluded that for efficient P management, it is important to take into account soil texture, the existing soil P level, SOC content, and soil calcareousness.     

Contrasting Effects of Four Plant Residues on Phosphorus Sorption-Desorption in Some Phosphorus Fertilized Calcareous Soils

Four types of plant residues (fruit waste, potato, sunflower, and wheat) with wide ranges of carbon to nitrogen (C/N) and carbon to phosphorus (C/P) ratios were added to the soil at the rate of 20 g kg^?1 (dry weight basis) and incubated for two months. In soils treated with plant residues, the P sorption ranged from 62.0% (potato) to 96.6% (wheat) and from 12.6% (fruit waste) to 50.6% (wheat) when 20 and 1500 mg P kg^?1 were added to the soils, respectively. In general, incorporation of plant residues decreased maximum P sorption capacity but increased bonding energy. The maximum P sorption capacity was reduced from 586 mg kg^?1to 500, 542, and 548 by wheat, fruit, and potato residues, respectively, but increased to 665 mg kg^?1 by sunflower residue. At higher P addition, the highest percentage of desorbed P was observed in soils treated with wheat residue (49.9%); followed by fruit waste (46.5%), potato (43.5%), sunflower (38.8%) and control soils (37.0%). It indicated that the P content of the organic residues had an important role in the sorption and desorption of P in calcareous soils. Among organic residues, sunflower residue showed high sorption and low desorption of P in soils, indicating a higher potential of this organic residue for P retention and reducing surface and groundwater contamination in calcareous soils.     

Poorly crystalline iron and aluminium oxides contribute to the carbon saturation and sorption of dissolved organic carbon in the soil

Soil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (<53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (>15 g kg^?1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil.     

Effects of soil organic matter on pH-dependent phosphate sorption by soils

Effects of soil organic matter (80M) on P sorption of soils still remain to be clarified because contradictory results have been reported in the literature. In the present study, pH-dependent P sorption on an allophanic Andisol and an alluvial soil was compared with that on hydrogen peroxide (H₂0₂)-treated, acid-oxalate (OX)-treated, and dithionite-citrate- bicarbonate (DCB)-treated soils. Removal of 80M increased or decreased P sorption depending on the equilibrium pH values and soil types. In the H₂O₂ OX-, and DCB-treated soils, P sorption was pH-dependent, but this trend was not conspicuous in the untreated soils. It is likely that 80M affects P sorption of soils through three factors, competitive sorption, inhibition of polymerization and crystallization of metals such as AI and Fe, and flexible structure of metal-80M complexes. As a result, the number of available sites for P sorption would remain relatively constant in the wide range of equilibrium pH values in the presence of 80M. The P sorption characteristics were analyzed at constant equilibrium pH values (4.0 to 7.0) using the Langmuir equation as a local isotherm. The maximum number of available sites for P sorption (Q _max) was pH-dependent in the H₂0₂-, OX-, and DCBtreated soils, while this trend was not conspicuous in the untreated soils. Affinity constants related to binding strength (K) were less affected by the equilibrium pH values, soil types, and soil treatments, and were almost constant (log K ≈ 4.5). These findings support the hypothesis that 80M plays a role in keeping the number of available sites for P sorption relatively constant but does not affect the P sorption affinity. By estimating the Q _max and K values as a function of equilibrium pH values, pH-dependent P sorption was well simulated with four or two adjustable parameters. This empirical model could be useful and convenient for a rough estimation of the pH-dependent P sorption of soils.     

Competitive sorption of cadmium and lead in acid soils of Central Spain

The bioavailability and ultimate fate of heavy metals in the environment are controlled by chemical sorption. To assess competitive sorption of Pb and Cd, batch equilibrium experiments (generating sorption isotherms) and kinetics sorption studies were performed using single and binary metal solutions in surface samples of four soils from central Spain. For comparisons between soils, as well as, single and binary metal solutions, soil chemical processes were characterized using the Langmuir equation, ionic strength, and an empirical power function for kinetic sorption. In addition, soil pH and clay mineralogy were used to explain observed sorption processes. Sorption isotherms were well described by the Langmuir equation and the sorption kinetics were well described by an empirical power function within the reaction times in this study. Soils with higher pH and clay content (characterized by having smectite) had the greatest sorption capacity as estimated by the maximum sorption parameter (Q) of the Langmuir equation. All soils exhibited greater sorption capacity for Pb than Cd and the presence of both metals reduced the tendency for either to be sorbed although Cd sorption was affected to a greater extent than that of Pb. The Langmuir binding strength parameter (k) was always greater for Pb than for Cd. However, these k values tended to increase as a result of the simultaneous presence of both metals that may indicate competition for sorption sites promoting the retention of both metals on more specific sorption sites. The kinetic experiments showed that Pb sorption is initially faster than Cd sorption from both single and binary solutions although the simultaneous presence of both metals affected the sorption of Cd at short times while only a minor effect was observed on Pb. The estimated exponents of the kinetic function were in all cases smaller for Pb than for Cd, likely due to diffusion processes into micropores or interlayer space of the clay minerals which occurs more readily for Cd than Pb. Finally, the overall sorption processes of Pb and Cd in the smectitic soil with the highest sorption capacity of the studied soils are slower than in the rest of the soils with a clay mineralogy dominated by kaolinite and illite, exhibiting these soils similar sorption rates. These results demonstrate a significant interaction between Pb and Cd sorption when both metals are present that depends on important soil properties such as the clay mineralogy.     

Capacity of soils for sorption of hydrogen sulfide

Abstract

Studies with 42 soils selected to obtain a wide range in properties showed that air‐dry and moist soils have substantial capacities for sorption of H₂S from air (averages, 9.8 and 12.5 g S kg^‐1 soil, respectively). Soil properties influencing the capacities of air‐dry soils for sorption of H₂S included sand and clay contents, DCB‐soluble Mn, exchangeable Na, DCB‐soluble Fe, and total DCB‐soluble metals. The corresponding capacities of moist soils were influenced by sand and clay contents, DCB‐soluble Mn, and surface area. It was possible to closely predict the H₂S sorption capacities of both air‐dry and moist soils (R² = 0.804 and 0.918, respectively) from consideration of their properties.     

Short-term phosphorus sorption and desorption in contrasting cropped Vertisols

Vertisols are important cropping soils in tropical and subtropical areas, but in many regions, decades of cropping has substantially reduced concentrations of plant-available phosphorus (P), especially in the subsoil layers. Phosphorus behaviour in P-depleted Vertisols has received comparatively little attention, and the availability of P following the addition of inorganic P fertilisers at different concentrations is poorly understood. In this study, we evaluated short-term P sorption and desorption behaviour in cropped Vertisols in relation to specific soil physical and chemical properties. We collected the surface and subsurface of 15 Australian soils with a broad range of physical and chemical properties, comprising nine Vertisols, three Ferralsols, two Lixisols and one Calcisol. For each soil, we generated sorption and desorption curves (fitted with a Freundlich equation), determined soil physical and chemical properties likely to influence P sorption and evaluated the relationships between the measured soil properties and the Freundlich equation sorption coefficients. The P sorption curves differed drastically between soils, with the sorption equation coefficients (a_S × b) significantly correlated with the P buffering index (PBI) and clay content. Clay content itself was correlated with citrate-extractable Fe and Al oxides and BET surface area. Vertisols formed on basaltic parent materials had greater Fe and Al oxide concentrations, resulting in an overall greater P sorption capacity. Sorption and desorption hysteresis were mostly small. The reacting materials in these soils probably had limited ability to continue to react with P. The Vertisols differed in their capacity to replenish P in the soil solution by desorbing different proportions of previously sorbed P, although the proportion of desorbable P generally increased with greater concentrations of sorbed P. These results suggest that for fertiliser management in these soils, smaller volumes of P enrichment combined with higher P concentrations may result in a greater P recovery by the crop.