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Soil and Water Science Dep., 106 Newell Hall, P.O. Box 110510, Univ. of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL 32611
Corresponding author (dag{at}mail.ifas.ufl.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: AEM, anion-exchange resin–impregnated membrane • CBD, citrate-dithionite-bicarbonate • Pyro-, pyrophosphate-extractable
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Spodosols are the prevalent soil order in Florida. These soils occur in a number of areas supporting extensive beef, dairy, citrus, and vegetable operations that constitute an important source of P to Florida's valuable ecosystems (Bottcher et al., 1999). Because many of these soils occur along a hydrologic continuum from uplands to wetlands and surface water, the biogeochemical behavior and transport of applied P in these soils could determine to a large extent the amount of P that moves into downstream waters.
Spodosols are characterized by sandy textures, fluctuating water table, and the presence of a spodic horizon (Bh) below an A–E horizon sequence. The A horizon is sandy with small amounts of organic matter and generally has a depth of 15 to 20 cm. The E horizon is highly eluted with thickness ranging from about 20 to 140 cm depending on the soil series. Below the E horizon is the Bh horizon in which C and associated metals (mainly Al) have accumulated (Soil Survey Staff, 1996). This characteristic composition coupled with low pH account for the propensity of the Bh horizon to sorb P. Phosphorus moving from the surface horizon is generally thought to accumulate in the Bh horizon. However, during the summer rainy periods, shallow water tables would not only restrict movement of P into the Bh horizon, but also, depending on the period of submergence, may trigger changes in pH and redox potential of the soil. These electrochemical changes could influence the dynamics of P in the Bh horizon and, consequently, its transport to watercourses.
Transport of P with water in soils depends largely on the extent of P retention by soil components. The high P sorption capacity of acid soils has often been linked with Fe and Al oxide surfaces. Significant correlations were found between P sorption and oxalate-extractable Al and Fe in Spodosols from British Columbia (Yuan and Lavkulich, 1994). Similarly, van der Zee and van Riemsdijk (1986) reported that P sorption in acid soils from the Netherlands was linearly related to the sum of oxalate-extractable Fe and Al. In a range of Danish soils, P sorption was a function of CDB-extractable Fe and oxalate-extractable Al and Fe (Borggaard et al., 1990). Citrate-dithionite-bicarbonate-extractable Fe and Al, oxalate-extractable Al, and clay content were major contributors to P sorption in Australian soils (Singh and Gilkes, 1991). Several researchers have reported positive relationships between the organic matter content of soils and P sorption (Owusu-Bennoah and Acquaye, 1989; Sanyal and De Datta, 1991). The role of organic matter in increasing the ability of soils to sorb P has been attributed to its association with cations such as Fe, Al, and Ca.
Quantitative descriptions of P sorption by soils have often been made with the Langmuir, Freundlich, and Temkin equations. Although these equations were originally derived to describe the adsorption of gases by solids, in many cases these models were able to provide adequate mathematical descriptions of P sorption in soils. By far, the most widely used model is the Langmuir equation, which has a distinct advantage over Freundlich and Temkin equations in that it allows estimation of sorption maximum and a constant that is related to the P binding strength. However, these sorption parameters do not provide specific information about the actual mechanism of a sorption process in soils. They are regarded simply as curve-fitting parameters, unless other independent data are presented to support their chemical significance (Sposito, 1982). Despite these limitations, the Langmuir equation has been used successfully in describing P retention properties of Spodosols (Nair et al., 1999; Zhou et al., 1997), wetland soils and stream sediments (Reddy et al., 1998), and lake sediments (Olila and Reddy, 1993).
This study was conducted to (i) compare the P sorption and release properties of Bh horizons from selected Florida Spodosols under aerobic and anaerobic conditions, (ii) evaluate the relationships among P sorption parameters and selected soil properties, and (iii) identify soil variables that best predict P sorption and release by these soils.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Various pools of soil Fe and Al were measured by selective dissolution. Reactive Al was estimated by extraction with 1 M CuCl2 · H2O (Juo and Kamprath, 1979). Aluminum and Fe associated with organic matter were extracted with 0.01 M Na4P2O7 (Wada and Higashi, 1976). Amorphous and crystalline forms of Al and Fe were dissolved in acidified 0.2 M (NH4)2C2O4 (McKeague and Day, 1966) and CDB (Mehra and Jackson, 1960), respectively. Exchangeable Al was extracted with 1 M KCl. Total Fe and Al were determined by digestion in nitric-perchloric acid mixture. Iron and Al in the extracts were analyzed using an inductively coupled argon plasma emission spectrophotometer.
Phosphorus Sorption Studies
Aerobic
Duplicate 1-g soil samples were placed in 50-mL centrifuge tubes and suspended in 20 mL of 0.01 M KCl solution containing 0, 1, 3, 6, 16, 32, 80, 161, 323, 1613, and 3226 µM P added as KH2PO4. Equilibration was done in an end-over-end shaker for 24 h at room temperature. The samples were centrifuged at 3620 g for 10 min, filtered through a 0.45-µm membrane filter, and analyzed for P on a Technicon Autoanalyzer II (Technicon Industrial, Tarrytown, NY) using the method of Murphy and Riley (1962).
Anaerobic
Duplicate 1-g soil samples were suspended in 10 mL of deionized water (O2–free) and incubated for 4 wk at room temperature. The tubes were purged three times each week with prepurified N2 gas. After incubation, 10 mL of deaerated 0.02 M KCl solution containing the same range of P as used in the aerobic studies were injected into the centrifuge tubes through a rubber septum fitted into a hole drilled in the cap. The samples were equilibrated in an end-over-end shaker for 24 h at room temperature, after which the tubes were centrifuged at 3620 g for 10 min. The supernatant liquid was withdrawn with a syringe and filtered through a 0.45-µm membrane filter under O2–free conditions. Phosphorus in solution was analyzed according to Murphy and Riley (1962) on a Technicon Autoanalyzer II.
Phosphorus Desorption Experiments
Phosphorus desorption isotherms were obtained by initially equilibrating duplicate 1-g soil samples with 20 mL of 0.01 M KCl containing 3226 µM P as KH2PO4. The amount of P sorbed by the soil after a 24-h equilibration represented the starting value for adsorbed P. These P saturated soils then were subjected to sequential desorption with 20 mL of 0.01 M KCl equilibrated in an end-over-end shaker for 24 h. Phosphorus in the extracts was analyzed as outlined above. Sequentially desorbable P was determined by summing total P desorbed after nine successive extractions.
Evaluation of Phosphorus Sorption Data
The term sorption refers here to removal of P from solution by either adsorption or precipitation. Sorption of added P, Si (mmol P kg-1) was calculated as
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Sorption data then were fitted into a linearized simple Langmuir equation
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Statistical Analysis
Data analyses were done using SAS program procedures (SAS Institute, 1990). Empirical relationships between P sorption parameters and soil properties were established using correlation and regression analyses.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Soil pH values in 1 M KCl were very acidic and were lower than those measured in water, indicating that all soils at their natural pHs are negatively charged (Table 2). Organic C contents ranged from 7.3 to 32.6 g kg-1, comparable with values reported for Bh horizons from New York (Laverdiere and Weaver, 1977) and British Columbia (Yuan and Lavkulich, 1994). Except for Vero, the soils were dominated by Al, with the mole ratio of total Al to Fe ranging from 1.2 to 42.7. Mean total Al and Fe contents were 153 and 16 mmol kg-1, respectively. Much of the total Al and Fe was amorphous (oxalate-extractable), comprising a combined average of 60% (Table 3). Similar numbers have been reported for Bh horizons from southern New Zealand (Parfitt and Childs, 1988), Canada (Yuan and Lavkulich, 1994; Wang et al., 1987), and the northeastern USA (Laverdiere and Weaver, 1977), though there was greater accumulation of Fe in the latter soils than in Florida Spodosols. The fraction of Fe and Al that dissolved in pyrophosphate was equally high. Pyrophosphate extracts organically complexed Fe and Al, while oxalate dissolves both amorphous and organically bound Fe and Al (Parfitt and Childs, 1988). Thus the difference between the two extractions is taken as the amount of amorphous inorganic Fe and Al oxides present (McKeague et al., 1971). However, in this study respective values were similar, suggesting that pyrophosphate may be extracting more than just organo-metallic complexes. Zhou et al. (1997) reported similar findings for five of eight Bh horizons from Florida Spodosols. Cupric chloride is another reagent used to extract Al associated with organic matter. This reagent gave values lower than pyrophosphate, which suggests a lack of selectivity for the pyrophosphate reagent. The values obtained from CDB extraction [(Al + Fe)d] were as large as for oxalate, in part because CDB is less selective than acid oxalate. Citrate-dithionite-bicarbonate solubilizes both crystalline and amorphous forms of Al and Fe. To obtain an estimate of crystalline forms of soil Al and Fe, the convention is to take the difference between CDB and oxalate extractions (Freese et al., 1992). Compared with oxalate- and pyrophosphate-extractable (Pyro-) Al and Fe, however, this fraction accounted for only a small portion of the total Al and Fe in our soils. Exchangeable Al was the smallest of the pools, with values ranging from 0.06 to 7.55 mmol kg-1.
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The Bh samples exhibited low equilibrium P concentrations (EPC) (Table 5). Excluding the high-P containing Cornelia, Narcoossee, and Ona soils, the mean EPC values of the soils was 0.3 mM P for aerobic soils and 0.4 mM P for anaerobic soils. These values are comparable with those reported by Nair et al. (1998) for Bh horizons from the native, unimpacted areas in the Lake Okeechobee watershed. The low EPC values associated with the Bh horizons in this study suggest that these horizons would probably act as sinks for P moving from the overlying horizons.
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The Bh horizons exhibited P sorption capacities that were consistent with their CDB-extractable Al contents. Mean Smax values of 7.55, 11.08, and 17.49 mmol P kg-1 were obtained for low-, medium-, and high-Al soils, respectively (Table 5). Anaerobic incubation decreased P sorption capacity of the low- and medium-Al soils, but slightly increased that of two high-Al soils. Nair et al. (1998) reported substantial reduction in P sorption capacities under anaerobic conditions for several Bh horizons of manure-impacted soils in the Lake Okeechobee basin. In acid soils, reduction in P sorption upon flooding is generally attributed to increased levels of solution P via reductive dissolution of Fe phosphates (Willett, 1986). Also, the rise in pH that accompanies soil reduction would tend to reduce the positive surface charge on soils, causing desorption of P from colloid surfaces (Willett, 1989; Yu et al., 1989). The increased P sorption in high-Al soils could be attributed to adsorption of P by Al–organo complexes (Haynes and Swift, 1989; Bloom, 1981) or to the formation of amorphous Al-hydroxy-phosphates (Bloom, 1981; Veith and Sposito, 1977).
Within a given soil group, Langmuir b values were smaller for soils high in initially sorbed P (Sn) (Table 5). Ona, for example, the soil with the lowest b value under aerobic condition, initially had 4.18 µmol P g-1 in the adsorbed phase. By comparison, Wauchula (sandy, siliceous, hyperthermic Ultic Alaquod) soil with the largest b value initially had no P in the adsorbed phase. Agbenin and Tiessen (1994) found a linear relationship between b and the degree of saturation of the sorption complex. The authors concluded that soils with relatively high concentrations of initial labile P had low b values because most of their reactive sites were saturated with P. Contrary to results reported by Nair et al. (1999) anaerobic incubation markedly increased b values of the soils used in this study. One would expect that reduced P sorption would result in lower b values, but some researchers (Sah and Mikkelsen, 1989; Willett, 1989) have attributed increased energy of adsorption upon flooding to increased activity of Fe oxides in sorbing P.
Maximum buffer capacity (MBC) describes the resistance to change in P concentration of the soil solution or the labile solid phase (Holford, 1979). High MBC values were associated with high P-sorbing soils such as Leon (sandy, siliceous, thermic Aeric Alaquod), Pomona, Smyrna (sandy, siliceous, hyperthermic Aeric Alaquod), and Wauchula (Table 5), and generally correspond with very low concentrations of P in solution. Thus, the mobility of P would be greater in soils of low MBC. Increased buffer capacity due to flooding has been attributed to P sorption by reprecipitated, poorly crystalline, ferrous hydroxides (Patrick et al., 1985).
Phosphorus Desorption Behavior
Desorbability of P from the Bh horizons was a function of their Al contents (Table 5). Regardless of the amount of newly adsorbed P, desorption was highest for low-Al soils. Desorption patterns were similar for medium-Al soils except for Leon, a soil with high P buffer capacity. Almost 50% of the newly sorbed P was released into solution during the first extraction. Desorption of P was nearly complete after nine successive 24-h extractions (Table 5). The high-Al soils retained a high proportion of the P adsorbed, except for Ona. Not including Ona, these soils released just over 25% of the newly sorbed P (Table 5). The high desorbability of P in Ona may have resulted from the high amount of P initially present in the adsorbed phase. When the amount of P sorbed by the soils at each desorption step was expressed as a percentage of newly sorbed P, the soils exhibited varying degrees of hysteresis (Fig. 2)
. Irreversibility was most pronounced for high-Al soils, where >70% of the added P remained in the adsorbed phase after nine desorptions. Zhou et al. (1997) reported greater desorbability of P for untreated Bh than for Bh samples in which Al–organic matter complexes were removed. They concluded that P sorption on metal organic complexes in Bh samples is more reversible than on inorganic metal oxides.
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Correlation of Phosphorus Sorption Parameters with Soil Properties
There was excellent agreement among the P sorption parameters obtained under aerobic and anaerobic conditions; thus, it was decided to limit the correlations to aerobic soils.
The P sorption maxima obtained from the Langmuir equation (Smax) correlated well with all the forms of Al (Table 6), with correlation being consistently highest for CuCl2–extractable Al (r = 0.781, P < 0.001). This suggests that Al associated with organic matter is the major sorbent of P in the Bh horizons studied. Similarly, organically bound Al extracted by pyrophosphate had significant (though lower) correlation with the P sorption (r = 0.373, P < 0.01). The sorption of P by Al–organic matter complexes in acid soils has been demonstrated by Haynes and Swift (1989). They attributed increased P sorption by Al–organic matter complexes with pH to enhanced hydrolysis and polymerization of hydroxy-Al associated with the organic matter. A positive effect of drying on P sorption was related to condensation of organic matter, resulting in increased accessibility of phosphate to adsorption sites on the hydroxy-Al. For Bh horizons that undergo alternate flooding and drying cycles, the latter could be a possible mechanism by which sorption capacities of these soils could be enhanced.
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Organic C did not show significant correlation with any of the P sorption parameters; however, it correlated highly with organically bound Al (r = 0.505, P < 0.001), implying an active participation of the organic matter in P sorption. The role of organic matter in augmenting P sorption in acid soils has often been attributed to the association of soil organic matter with Fe and Al hydroxides, which provides an active surface for P sorption (Sanyal and De Datta, 1991).
The binding strength constant b and the maximum buffer capacity MBC had positive correlations with the different forms of Al but not with any of the Fe forms (Table 6). This was viewed as more a function of concentration than the affinity of the metals to bind P. There was a negative correlation between b and the amount of P initially present in the adsorbed phase (Sn) (r = -0.520, P < 0.001). This is consistent with the theory that the more saturated the sorption complex is, the lower the energy of subsequent sorption.
The amount of sequentially desorbed P (DESP) was negatively correlated with all forms of Al, indicating lower desorbability of P for Bh samples high in Al. The positive correlation of DESP with exchangeable Ca (r = 0.293, P < 0.05) and Sn (r = 0.593, P < 0.001) suggests increased desorbability of newly sorbed P by the soils in the presence of Ca and P originally sorbed on the solid phase.
Prediction of Phosphorus Sorption Based on Soil Properties
The relative contributions of soil properties to P sorption were examined using a stepwise model-building procedure known as the maximum R2 improvement technique (SAS Institute, 1990).
Consistent with correlation results, CuCl2–Al accounted for most of the variation in all P sorption parameters (Table 7). The combination of CuCl2–Al, Pyro-Al, and CDB-Al explained 
80% of the variability in P sorption, 61% of which came from CuCl2–Al alone. The contribution of CDB-Al was small but significant. Holding CuCl2–Al and Pyro-Al constant, the model predicted CDB-Al to increase Smax by a factor of 0.07 for every unit of CDB-Al added. The presence of Pyro-Al, however, was predicted to have a small negative effect on Smax. This was due to having two variables with overlapping properties in the model. Based on the simple correlation, Smax correlated much more strongly with CuCl2–Al (r = 0.781, P < 0.001) than with Pyro-Al (r = 0.373, P < 0.01).
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70% of the variability in b. According to the model, CuCl2–Al and BaCl2–extractable Ca would increase b by a factor of 0.13 and 0.26 for every unit of CuCl2–Al and Ca, respectively. As expected, the binding strength was predicted to decrease with the amount of initially sorbed P (Sn). As was the case for Smax, CuCl2–Al accounted for most of the variability (67%) in MBC. Again, the negative effect of having both Pyro-Al and CuCl2–Al in the model is manifested. The addition of clay in the model increased the R2 value by 2% for MBC.
About 84% of the variability in the amount of sequentially desorbed P was accounted for by the combination of CuCl2–Al, Pyro-Al, and Sn. Consistent with the correlation of DESP with CuCl2–Al and Pyro-Al, both variables were predicted to decrease release of P by these soils. Desorbability of newly sorbed P was expected to increase with the amount of previously adsorbed P.
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Phosphorus sorption–desorption reactions for Bh horizons were influenced largely by Al. Correlation and regression analyses show that CuCl2–extractable Al was the single most important chemical property contributing to P retention in these soils, accounting for >60% of the variation in P sorption. This suggests that sorption and desorption of P by the Bh horizon to a large extent involve Al–organic matter complexes. The relationship between Al and P retention capacities of Bh samples must be considered when formulating environmentally sound management systems for Florida Spodosols.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Received for publication April 23, 1999.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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