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Published online 9 August 2006
Published in J Environ Qual 35:1855-1862 (2006)
DOI: 10.2134/jeq2005.0420
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Waste Management

Sorption Dynamics of Organic and Inorganic Phosphorus Compounds in Soil

A. S. Berg and B. C. Joern*

Department of Agronomy, 915 W. State Street, Purdue University, West Lafayette, IN 47907

* Corresponding author (bjoern{at}purdue.edu)

Received for publication November 4, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Phosphorus retention in soils is influenced by the form of P added. The potential impact of one P compound on the sorption of other P compounds in soils has not been widely reported. Sorption isotherms were utilized to quantify P retention by benchmark soils from Indiana, Missouri, and North Carolina when P was added as inorganic P (Pi) or organic P (ß-D-glucose-6-phosphate, G6P; adenosine 5'-triphosphate, ATP; and myoinositol hexaphosphate, IP6) and to determine whether soil P sorption by these organic P compounds and Pi was competitive. Isotherm supernatants were analyzed for pH and total P using standard protocols, while Pi and organic P compounds were assayed using ion chromatography. Under the controlled conditions of this study, the affinity of all soils for P sources followed the order IP6 > G6P > ATP > Pi. Each organic P source had a different potential to desorb Pi from soils, and the order of greatest to least Pi desorption was G6P > ATP > IP6. Glucose-6-phosphate and ATP competed more directly with Pi for sorption sites than IP6 at greater rates of P addition, but at the lesser rates of P addition, IP6 actually desorbed more Pi. Inositol hexaphosphate was strongly sorbed by all three soils and was relatively unaffected by the presence of other P sources. Decreased total P sorption due to desorption of Pi can be caused by relatively small additions of organic P, which may help explain vertical P movement in manured soils. Sorption isotherms performed using Pi alone did not accurately predict total P sorption in soils.

Abbreviations: AlCBD, citrate–bicarbonate–dithionite extractable aluminum • Alox, ammonium oxalate extractable aluminum • ATP, adenosine 5'-triphosphate • DSEP, dilute-salt extractable phosphorus • FeCBD, citrate–bicarbonate–dithionite extractable iron • Feox, ammonium oxalate extractable iron • G6P, ß-D-glucose-6-phosphate • IP6, myoinositol hexaphosphate • Pox, ammonium oxalate extractable phosphorus • WSP, water-soluble phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS is present in many naturally occurring organic compounds. Organic P constitutes 29 to 65% of total soil P (Harrison, 1987). The soil organic P fraction is dominated by inositol phosphates, nucleic acids, phospholipids, and small quantities of other phosphate esters that arise primarily from microorganisms (Anderson, 1980). Inositol phosphates can comprise up to 50% of the total organic P in soils, with penta- and hexaphosphates predominating (Anderson, 1967). Phospholipid content varies from 0.5 to 7.0% of total soil organic P (Anderson and Malcolm, 1974) and generally <3% of soil organic P is attributed to nucleic acids and their derivatives (Anderson, 1967).

Soil organic P dynamics are influenced by many factors including organic matter content and mineralization, drainage, pH, cultivation, and organic P additions. Animal manure is a widely used fertilizer material that can supply a substantial amount of organic P to the soil. Manure organic P content varies with animal species, age, and diet, but typically varies between 5 and 25% of total P (Sharpley and Moyer, 2000). The principal components of organic P in manure are inositol phosphates, nucleic acids, and phospholipids. Inositol phosphates have been reported to vary between 2 and 60% of the total organic P in manure (Caldwell and Black, 1958; Baxter et al., 2003).

Inositol hexaphosphate (IP6) has been shown to reduce the sorption of inorganic P (Pi) in soils (Anderson et al., 1974), and adding IP6 to sandy loam soil columns displaced Pi and resulted in Pi leaching from the columns (Bowman et al., 1967). Extensive research has evaluated the sorption of different organic and inorganic P sources on soils and individual clay components; however, we are not aware of studies that have evaluated how these compounds interact in soils when added simultaneously. Our objectives were to quantify P sorption by benchmark soils in the USA from Indiana, Missouri, and North Carolina when P was added as inorganic P, Pi (NaH2PO4·H2O), or three organic P compounds [ß-D-glucose-6-phosphate, G6P (NaC6H13O9P); adenosine 5'-triphosphate, ATP (Na2C10H16N5O13P3); and myoinositol hexaphosphate, IP6 (Na12P6C6H6O24)], and to determine whether sorption of these organic P compounds and Pi by soils is competitive. Organic P sources were chosen based on their predominance in soils and manure and differences in P content.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Analyses
Three benchmark soils were used in this study: a Cecil clay loam (clayey, kaolinitic, thermic Typic Kanhapludult) from North Carolina, a Creldon silt loam (fine-loamy, mixed, active, mesic Oxyaquic Fragiudalf) from Missouri, and a Sleeth silt loam (fine-loamy, mixed, active, mesic Aeric Ochraqualf) from Indiana (Table 1). The Cecil clay loam and Creldon silt loam soils were collected from fields under pasture management, whereas the Sleeth silt loam soil was collected from a field under row-crop management. The surface 20 cm of each soil was collected, air-dried, ground, and sieved (<2 mm). Soils were analyzed for pH using a 1:1 soil/solution ratio (m/v). Dilute-salt–extractable P (0.01 M CaCl2, DSEP) and water-soluble P (WSP) were determined using a 1:10 soil/solution ratio. Soil solutions were shaken for 1 h on an inline shaker at 120 excursions per minute (epm) at 25°C, centrifuged at 2000 x g for 10 min, and filtered through a 0.45-µm membrane filter.Soils were analyzed for Bray P1 (Bray and Kurtz, 1945), Mehlich III P (Mehlich, 1984), and total Kjeldahl P (Lachat Instruments QC8000 method 13-1105-01-1-B). All of these soil P measures were determined colorimetrically using the molybdate blue method (Murphy and Riley, 1962). Oxalate-extractable Fe (Feox), Al (Alox), and P (Pox), and citrate–bicarbonate–dithionite (CBD)–extractable Fe (Fecbd) and Al (Alcbd) were determined by the methods of Schwertmann (1964) and Mehra and Jackson (1960), respectively, and analyzed by inductively coupled plasma–atomic emission spectrometry (ICP–AES).


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

 
Sorption Isotherm Experiments
Sorption isotherms were used to determine noncompetitive and competitive sorption of Pi, G6P, ATP, and IP6 by a procedure similar to Nair et al. (1984). Structural models of these compounds are presented in Fig. 1 . Phosphorus solutions were prepared in 0.01 M NaCl and adjusted to the soil–water pH of the soil using additions of 0.1 M HCl or 0.1 M NaOH as necessary. All isotherms were performed in duplicate using a 1-g soil sample, 1:25 soil/solution ratio (m/v), and 24 h equilibration time on an inline shaker at 120 epm at 25°C. Chloroform was added to all solutions (20 g L–1) to inhibit microbial activity. For the noncompetitive isotherms, six P concentrations [0, 6.45 (4.99), 16.13 (12.49), 32.26 (24.98), 161.3 (124.9), and 323 (250.1) µmol P L–1 (mg kg–1)] were used for each P source. Experiments designed to investigate competitive sorption between Pi and each organic P source included seven total P concentrations [0, 6.45 (4.99), 16.13 (12.49), 32.26 (24.98), 161.3 (124.9), 323 (250.1), and 646 (500.2) µmol P L–1 (mg kg–1)] with equimolar P contributions from Pi and the organic P source. To determine competition among all P sources studied, an isotherm was performed using seven total P concentrations [0, 12.9 (9.99), 32.26 (24.98), 64.52 (49.96), 322.6 (249.8), 646 (500.2), and 1292 (1000) µmol P L–1 (mg kg–1)] with equimolar P contributions from all P sources (Pi, G6P, ATP, and IP6). After equilibration, the Sleeth and Creldon soil solutions were centrifuged at 670 x g for 10 min and filtered through a 0.45-µm membrane filter, whereas the Cecil soil solutions were centrifuged at 17500 x g for 10 min and filtered through a 0.2-µm membrane filter to remove fine clay particles.


Figure 1
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  		Fig. 1. Ball-and-stick structural model of (A) phosphate, (B) glucose-6-phosphate, (C) adenosine triphosphate, and (D) inositol hexaphosphate. Black balls are O atoms, dark gray balls are P, light gray balls are C, white balls are H, and the dark gray balls of adenine in the ring of adenosine triphosphate are N.

 
Organic and inorganic P concentrations in the equilibrium solutions were determined by ion chromatography (Dionex DX-600 Ion Chromatography Workstation, Dionex, Sunnyvale, CA). The chromatography system included a GP50 gradient pump, an AS50 autosampler with a 25-µL injection loop, and a conductivity cell connected to an ED50 electrochemical detector. An ASRS-ULTRA (4 mm) micromembrane suppressor was used in chemical suppression mode and was continuously regenerated with 60 mM H2SO4. Separations were performed using a Dionex Omnipac PAX-100 analytical column (250 by 4 mm i.d.) and guard column (50 by 4 mm i.d.). A Dionex trap column (ATC-3) was used to remove any carbonate contamination from the mobile phase. The three mobile phases used for the analysis included: Eluent A, deionized water with a specific resistance >17.8 M{Omega} cm–1; Eluent B, 200 mN NaOH; and Eluent C, 50% (v/v) isopropanol (CH3CHOHCH3) (Baxter, 2003). Total dissolved P was determined by modified Kjeldahl digestion with P analysis by the molybdate blue method (Murphy and Riley, 1962). The quantity of P adsorbed was calculated as the difference between the amount of P added and the solution P concentration after 24 h. Sorption maxima were determined by fitting data to the linear form of the Langmuir equation:

Formula 1[1]
where C is the concentration of P in the equilibrium solution (mg L–1), X is the total amount of P sorbed, k is the affinity constant or binding energy (L mg–1), and Xm is the P sorption maxima (mg kg–1). The plot of C/X vs. C gives a straight line with slope 1/Xm and intercept 1/(kXm) (Olsen and Watanabe, 1957).

It is important to mention that the Langmuir equation and its application in describing Pi adsorption has been widely criticized. Veith and Sposito (1977) reported the usefulness of the Langmuir equation only when evidence exists that adsorption, not precipitation, is controlling Pi fixation. Phosphate adsorption increases the negative charge of the surface, limiting the application of the Langmuir equation, which assumes constant energy of adsorption (Barrow, 1978). Sposito (1986) emphasized that Pi sorption data described by adsorption isotherm equations do not imply a mechanism or mechanisms of sorption and often the maximum adsorption value is exceeded at high concentrations.

Correlation between potential Pi sorption and soil test parameters was performed using the linear regression procedure PROC REG of SAS (SAS Institute, 2001). Significant differences among means were determined using the Fisher least significant difference (LSD) method ({alpha} = 0.05).

Selected samples and standards from all G6P isotherms were checked for glucose in solution using glucose oxidase (Glucose Trinder, Sigma Chemical, St. Louis MO; Product 315-100), with all results being negative for the presence of glucose. Phosphorus sorption isotherms also were performed to investigate competition between {alpha}-D-glucose-1-phosphate (G1P, C6H13O9P) and Pi. Significant degradation of G1P, indicated by glucose in solution, occurred during the equilibration period; therefore, those results are not presented.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soils
The three soils used in this experiment differed greatly in P content and mineralogy (Table 1). The Sleeth silt loam contained greater P concentrations than the Creldon and Cecil soils as measured by Bray P1, Mehlich III, WSP, DSEP, and ammonium oxalate extractions. The Cecil soil had significantly greater WSP, Bray P1, Mehlich III, and ammonium oxalate P than the Creldon soil; however, DSEP was not significantly different between the Cecil and Creldon soils. Water-soluble P was 7.5 and 12 times greater in the Sleeth soil than in the Cecil and Creldon soils, respectively. Clay content has been shown to be associated with P adsorption of soils. Loganathan et al. (1987) and Fox and Kamprath (1970) showed positive correlations between clay content and P adsorption capacity of soils. Our particle size analysis showed that clay content was greatest for the Cecil soil (287 g kg–1), and that the Cecil soil also had the greatest potential of all soils to sorb additional Pi, despite the significantly greater soil test P values for the Cecil soil than for the Creldon soil (Table 2).


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  		Table 2. Maximum additional inorganic P sorption for each soil and isotherm calculated by the Langmuir equation.

 
The Creldon and Cecil soils had significantly greater Alox than the Sleeth soil, whereas the Sleeth soil had significantly greater Feox than the Creldon and Cecil soils. Oxalate-extractable Al was not significantly different between the Creldon and Cecil soils; however, the Creldon soil had about two times greater Feox. The Sleeth soil is a moderately weathered, poorly drained soil, and under typical climatic conditions in Indiana where seasonal saturation and rapid oxidation is common, the formation of significant quantities of amorphous Fe oxides is expected. The moderately well-drained Creldon soil and well-drained Cecil soil are more highly weathered and generally do not have the same extent of seasonal saturation, and therefore they have less amorphous Fe oxides and consequently lower Feox content than the Sleeth soil. Citrate–bicarbonate–dithionite–extractable Al was three times greater for the Cecil soil than the Creldon and Sleeth soils, and Fecbd content was two times greater for the Cecil soil than for the Creldon and Sleeth soils. Citrate–bicarbonate–dithionite–extractable Al was 15% greater for the Creldon soil than the Sleeth soil, but the Sleeth soil had 16% greater Fecbd than the Creldon soil. The ratio of Alox to Alcbd was 0.5, 0.6, and 0.2, and the ratio of Feox to Fecbd was 0.7, 0.4, and 0.1 for the Sleeth, Creldon, and Cecil soils, respectively. These data indicate that amorphous Fe and Al oxides are important P sorbing surfaces for the Sleeth and Creldon soils, while P sorption in the Cecil soil is largely attributed to crystalline Fe and Al oxides. The Sleeth soil had significantly greater WSP, Bray P1, Mehlich III, and Pox content than the Cecil soil; however, the soils each have degree of P saturation (DPS) values of 24%. The calculation of DPS assumes that amorphous Fe and Al oxides largely control P sorption. Since the P sorption capacity of the Cecil soil was predominately influenced by crystalline Fe and Al oxides, the DPS did not accurately assess the P sorption capacity of this soil.

Inorganic Phosphorus Sorption
Inorganic P sorption differed greatly for the soils used in this experiment. Table 2 shows the additional Pi that could be sorbed by each soil after adding Pi alone or in conjunction with the organic P sources. The potential for these soils to sorb additional Pi followed the trend Sleeth (11.4 mg kg–1) < Creldon (78.1 mg kg–1) < Cecil (115.0 mg kg–1). Additional Pi sorption capacity for each soil was inversely related to soil WSP, DSEP, and Pox; however, soil total P, Bray P1, and Mehlich III P were not significantly correlated to additional P sorption capacity (Table 3). Studies have shown a linear relationship between WSP or Pox and dissolved reactive P lost in runoff (Pote et al., 1996, 1999; Pautler and Sims, 2000). Pautler and Sims (2000) studied a range of soils from Delaware and found that as soil test P levels increased, the ratio of labile P forms to total P increased. Laboski and Lamb (2004) found a linear relationship between WSP and P saturation of mollisols and alfisols in Minnesota. They also showed that WSP was negatively correlated with P sorption strength (Langmuir equation parameter k, or affinity constant), and concluded that P affinity for adsorbing surfaces decreased with increased WSP levels. The Langmuir affinity constants for our data, however, were not significantly different.


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  		Table 3. Linear regression of potential Pi sorption on initial soil test P parameters.

 
In this study, the Sleeth soil had the greatest sum of Alox and Feox, which is commonly associated with P sorption capacity; however, the elevated levels of WSP, DSEP, Pox, and DPS indicate saturation of P sorption sites. Maguire et al. (2001) showed a decreased potential for further Pi sorption with increased Pox values for several acidic northern Ireland soils with a wide range of Alox and Feox contents. Sharpley (1995) found a linear relationship between DPS and P runoff. Laboski and Lamb (2004) showed a positive linear relationship between DPS and WSP for Minnesota soils with and without previous manure applications. They indicated that soils with DPS > 21.7% would have WSP > 1 mg L–1. Our data showed that the Sleeth and Cecil soils each had DPS of 24%, but that the Sleeth soil had 7.5 times greater WSP than the Cecil soil (0.97 vs. 0.14 mg P L–1). The ratios of Alox to Alcbd and Feox to Fecbd for the Sleeth (0.5 and 0.7, respectively) and Cecil (0.2 and 0.1, respectively) soils indicate relatively more P sorption by amorphous Fe and Al in the Sleeth soil. The greater WSP content of the Sleeth soil indicates that this soil is more P saturated than the Cecil soil. The DPS did not accurately predict the total sorption sites available in the Cecil soil because crystalline Fe and Al oxides predominate.

The Creldon and Cecil soils had similar amounts of WSP and DSEP, but the Creldon soil had significantly lesser clay content and Alcbd and Fecbd content than the Cecil soil. Fontes and Weed (1996) concluded that crystalline Fe oxides (goethite and hematite), crystalline Al oxide (gibbsite), and amorphous Al oxides of total clay samples from oxisols were the important components for P adsorption. The Cecil soil had the greatest amount of Fe and Al oxides, as indicated by Fecbd and Alcbd, which increased the total P sorption sites of this soil relative to the Creldon and Sleeth soils.

Effect of Inorganic and Organic Phosphorus Competition on Inorganic Phosphorus Sorption
The potential for soils to sorb additional Pi was affected by additions of all organic P sources (Table 2). All competitive isotherms caused desorption of Pi from the Sleeth soil in the presence of IP6, ATP, or G6P. The maximum additional Pi sorption presented in Table 2 occurred at low final P concentrations for the competitive isotherms in all soils. As final P concentrations increased, Pi sorption decreased in all soil and P source combinations except the Cecil soil Pi + IP6 isotherm. Each organic P source decreased each soil's capacity to sorb additional Pi by different amounts but consistently followed the order IP6 < ATP < G6P. We hypothesize this order is reflective of the competition between each organic P source and Pi for sorption sites. The competition between IP6 and Pi for sorption sites has been investigated. Anderson et al. (1974) showed that adding IP6 before or simultaneously with Pi reduced sorption of Pi, with little differences between pretreatment and simultaneous treatment. Celi et al. (1999) showed that goethite, illite, and kaolinite retain more IP6 than Pi. Leytem et al. (2002) determined the Langmuir sorption maxima of Pi, ATP, and IP6 of a sand, sandy loam, and sandy clay loam from North Carolina and found that P sorption maxima followed the order Pi < ATP < IP6. Ognalaga et al. (1994) showed that similar amounts of G1P and Pi were adsorbed on synthetic goethite, whereas Goring and Bartholomew (1950) showed adsorption of G1P to be lower than Pi on clays (bentonite, illite, kaolinite, and subsoil clay samples). Glucose-6-phosphate and G1P sorption also has been shown to be lower than Pi sorption on soils as well as Al and Fe precipitates (McKercher and Anderson, 1989; Shang et al., 1990). These studies, however, measured glucose phosphates in solution by digestion followed by colorimetric analysis of total Pi in solution; therefore, desorbed Pi would be attributed to the glucose phosphate compound. Our study also showed degradation of G1P after the 24-h equilibration period. The glucose phosphates and inorganic P in solution in this study were measured directly by ion chromatography.

For the noncompetitive IP6, ATP, and G6P isotherms, we calculated the difference between organic P sorbed and Pi in solution after the equilibration period. This value represents the organic P sorbed without desorbing Pi or organic P sorbed at sites not previously occupied by Pi. The portion of organic P sorbed without desorbing Pi divided by total organic P sorbed at each isotherm P rate is presented in Table 4. At low organic P additions (6.45 and 16.13 µmol P L–1), IP6 caused the greatest desorption of Pi. Increased Pi desorption with low additions of IP6 also is presented in Fig. 2 . Glucose-6-P showed the greatest competition with Pi for similar sorption sites in all soils at the greater P addition rates (32.26, 161.3, and 323 µmol P L–1) whereas Pi desorption with ATP was consistently between G6P and IP6 at these rates of P addition. Greater competition for sorption sites was observed between G6P and Pi at high addition rates, but with low addition rates IP6 may result in a greater release of Pi into solution.


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  		Table 4. Fraction of organic P sorbed without desorbing Pi.

 

Figure 2
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  		Fig. 2. Inorganic phosphorus desorption for inositol hexaphosphate (IP6), adenosine triphosphate (ATP), and glucose-6-phosphate (G6P) noncompetitive isotherms. Error bars indicate the standard error of the mean.

 
Effect of Inorganic and Organic Phosphorus Competition on Total Phosphorus and Organic Phosphorus Sorption
Total P sorption differed for each soil and P source. Data for total P sorbed for all isotherms at the greatest addition (323 µmol P L–1) of Pi or organic P used in the study are presented in Table 5. Figure 3 shows total P sorbed with increased P additions for all isotherms and soils. The greatest differences in P sorbed among the different isotherms occurred with the greatest P addition (323 µmol P L–1). Total P sorbed for all organic P isotherms includes sorption of organic P sources as well as sorption and/or desorption of Pi. For the organic P compounds, each soil showed the greatest total P sorption for the IP6 isotherm, followed by the ATP isotherm, and lastly the G6P isotherm.


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  		Table 5. Total P sorption for each isotherm after adding 323 µmol P L–1 total P.

 

Figure 3
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  		Fig. 3. Total P sorbed for each isotherm at equivalent total P additions. Total P additions for competitive isotherms have equimolar contributions from each P source. Error bars indicate the standard error of the mean.

 
Soils had different capacities to sorb organic P sources and Pi. At the 323 µmol P L–1 addition, the Cecil soil showed a similar capacity to sorb total P in the Pi isotherm (107.5 mg kg–1), G6P isotherm (105.7 mg kg–1), and Pi + G6P isotherm (107.6 mg kg–1), indicating the Cecil soil has the same capacity to sorb Pi and G6P (Table 5). Table 6 shows organic P sorption for each organic P source at the greatest organic P addition (323 µmol P L–1). Small differences were observed between G6P sorption in the G6P isotherm, 247.4, 242.0, and 241.3 mg kg–1, compared with the Pi + G6P isotherm, 236.6, 246.4, and 250.1 mg kg–1, for the Sleeth, Creldon, and Cecil soils, respectively. The phosphate group of G6P likely competes with Pi for similar sorption sites because the phosphate group extends from the glucose molecule (Fig. 1). The G6P, ATP, and IP6 isotherms showed that organic P sources caused desorption of previously sorbed Pi in all soils (Fig. 2). This graph shows that additions of G6P in each soil caused the greatest desorption of Pi. These data suggest that G6P and Pi compete for the same sorption sites, but they suggest that G6P has a greater affinity for those sites than Pi.


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  		Table 6. Organic P sorption for each isotherm at 323 µmol P L–1 organic P additions.

 
Total P sorption for the ATP and Pi + ATP isotherms was consistently lesser than the IP6 isotherms and consistently greater than the G6P isotherms in all of the soils studied (Table 5). This trend is also true for Pi desorbed for the ATP isotherm (Fig. 2). Total P sorption for the ATP isotherms was greater than the G6P isotherms because significantly less Pi desorption occurred with ATP additions. Adenosine triphosphate has been shown to be adsorbed by montmorillonite and kaolinite (Graf and Lagaly, 1980). Cortez and Schnitzer (1981) also reported sorption of the nucleic acid base adenine on illite, montmorillonite, kaolinite, Fe and Al oxides (gibbsite and goethite), and a prepared fulvic acid–montmorillonite complex. Adenosine triphosphate sorption was significantly decreased when Pi was simultaneously added in solution (Table 6). Despite the potential binding of three phosphate groups on ATP, the large organic component of ATP (Fig. 1) may cause a steric hindrance or limit the access of ATP to certain sorption sites, allowing Pi to be a better competitor for some sorption sites. Adenosine triphosphate and Pi compete for sorption sites, and Pi and ATP have more comparable affinities for sites than Pi and G6P.

The IP6 isotherm showed the least Pi desorption (Fig. 2) and the greatest total P sorption (Table 5) in all soils. Apparently the unique structure of IP6 (Fig. 1) accesses sorption sites not available to Pi and other organic P sources. Celi et al. (1999) and Ognalaga et al. (1994) showed that the inositol moiety of IP6 is not involved in adsorption on synthesized goethite, and Celi et al. (1999) reported that it may create a steric hindrance. Preference for IP6 on synthetic goethite has largely been attributed to the multiple phosphate groups available for inner-sphere complexes (Ognalaga et al., 1994; Celi et al., 1999); however, sorption sites available in soils are likely significantly different from those on pure mineral specimens due to the influence of organic matter and other soil constituents. Inositol hexaphosphate sorption remained high under all conditions (IP6, Pi + IP6, and Pi+ IP6 + ATP + G6P isotherms), indicating the strong affinity of IP6 by all three soils used in this study (Table 6). Leytem et al. (2002) found that IP6 sorption increased to a maximum after which no additional sorption occurred, whereas Anderson et al. (1974) showed IP6 sorption reached a maximum and then began to desorb at higher P additions. Sorption of IP6 on short-range ordered Al precipitates also increased to a maximum, after which desorption occurred (Shang et al., 1996). In the previous studies, greater P additions were used in constructing isotherms than in the present study.

Inorganic Phosphorus, Glucose-6-Phosphate, Adenosine Triphosphate, and Inositol Hexaphosphate Competitive Isotherm
The combined (Pi + G6P + ATP + IP6) isotherm showed the following trend from greatest to least P sorption of P sources in all three soils: IP6 > G6P > ATP > Pi. Research comparing sorption maxima of different P compounds from isotherms performed separately have consistently shown the large capacity of soils to sorb IP6 over other organic P sources and Pi, as well as soils consistently sorbing more ATP than Pi (Leytem et al., 2002; McKercher and Anderson, 1989). The sorption of all P compounds for the combined isotherm and each corresponding noncompetitive isotherm for the Sleeth soil (results for Creldon and Cecil soils were similar) is presented in Fig. 4 . Data are presented as P compound sorption as a function of the quantity of P added from each compound (not total P added). The greater sorption of G6P and ATP is due to fewer phosphate groups on these compounds relative to IP6. Glucose-6-phosphate, ATP, and Pi sorption was significantly reduced for the combined isotherm relative to the noncompetitive isotherms, whereas sorption of IP6 was unchanged under the two conditions. Inositol hexaphosphate sorption was not affected by the other P sources used in this study; however, the sorption of G6P and ATP decreased under the competitive conditions of the combined isotherm. The greatest effect of the combined isotherm was the large desorption of Pi, indicating the possibility of high soluble P concentrations in soils with large additions of organic P compounds.


Figure 4
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  		Fig. 4. Sodium phosphate, Na–ATP, Na–G6P, and Na–IP6 noncompetitive and combined isotherms sorbed at equivalent P additions for the Sleeth soil. Phosphorus additions for the combined isotherm are based on the P added by each source. Error bars indicate the standard error of the mean.

 
For all soils in this study, the potential for organic P sources to desorb Pi followed the trend G6P > ATP > IP6. Competition among organic P sources in the Pi + G6P + ATP + IP6 isotherm also occurred. Interactions among P sources complicate the assessment of potential P losses from soils. The Pi + G6P + ATP + IP6 isotherm demonstrates the greater potential of Pi release when organic P sources are added to soils, and that Pi loss potential is affected by the form and quantity of organic P added. Although ATP and G6P have limited stability in soils (Leytem et al., 2002; McKercher and Anderson, 1989), these compounds are ubiquitous in soils because ATP is an energy source and G6P is a metabolic intermediate. Inositol hexaphosphate is stable in soils and is present in manure (Turner et al., 2002). In this study IP6 generally desorbed significantly less Pi than ATP and G6P, except under low P addition rates where IP6 actually desorbed more Pi than either ATP or G6P (Table 4 and Fig. 2). Inositol hexaphosphate sorption was relatively unaffected by competition with other organic P sources, while the sorption of ATP and G6P were decreased significantly when IP6 was present. Total P sorption and solution P concentrations in the soils of this study were affected by total P added, source of P added, and competition among different P sources.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Characterizing potential P loss from soils depends on many factors and interactions and is influenced by the competition between organic and inorganic P sources for P sorption sites. Sorption isotherms performed using Pi alone do not accurately predict total P sorption in soils. Under the controlled conditions of this study, the affinity of all soils for P sources followed the order IP6 > G6P > ATP > Pi. Inositol hexaphosphate was strongly sorbed by all three soils and was relatively unaffected by the presence of other P sources, which suggests multiple mechanisms of P sorption with differences among the P sources. Each organic P source had a different potential to desorb Pi from soils, and the order of greatest to least Pi desorption was G6P > ATP > IP6 at the greater P addition rates. G6P and ATP competed more directly for Pi for sorption sites, except with low P additions where IP6 showed the greatest Pi desorption. The presence of Pi decreased sorption of ATP. Decreased total P sorption due to desorption of Pi can be caused by low additions of organic P, which may help explain vertical P movement in manured soils. Soils with elevated levels of labile P have less potential to sorb additional Pi and may more readily desorb Pi in the presence of organic P. Research on the competitive sorption dynamics between added P compounds without microbial inhibition may further our understanding of P movement in soils.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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