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DIVISION S-2—SOIL CHEMISTRY

Hydrolysis of Cyclotri- and Cyclotetraphosphate in Soil

^a Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843
^b Texas A&M Univ. Research and Extension Center, Route 3, Box 219, Lubbock, TX 79401

^* Corresponding author (h-shahandeh{at}tamu.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Cyclotriphosphate (Na₃P₃O₉, C3P) and cyclotetraphosphate (Na₄P₄O₁₂, C4P) are not appreciably sorbed by soil constituents. This property combined with a reduced rate of hydrolysis of C4P and its hydrolysis products may maintain P availability for plant uptake. The primary objective of this study was to determine the residence times of C3P and C4P and their hydrolysis products in four soils. Hydrolysis of C3P and C4P was dependent on soil moisture, temperature, biological activity, and time. Cyclic P hydrolysis followed apparent first-order reaction. Recovery of hydrolysis products of C3P and C4P was higher in soils, such as Falba sandy loam (sl), which demonstrated limited sorption for mono-, di-, and triphosphate. The presence of hydrolysis products may explain the decreasing rate constants observed in soils incubated for longer than 48 h. Cyclotetraphosphate was more stable than C3P in all comparable treatments, often by three to six times. The energy of activation (E_A) for C3P and C4P was 45.8 and 61.1 kJ mol^–1, respectively, in a Falba sl, and 38.6 and 51.2 kJ mol^–1, respectively, in a Branyon clay (c). Cyclotriphosphate was more readily hydrolyzed in soils compared with C4P. These results indicate a potential advantage of C4P over C3P because of its slower hydrolysis in soils, and over all linear phosphates, which are strongly sorbed by soil constituents.

Abbreviations: c, clay • C3P, cyclotriphosphate • C4P, cyclotetraphosphate • E_A, energy of activation • sil, silt loam • sl, sandy loam

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
CYCLOPHOSPHATES are of interest because they might not be appreciably affected by sorption and/or be slowly hydrolyzed. These characteristics can be important to maintain P availability for plant uptake when cyclophosphates are used as fertilizers. Lucci (1967) and Blanchar and Hossner (1969a) reported that C3P (Na₃P₃O₉) was not appreciably adsorbed by soil. Cyclotetraphosphate (Na₄P₄O₁₂) was slightly retained by soils (Lucci, 1967), however, no studies have been conducted to determine to what degree this may occur. Also no comparisons are available to evaluate the kinetics of C4P hydrolysis in soils and possible advantages of C4P over C3P as a P amendment.

Cyclotriphosphate had a half-life of 5 h in an Elliot soil (Blanchar and Hossner, 1969b). Reaction kinetics of C3P hydrolysis generally follows first order in C3P although a few workers have shown second-order kinetics under highly acid or alkaline conditions (Kalliney, 1972). The kinetics of C3P hydrolysis in Iowa soils, as well as the role of biological (enzymatic) components in C3P hydrolysis, were reported by Busman and Tabatabai, (1985).

Busman and Tabatabai (1985) determined the E_A for C3P in soils ranging from 29 to 39 kJ mol^–1. Some kinetic reaction values were reported for C3P as follows: E_A, 14.9 to 39.9 kJ mol^–1; two first-order reactions, one following the other; rate constants of 0.001 to 0.01 h^–1 (slightly lower for saturated soil); and Q₁₀ < 2.0 for all soils (Dick and Tabatabai, 1986b). The range of E_A values in soils was less than that for C3P in solution (90 kJ mol^–1) (Kura, 1982). Lower values are expected in soils because of microbial activity and the presence of known catalysts (e.g., Ca, Mg, Al, Fe) in soils (Van Wazer, 1958). Microbial activity was a dominant factor in the hydrolysis of C3P as Q₁₀ values were 2, and biological hydrolysis of C3P in most soils was greater than chemical hydrolysis (Busman and Tabatabai, 1985).

The objectives of this research were to determine the rate of hydrolysis of C4P compared with C3P and to determine the residence time of the C4P hydrolysis products in four soils.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soils were selected to provide a range of pH, texture, organic C, reactive oxides of Fe and Al, and cation-exchange capacity (Table 1). These properties have been reported to influence polyphosphate hydrolysis.

Sorption Isotherms of C3P and C4P
Sorption experiments utilizing monophosphorus, triphosphorus, C3P, and C4P were conducted following the method of Blanchar and Hossner (1969b). Thirty milliliters each of 0.040, 0.120, 0.360, and 1.080 g P L^–1 solutions were added in triplicate to 3.00-g samples (water/soil ratio of 10:1) of autoclaved Branyon c and Falba sl in a 125-mL Erlenmeyer flask. Autoclaved soil was used to minimize microbial hydrolysis. Phosphorus species were equilibrated with the soil for 24 h on a reciprocating shaker at 25°C. The soil/water mixture was transferred to a 100-mL centrifuge tube and the solution phase separated by centrifugation. Total P in the solution phase was determined following the method of Murphy and Riley (1962) after hydrolysis of condensed P species by heating in 1 M H₂SO₄. Log-log plots were prepared to determine the amount of P remaining in the solution against that added at the beginning of the shaking time.

Hydrolysis of Cyclophosphates
Hydrolysis studies of C3P and C4P were conducted in four soils under a variety of conditions. The main treatments were one application rate of 0.100 g cyclic P kg^–1 soil for two P sources (C3P and C4P) at 0.033 MPa moisture content in four soils. Each treatment was prepared in triplicate and sampled at 0, 4, 8, 16, 24, 48, 96, 168, 336, and 552 h or until hydrolysis was complete. Additional treatments included autoclaved soil and saturation moisture content (water was added until the soil was saturated and the soil surface just glistened with excess water), factors that have been shown to have an effect on P hydrolysis rates. Sterilized solutions were used to prepare stock P solutions and to adjust soils to the appropriate moisture content for the autoclaved treatments. These treatments were confined to Branyon c and Falba sl soils. The effect of temperature on cyclophosphorus hydrolysis was examined by incubating soil samples at 10, 20, and 30°C. Finally, because previous work has been conducted at P application rates higher than 0.100 g P kg^–1 soil, a rate of 0.400 g P kg^–1 soil was included for Branyon c and Falba sl soils.

Cyclophosphates and their hydrolysis products were extracted from soil using a sequential water/0.5 M H₂SO₄/1.0 M NaOH procedure which maximized P extraction and minimized hydrolysis of cyclic and linear phosphates. Gradient elution chromatography (Dionex Model 4000I Ion Chromatograph, Dionex Corp., Sunnyvale, CA) was used to separate cyclic phosphates and their hydrolysis products (Dionex Corp., 1987). Separation and direct quantitative analysis of linear and cyclic polyphosphates were accomplished in <15 min.

Data for the amount of cyclophosphorus remaining in each treatment were analyzed for kinetic behavior. Plots of ln P verses time were constructed for the examination of first-order fit and evaluation of slope where k is the first-order rate constant. Curves of ln P concentration versus time were examined in the manner of Dick and Tabatabai (1986b) for two straight-line portions (two successive first-order reactions). Points were chosen at which to divide the curve into two portions, each giving the best straight-line fit. No division of points occurred unless a minimum of three points could fit on each portion of the graph. Rate constants are described as k_1a and k_1b rather than k₁ and k₂ (Dick and Tabatabai, 1986b) to reduce confusion over the possible meaning as k₂, which is often used to designate second-order rate constants. The half-life, t_1/2, was determined by t_1/2 = 0.693/k. Further analysis indicated that the time to one-half decomposition (1/2t_d) might be a more appropriate tool to compare treatments. These values were determined using an interpolation procedure (Stineman, 1980).

Energy of Activation
The E_A a measure of the energy barrier that must be overcome before hydrolysis occurs, is derived from the temperature dependence of rate constants for chemical reactions. The temperature influence can be described by the linear form of the Arrhenius equation (Atkins, 1982),

Where k is the rate constant, A is the pre-exponential factor, E_A is the energy of activation, R is the gas constant, and T is the temperature in °K. The activation energy can be calculated from the slope of the linear form of the equation when ln k is plotted against 1/T.

Temperature Dependence
The temperature dependence of the reaction, Q₁₀, (Dick and Tabatabai, 1986b) was determined for Branyon c and Falba sl soils at two application rates of P.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Sorption Isotherms of C3P and C4P
Results of sorption studies with C3P and C4P clearly showed that neither was appreciably sorbed by Branyon c (Fig. 1) and Falba sl (data not shown) after 24 h of shaking in autoclaved soil. There was a 1:1 relationship between C3P and C4P remaining in solution after shaking and the initial C3P and C4P in solution.

View larger version (20K): [in this window] [in a new window]   Fig. 1. Sorption of monophosphorus, diphosphorus cyclotriphosphate (C3P), and cyclotetraphosphate (C4P) by a Branyon clay soil.

Speciation of Hydrolysis Products
Speciation and P recovery were documented for C4P hydrolysis over a period of 552 h (23 d) for the Falba sl soil (Fig. 2) and 336 h (14 d) for the Branyon c soil (Fig. 3) . Total recovery of applied P from the Falba sl was close to 100% for the duration of the incubation period but was variable for the Branyon c, generally between 75 and 85% after 12 h. High recovery of applied P from the Falba sl indicates that the non-cyclic species, which are subject to retention in most soils were fully recovered. Hydrolysis of C4P was faster in the Branyon c than the Falba sl. Detectable quantities of C4P were still present at 96 h in the Falba soil (Fig. 2), but there was no measurable C4P in the Branyon soil after 12 h (Fig. 3). A decline in total P recovery was observed in extracts of Branyon c soil due to sorption and the difficulty of extracting P from the fine-textured soils. Quantitative analysis of the hydrolysis products of C4P (tetraphosphorus, triphosphorus, diphosphorus, and monophosphorus) was accomplished in the sequential water/0.5 M H₂SO₄/1.0 M NaOH soil extracts. Standard errors about the mean values were low and the sequence of reaction products was as expected in both soils.

View larger version (26K): [in this window] [in a new window]   Fig. 2. Speciation of P in Falba sandy loam soil after application of 0.100 g cyclotetraphosphate (C4P) P kg–1 soil and incubation for 552 h at 0.033 MPa moisture content and 20°C. Bars represent the standard error of the mean.

View larger version (22K): [in this window] [in a new window]   Fig. 3. Speciation of P in Branyon clay soil after application of 0.100 g cyclotetraphosphate (C4P) P kg–1 soil and incubation for 336 h at 0.033 MPa moisture content and 20°C. Bars represent the standard error of the mean.

View larger version (13K): [in this window] [in a new window]   Fig. 4. The relationship between ln cyclotetraphosphate (C4P) concentration and time following application of 0.100 g C4P P kg–1 soil in Falba sandy loam and incubation at 0.033 MPa moisture content and 20°C.

A possible explanation of the observed kinetics of cyclic P hydrolysis is the inhibition of a first-order reaction by the hydrolysis products either directly or by the binding of a catalyst. The linear polyphosphorus produced from cyclophosphorus hydrolysis may react with Fe and Al oxides to reduce hydrolysis rate. Iron and Al oxides have been positively correlated with polyphosphorus hydrolysis rates (Dick and Tabatabai, 1986a). Formation of strong complexes with Ca and Mg by diphosphorus, triphosphorus, and tetraphosphorus, compared with cyclic P, which forms weaker complexes (Kura et al., 1974), could reduce the effect of these known catalysts on C3P and C4P hydrolysis (Van Wazer, 1958). It is believed that C3P and C4P hydrolysis in soils is first order, however, the complexity of the soil matrix complicates the study of hydrolysis compared with hydrolysis in solution.

Variability among the data was examined by determining the CV for each treatment (Table 2). All but one of the values was below 15% and probably represents variation due more to extraction of soil P and ion chromatography analysis. The CV for k_1a for C4P hydrolysis in Wilson soil was 21.9%. Because only three points were used in the determination of some individual rate constants, one value that is considerably different from the other two can cause an inflated CV.

These studies suggest that the hydrolysis of C4P and tetraphosphorus may involve more than ring degradation or the removal of a P unit from the end of the chain. Hydrolysis of C4P in some treatments indicated that triphosphorus and diphosphorous formed more quickly than was expected relative to the level of tetraphosphorus in the soil. The role of phosphatase enzymes in hydrolysis of cyclicphosphorus and polyphosphorus is not clearly understood, but it is believed that the presence of phosphatases may alter the traditional mechanisms of chemical polyphosphorus hydrolysis (clipping of terminal P units).

Data in Table 3 show the time of one-half decomposition (1/2t_d) of soil-applied C3P and C4P under varying conditions of temperature (10, 20, and 30°C) and biological activity (autoclaved compared with unautoclaved). Time of one-half decomposition value was chosen as a more accurate representation of what was occurring in the soil than the kinetic half-life, t_1/2. Blanchar and Hossner (1969b) reported that C3P had a half-life of 5 h in an Elliot sil soil, and that all of the material was hydrolyzed in 24 h. Further work showed that C3P persisted over 24 h in only two of 32 Midwestern U.S. soils (Blanchar and Hossner, 1969a). Dick and Tabatabai (1986b) indicate that C3P was present for a minimum of 168 h, but they used very high P application rates and measured only monophosphorus.

The increase in 1/2t_d for C4P at 0.100 g P kg^–1 soil was from 3.3 to 7.7 times more than that of C3P in Branyon c (Table 2). Comparable results favoring C4P in Falba soil are from 1.1 to 2.4 times more than the 1/2t_d of C3P. Increases in 1/2t_d for C4P over C3P were at least 6.3 times greater in Branyon c and 2.6 times in Falba sl at the 0.400 g P kg^–1 rate (data not shown). Virtually all treatments indicated a substantial increase in the persistence of C4P relative to C3P in soils. Higher stability of C4P was expected based on the longer half-life (Kura et al., 1982) and higher E_A (Watanabe et al., 1975) of C4P compared with C3P reported in the literature.

Decomposition of cyclic P was slowest in saturated, autoclaved soil (Fig. 5) . The time required for complete hydrolysis of C3P was longer as compared with C4P in the saturated autoclaved Falba soil. The most rapid hydrolysis of cyclic P was for unautoclaved soil at field capacity moisture content. The presence of cyclic P was detected up to six times longer in selected autoclaved treatments compared with the unautoclaved treatments.

View larger version (20K): [in this window] [in a new window]   Fig. 5. Decline in cyclotri- (C3P) and cyclotetraphosphate (C4P) in Falba sand loam soil as influenced by autoclaving and moisture conditions. Bars represent the standard error of the mean.

View larger version (12K): [in this window] [in a new window]   Fig. 6. Plot of ln k1 versus reciprocal temperature (1/K) to determine energy of activation (EA) (slope) for cyclotri- (C3P) and cyclotetraphosphate (C4P) at an application rate of 0.100 g P kg–1 soil and 0.033 MPa moisture content in Branyon c soil.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

Examination of kinetic parameters in soil indicated that hydrolysis of C3P and C4P followed first order but reaction kinetics was complicated by secondary reactions in the soil matrix. The presence of hydrolysis products and their possible effect on catalysts (Ca, Fe, Al) may explain the decreased rate constants and hydrolysis observed in soil incubated for >2 d.

The time of one-half decomposition (1/2t_d) was evaluated for each treatment. Cyclotetraphosphate was more stable than C3P in all comparable treatments, often by three to six times. These results indicate a potential advantage of C4P over C3P because of its slower hydrolysis in soils, and over all linear phosphates, which are sorbed by soil constituents.

The E_A was 45.8 and 61.1 kJ mol^–1, respectively, for C3 and C4P in Falba sl, and 38.6 and 51.2 kJ mol^–1, respectively, for the same treatments in Branyon c. Lower E_A values for C3P indicate its comparatively easier hydrolysis in soils.

The longer half life for C4P and extended times for hydrolysis of reaction products to monophosphorus would be expected to favor higher utilization of P by plants from cyclic phosphates. In addition, the cyclophosphates are more mobile in the soil before hydrolysis. Controlled studies on plant uptake of P from cyclophosphates are needed to determine the magnitude of P efficiency from these compounds, as compared with monophosphorus sources over a range of soil conditions.

Received for publication May 14, 2003.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Hossner, L. R. Articles by Shahandeh, H. Search for Related Content PubMed Articles by Hossner, L. R. Articles by Shahandeh, H. Agricola Articles by Hossner, L. R. Articles by Shahandeh, H. Related Collections Soil Fertility and Productivity Plant Nutrition Soil Chemistry