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Published online 4 August 2005
Published in Soil Sci Soc Am J 69:1405-1411 (2005)
DOI: 10.2136/sssaj2004.0266
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Soil Chemistry

Do Decomposing Organic Matter Residues Reduce Phosphorus Sorption in Highly Weathered Soils?

Christopher N. Guppya,*, Neal W. Menziesb, F. P. C. Blameyb and Phil W. Moodyc

a School of Rural Science and Agriculture, The Univ. of New England, Armidale, NSW, Australia, 2351
b School of Land and Food Sciences, The Univ. of Queensland, St Lucia, QLD, Australia, 4067
c Queensland Dep. of Natural Resources, Mines and Energy, 80 Meiers Rd, Indooroopilly, QLD, Australia, 4068

* Corresponding author (cguppy{at}une.edu.au)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Many studies have shown a reduction in P sorption in highly weathered soils when organic matter (OM) is applied, suggesting competition between OM decomposition products and P for sorption sites. However, such studies seldom consider the P released from the added OM. To delineate the effects of OM addition on P availability through sorption competition and P addition, water leachate from incubated soybean (SB) [Glycine max (L.) Merr.] and Rhodes grass (RG) (Chloris gayana Kunth cv. Callide) was used in competitive P sorption studies both undiluted and after acidification (i.e., the fulvic acid [FA] component). Addition of two rates (0.2 and 2 mL) of SB leachate to an Oxisol significantly increased P sorption at the higher rate, while a similar trend was observed following RG leachate addition at the same rates. Extending the range of highly weathered soils examined (two Oxisols, an Ultisol, and an acidic Vertisol) resulted in no observed decrease in P sorption following addition of OM leachate. Surprisingly, SB leachate transiently increased P sorption in the two Oxisol soils. Addition of the FA component of the leachates resulted in a transient (<6 d) decrease in P sorption in three of the four soils examined and constituted the only evidence in this study that decomposing OM residues reduced P sorption. This research provides further evidence contradicting the long held assumption that inhibition of P sorption by dissolved organic compounds, derived from decomposing OM, is responsible for increased P phytoavailability when P fertilizer and OM are applied together.

Abbreviations: AL, acidified leachate • CL, complete soybean leachate • DOC, dissolved organic carbon • FA, fulvic acid • HA, humic acid • LOA, low molecular weight organic acids • OM, organic matter • rcf, relative centrifugal force • RG, Rhodes grass • SB, soybean • TDI, triple deionized


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
HIGHLY WEATHERED SOILS of the tropics and subtropics are typically dominated by sesquioxides and hence have a high P sorption capacity. Sorption of applied P limits plant availability resulting in decreased productivity. Many studies have shown a reduction in P sorption (increased solution P concentration following small P additions) in highly weathered soils when OM is applied, suggesting competition between OM decomposition products and P for sorption sites (Singh and Jones, 1976; Bumaya and Naylor, 1988). These interactions are the subject of a recent review article (Guppy et al., 2005). The interaction of low molecular weight organic acids (LOAs), humic (HA) and fulvic acids (FA) with P, resulting in reduced P sorption, is well known (Leaver and Russell, 1957; Nagarajah et al., 1970; Appelt et al., 1975; Lopez-Hernandez et al., 1986; Sibanda and Young, 1986; Hue, 1991; Bolan et al., 1994). However, HA and FA not only compete with P for sorption sites in soil, but sorb P themselves in many instances (Levesque and Schnitzer, 1967; Appelt et al., 1975; Perrott, 1978; Borie and Zunino, 1983). The propensity for HA to sorb P is closely related to its Fe and Al content, particularly that of Al (Larsen et al., 1959; Heng, 1989). The known effects of LOAs, HA, and FA on P sorption when studied in isolation do not diminish the need to examine the interaction of P with water-soluble dissolved organic carbon (DOC) compounds derived from OM additions. Indeed, the whole suite of compounds produced during decomposition may interact and behave differently with respect to P sorption than the individual components.

Few researchers, with the exception of Othieno (1973), Ohno and Crannell (1996), and Ohno and Erich (1997) have focused specifically on the effects of OM-derived DOC compounds on P sorption. Ohno and Crannell (1996) compared the effects of DOC from vetch (Vivia billosa L.) and clover (Trifolium incarnatum L.) residues on the sorption of 2500-mg kg–1 additions of P in a Spodosol. Addition of extracts from these residues reduced subsequent P sorption by 1.0 to 2.5% where the residue extract addition rate produced a 5-mM DOC solution, increasing to 4 to 7.5% where the extract addition produced a 20-mM DOC solution. In comparison, P sorption was reduced by 10 and 25% in the presence of 5- and 20-mM citrate solutions. The decrease in P sorption was related to the amount of Al released from soil colloids by the DOC, suggesting that complexation of Fe and Al from oxides may have contributed to the reported decreases in P sorption. This was subsequently confirmed through examination of P sorption kinetics (Ohno and Erich, 1997). However, one of the weaknesses of using a single point estimate of changes in P sorption is that the slope of a sorption isotherm is dependent on the equilibrium solution P concentration at which it is measured, and this concentration will depend on the size of the P rate chosen. Othieno (1973) compared the effects of surface mulching, and the application of leachate derived from the surface mulch, to determine if improved fertilizer P phytoavailability was the result of changes in either physical or chemical factors associated with OM addition. Although concluding that up to 30% of improved plant yield and P uptake could be attributed to chemical effects, such as competitive sorption of DOC compounds with P, the experiments as reported did not consider the P released rapidly from the OM material. The sorption of P released from OM, but unaccounted for, would result in an apparent decrease in P sorption. This P would also be available for plant uptake resulting in improved yields.

Experiments examining the effects of plant-derived DOC have reported reductions in P sorption (Othieno, 1973; Ohno and Crannell, 1996; Ohno and Erich, 1997). Although these reductions have been small, the results of these experiments have contributed to the view that OM decomposition improves P availability through competition for sorption sites. However, as with the reactions between P and HA or FA, the potential for P to adsorb to these compounds themselves, particularly through metal linkages, creates ambiguity as to their net effect on P sorption. In this study, we examine the hypothesis that OM decomposition products reduce P sorption in highly weathered soils, and consider the role of FA-like compounds in mediating this effect.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Soils
Four highly weathered soils with a range of physical and chemical properties were used in a laboratory study (Table 1). The two Oxisols had differing clay contents (OX1 32% and OX2 51%) but were both dominated by kaolinite, with goethite and haematite also present. The Ultisol (ULT) had 12% clay dominated by kaolinite while the Vertisol (VERT) was 49% smectite dominated clay. All soils were acidic and had low (<7 µg g–1) bicarbonate-extractable P contents (Colwell, 1963).


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  		Table 1. Selected properties of four highly weathered soils used in P sorption experiments.

 
Organic Matter
The OM sources used were soybean trash (SB) and Rhodes grass hay (RG). The SB was threshed in the field and large stalks (<5 mm) removed. The RG was ground to pass a 5-mm sieve. The C and N concentrations were determined on a Leco 2000 combustion analyzer (Leco Corp., St. Joseph, MI), while the P concentration was determined by inductively coupled plasma atomic emission spectroscopy (ICP–AES), following digestion in HNO3/HClO4 acid (5:1) (Martinie and Schilt, 1976). The SB contained 2.0% N with a C/N ratio of 21, while the RG had 1.1% N and a C/N ratio of 37. Both materials had a P concentration of 0.16%.

Leachate Collection
Perspex columns, each 65 mm in diameter and 200 mm long, were used to collect leachate from a sand and OM mixture. A mixture of 200 g of acid-washed sand (P sorption capacity of <3 µg P g–1) and 10 g of either SB or RG was added to each column, and a cellulose filter paper placed on top. All columns were spiked with 10 mL of a soil extract inoculum to ensure microbial activity. The inoculum extract was obtained by shaking 10 g of fresh soil with 200 mL of triple deionized (TDI) water for 1.5 h and centrifuging for 10 min at 200 x g relative centrifugal force (rcf). Ten milliliters of the supernatant (containing ~3 µg P) was added with the first addition of TDI water to each column, and the leachate filtered through a fiberglass filter paper. A total volume of 50 mL of TDI water was passed through each column to wet the mixture, collected and recycled until no further leachate exited the columns. The retained volume of the acid washed sand with RG or SB was ~50 mL. After 1-, 7-, 14-, and 21-d incubation, 50 mL TDI water was passed through all columns. The extracts were collected, mixed, and analyzed for inorganic P (Pi) using malachite green (Motomizu et al., 1980), and total P (Pt) and DOC (Oweczkin et al., 1995) using ICP–AES (Table 2). Organic P (Po) was determined by difference. The collected leachate was then used in subsequent P sorption experiments.


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  		Table 2. Composition of leachates from soybean (SB) and Rhodes grass (RG) after incubation for up to 21 d and basic properties of a 1-d leachate used in subsequent sorption curves.{dagger}

 
Leachate Composition
The leachate collected from incubated OM was dark brown initially, with visible hydrophobic components (SB > RG), presumably waxes and lignins, becoming pale yellow to clear as the period of incubation increased from 1 to 21 d. Both SB and RG released <10% of their total P (16 mg) as a flush of Pi after 1 d (Table 2). Over 21 d total P release rose to 24% for SB (3.8 mg P) and 20% for RG (3.2 mg P). A rapid decrease in the Pi released from both the SB and RG leachate was observed from 1 to 7 d (Table 2). The main difference between the two materials was in the amount and manner of DOC release. After 1 and 7 d of incubation up to five times more DOC was released by the SB than by RG, this decreased to 55% more after 21 d. For the SB leachate, C/P ratio decreased with time, as the DOC released halved with successive leaching events, while leachate P concentration remained relatively constant. The RG material also released half the amount of DOC at 7 d as was released after 1 d, but DOC release remained constant over the next 14 d. The C/P ratio in the RG extract increased slightly over time, as the leachate P concentration slowly decreased.

Phosphorus Sorption
Phosphorus sorption was determined using 2 g of air-dried and ground (<2 mm) soil shaken end-over-end with 20 mL 0.01 M CaCl2 solution and various P additions (50, 100, 200, and 400 µg P g–1) for 1.5 h. Short reaction time curves were adopted to limit the confounding effects of microbial activity in the C-rich solutions (Zhou and Wong, 2000). After shaking, the tubes were centrifuged at 900 x g rcf for 10 min to clear the supernatant. From each tube, 3 mL of supernatant was removed and analyzed for Pi using malachite green (Motomizu et al., 1980). Phosphorus sorption curves were then plotted using the Tempkin plot [P sorbed (Psb) = a log solution P (Psln) + b] and a linear regression for this relationship calculated. Statistical analysis was undertaken by comparing the parameters of the linear Tempkin regressions using Student's t test (Cochran and Cox, 1957). Differences were considered significant at P < 0.05.

A preliminary experiment had shown no differences in P sorption due to the order of addition of P and OM leachate. Hence for all experiments, simultaneous addition of OM leachate and P to soil was used. In another preliminary experiment, the effect on P sorption of leachate collected after different periods of incubation was assessed. Only leachate collected in the first 3 d of incubation affected P sorption. No P sorption response was observed following addition of leachate collected after 7, 14, or 21 d, hence all experiments used leachate collected after 1 d of incubation as this contained the highest DOC concentration. The Pi already present in the OM leachate was accounted for by measuring the total amount supplied in the respective addition (i.e., 2.8 µg Pi in 0.2 mL SB leachate and 10.4 µg Pi in 0.2 mL RG addition) and including this in calculation of the proportion of P sorbed. The Pi added by the OM leachate constituted <4% of Pi supplied in sorption curve formation in most cases.

Rate of Leachate Application
An experiment was conducted on the OX1 soil to compare the effects of DOC concentration, controlled by altering the volume of OM leachate added, on P sorption. Calculations indicated that the DOC contained in 0.2 mL of OM extract would be equivalent to that released to the soil solution from a field application of ~10 Mg OM ha–1, assuming that all the soluble OM breakdown products were displaced by one retained volume of leachate. To compare the effects of DOC concentration on P sorption, either 0.2 or 2 mL of RG or SB leachate was added with Pi.

Leachate Application (Soybean) to Four Highly Weathered Soils
Following the results of the rate of leachate experiment, examination of the effects of the more reactive SB leachate (0.2-mL additions) on P sorption was undertaken on all four highly weathered soils (Table 1). Lower rates of Pi addition (0–250 µg P g–1 for the OX1 and ULT soils and 0–800 µg g–1 for the OX2 and VERT soils) were used to create sorption curves with solution P concentrations closer to those found in the soil solution of highly weathered soils. A comparison with 6-d sorption curves was performed by extending the reaction time of some of the 1.5-h curves. Samples were spiked with two drops of CHCl3 to inhibit microbial activity during the 6-d incubation period. All P sorption curves were established and analyzed as described above.

Effects of the Acidified Fraction of Soybean Leachate
A portion of the complete SB leachate (CL) was acidified to pH 1.5, allowed to stand for 10 min, during which ~70% of the larger molecular weight C compounds precipitated, and then centrifuged at 200 x g rcf for 15 min. The acidified leachate (AL) fraction remaining in the supernatant was then used in competitive P sorption studies. Both 1.5-h and 6-d P sorption curves were established and analyzed as described above. Functionally, the AL fraction can be thought of as a FA-like fraction.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION AND CONCLUSIONS
 REFERENCES
 
To address the results fully, some consideration of the interpretation of changes in P sorption curves is necessary. Displacement of a P sorption curve to the right indicates there is more P in solution for a given amount of adsorbed P, and may occur due to competitive inhibition of P sorption. Conversely, displacement of a P sorption curve to the left indicates that there are more sorption sites, but of the same suite of adsorption energies as was already present in the soil. A change in the slope of a P sorption curve demonstrates that more (or less) P is sorbed per unit change in P concentration, indicating that a fundamental change in the suite of sorption sites present has occurred. Where an increase in slope is observed, new sorption sites have been created; while a decreased slope may occur due to permanent blocking of P sorption sites by other entities. These theoretical propositions are derived from the mechanistic model of adsorption developed by Bowden et al. (1980a). For P adsorption, (i) surface charge becomes more negative as pH rises and (ii) charging curves are steeper at high than at low ionic strength. Hence, for variable charge minerals at constant pH, increasing the solution activity of phosphate will increase the surface activity (or adsorption) of phosphate. However this will not be a straight linear response because of the twin effects of reduced number of sorption sites and increasingly negative surface charging as adsorption proceeds. If an increase in sorption at a similar solution P activity is observed, then it relates to a change in one of these two properties (sorption sites or surface charging [affinity]).

Rate of Leachate Application
Addition of 2.0 mL of SB leachate to the OX1 soil significantly increased the slope of the P sorption curve and displaced the curve to the left, while a 0.2-mL addition did not significantly alter the slope (Fig. 1). A similar, though not significant, trend was observed following reaction of the OX1 soil with RG leachate (Fig. 1). The difference between the effects of the SB and RG leachate at the same volume addition was probably due to the RG leachate having <40% of the DOC of the SB (Table 2). These results (i.e., the absence of any competition between OM and P for sorption sites) stand in marked contrast to most reports of decreased P sorption due to competition of P with OM derived DOC compounds (Ohno and Crannell, 1996; Ohno and Erich, 1997).



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  		Fig. 1. Effects of incubated soybean (SB) or Rhodes grass (RG) leachate and P applied on the P sorption after 1.5 h of an Oxisol (OX1) soil (____ Control; SB: _ _{blacktriangleup}_ _ 0.2 mL, ...{triangleup}... 2 mL; RG: --{blacksquare}-- 0.2 mL, ... {square} ... 2 mL). Significant differences in curves- Control: Psb = 101(±14) log Psln – 69(±29); 2 mL SB Psb = 141(±17)*log Psln – 98(±31) where Psb indicates P sorbed and Psln indicates P in solution.

 
Application of Soybean Leachate to Four Highly Weathered Soils
Addition of 0.2 mL of SB leachate significantly increased the slope of the P sorption curve and displaced the curve to the left in the OX1 soil after a 6-d reaction (Fig. 2). However in the OX2 soil, while 0.2 mL of SB leachate displaced the curve to the left, the slope of the curve was decreased after a 6-d reaction. There was no effect of SB leachate on P sorption curves in the ULT and VERT soils. After a 1.5-h reaction there was no significant difference observed in any of the soils, indicating potential limitations in using short-term P sorption curves for DOC competition studies. The kinetics of P reactions over time should be considered in future P competition studies. This experiment provided no evidence that decomposing OM residues decreased P sorption in the four highly weathered soils studied (Fig. 2). Rather than decreasing P sorption, the observed trend was toward increased P sorption in the OX1 soil following competitive sorption with OM leachate (Fig. 2).



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  		Fig. 2. Effects of 0.2 mL addition of soybean (SB) leachate and P on the P sorption of two Oxisols (OX1 and OX2), an Ultisol (ULT) and a Vertisol (VERT) after 1.5 h (____ Control; _ _{blacktriangleup}_ _ SB) and 6 d (____ Control; _ _ {triangleup} _ _ SB). Significant differences in curves– OX1 curves- Control Psb = 65(±5) log Psln + 53(±3); SB Psb = 92(±4)** log Psln + 71(±1)**; OX2 curves- Control Psb = 233(±16) log Psln + 73(±3); SB Psb = 173(±28)* log Psln + 162(±14)** where Psb indicates P sorbed and Psln indicates P in solution.

 
Effects of Acidified Fraction of Soybean Leachate
Separation of the effect of CL and AL components on P sorption may permit the identification of the active components affecting P sorption in these soils. The response of the soils to the CL varied with soil type and length of reaction. A significant reduction in P sorption after 1.5 h was observed following AL addition in the OX1, ULT, and VERT soils when compared with both the control and CL solution (Fig. 3). The addition of the AL component shifted the curves significantly to the right, although the slope of the curve remained unaffected. When the reaction time was extended to 6 d, however, there was no significant difference between the effect of AL and CL SB leachate additions on P sorption in three of the four soils (data not shown). The transient decrease in P sorption following addition of the AL component of SB leachate constitutes the first evidence in these experiments that decomposing OM residues decreases P sorption in these four highly weathered soils.



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  		Fig. 3. Comparison of 0.2 mL additions of complete soybean (SB) leachate (____ Control; _ _{blacktriangleup}_ _ CL-SB) and the acidified leachate (AL) component (...{triangleup}... AL-SB) on the P sorption of two Oxisols (OX1 and OX2), an Ultisol (ULT) and a Vertisol (VERT) soil after 1.5 h reaction. Significant differences in curves– OX1 curves- Control Psb = 58(±4) log Psln – 2(±5); CL SB Psb = 58(±6) log Psln – 12(±5); AL SB Psb = 60(±4) log Psln + 3(±6)*; ULT curves- Control Psb = 73(±4) log Psln – 25(±5); CL SB Psb = 79(±5) log Psln – 26(±6); AL SB Psb = 75(±3) log Psln – 38(±5)**; VERT curves- Control Psb = 225(±8) log Psln – 19(±8); CL SB Psb = 232(±12) log Psln – 9(±11); AL SB (VERT) Psb = 242(±19) log Psln – 60(±23)** where Psb indicates P sorbed and Psln indicates P in solution.

 

   DISCUSSION AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
REFERENCES
 
These studies addressed the question of whether decomposing OM residues reduce P sorption in highly weathered soils. The increase in slope and displacement of the P sorption curve to the left, observed where 2 mL of SB leachate was applied to the OX1 soil (Fig. 1), indicates both an increase in the number of sorption sites and changes in the affinity of sorption sites for P due to changes in surface charging (according to the mechanistic model of Bowden et al. [1980b]). An increase in the slope of the curve indicates a decrease in surface negative charge and an increased affinity for P sorption. These changes are RELATIVE to the changes in surface charge where OM leachate is not added and may involve metal chelate linkages. Similarly, P sorption curves were displaced to the left in both the OX1 and OX2 soil following a 0.2-mL addition of SB leachate and a 6-d reaction (Fig. 2), again indicating an increased number of sorption sites. This effect may have arisen through reaction of P with metal-chelate linkages on the external surfaces of DOC compounds (Borie et al., 1989; Gerke, 1992). These reactions with DOC compounds may occur either through solid phase sorption of DOC and subsequent metal linkages, or through reaction of P with DOC-metal linkages in solution. If the DOC compounds in the OM leachate were similar to FA or HA, reaction with soluble Fe and Al may have increased the number of potential sorption sites, and hence P sorption (Larsen et al., 1959; Levesque and Schnitzer, 1967; Heng, 1989). That these reactions can and do occur in soil was demonstrated by Gerke (1992) and Guggenberger et al. (1996a)(1996b). Whether these are solid phase or solution reactions of P with metal-chelate linkages, either way they are not malachite green reactive and present as increased P sorption. Alternatively, some of the increase in P sorption could have been due to increased solution ionic strength as a result of the higher DOC concentration applied (Bolan et al., 1986). The response curves developed in this study cover the range of critical solution P concentration for plant growth, and demonstrated no decrease in P sorption following reaction with SB leachate in four highly weathered soils (Fig. 2). In fact, an increase in P sorption was observed in the two Oxisols. These results are in contrast to those of Ohno and Erich (1997) and cast doubt on generalized assertion of inhibited P sorption in the presence OM breakdown products.

The question remains as to why only the two Oxisols demonstrated variation and increased P sorption responses in the presence of OM leachate. Interaction with variable charge colloids seems critical in this response. The response requires time to develop and is not observed where reactive surface areas are lower (ULT 12% clay) or in soils where permanent charge predominates (VERT). Further characterization of the DOC is necessary before firm conclusions can be drawn, however the nature of the DOC itself does not result in increased P sorption. Interaction of DOC with variable charge colloids in the two Oxisols was important; indeed approximately 1/3 of the added SB DOC was removed from solution in the OX1 soil (data not shown). Hypotheses relating the increased P sorption to metal-chelate linkages and increased sorption sites can only be tentatively proposed as increased Fe and Al concentrations in the OX1 soil were not observed (Table 1). Future research will focus on identification of the specific properties of DOC leachates as they interact with variable charge colloids and their effect on P sorption.

The displacement of the P sorption response curve to the right (Fig. 3) following the addition of AL, constituted the only evidence of competitive inhibition of P sorption by OM leachate in highly weathered soils in this study, a finding in agreement with that of Sibanda and Young (1986). However, the lack of effect of AL on P sorption following the longer reaction time of 6 d indicates that the effects of AL on P sorption were transient at best. This transient inhibition of P sorption, applied at 1000 mg P kg–1 to an Oxisol, by large quantities of FA was also observed by Leaver and Russell (1957). When a similar experiment was performed using pure Fe oxide, the inhibition of P sorption at 10 d was only half that at 2.4 h. Leaver and Russell (1957) attributed the transience of P sorption inhibition to hydrolysis of FA, despite the addition of toluene to all samples to inhibit microbial activity. The results presented in this paper indicate that direct competition between AL (considered to FA-like in function) compounds and P for soil sorption sites, resulting in decreased P sorption, occurred initially (Sibanda and Young, 1986; Sibanda and Young, 1989). However the transience of these reactions suggests that the net effect of the AL compounds on P sorption was neutral or positive.

In conclusion, the results of this study suggest that in most instances soluble RG and SB breakdown products, at concentrations likely to be encountered in the field, would have no beneficial effect on P phytoavailability. Indeed, a short-term increase in the P sorption capacity of the OX1 soil was observed. The AL component of the SB leachate produced a transient inhibition of P sorption, present after 1.5 h, but this inhibition was not evident after 6 d, indicating that long-term competitive inhibition is unlikely. This study provides evidence contradicting long held assumption that the inhibitory effect on P sorption of DOC compounds, derived from decomposing OM, is responsible for increased P phytoavailability when P and OM are added to soil together. By considering the P released from the OM itself in P sorption calculations, reported decreases in P sorption from OM/P mixtures (Singh and Jones, 1976; Bumaya and Naylor, 1988) may depend simply on the amount of P contained in the residues and the rate of its release to the soil (Guppy et al., 2005). Further consideration of interactions between DOC and P in soil solution is warranted before attributing increased P phytoavailability when OM and P fertilizer are applied to highly weathered soils.


   ACKNOWLEDGMENTS
 
The authors thank the Australian Centre for International Agricultural Research (ACIAR Project 9414) for funding the research reported in this paper.


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




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