5
DIVISION S-7-FOREST & RANGE SOILS Influence of Oxalate Loading on Phosphorus and Aluminum Solubility in Spodosols T. R. Fox* and N. B. Comerford ABSTRACT Organic anions can increase the solubility of P and AI in soils and may influence nutrient availability and soil weathering. In Spodosols of the southeastern USA, oxalate has been identified as the dominant organic anion. Therefore, the objectives of this study were to examine the influence of oxalate on the release of inorganic P, organic P, and AI from A, and Bh horizons of two forested Spodosols from the Lower Coastal Plain. Oxalate loading rates ranging from 0 to 125 mmol kg-' soil were examined in a series of batch experiments using single, one- time additions and repeated sequential additions of oxalate. Oxalate had a negligible effect on P and AI release from the A horizon soils. In contrast, oxalate had a large effect on both inorganic and organic P, as well as AI release from the Bh horizon soils. The effect of se- quential additions of oxalate on P and AI release from these soils was found to be cumulative. The amount of AI or P released was primarily controlled by the cumulative oxalate loading rate. In addition, the manner of oxalate loading (single vs. sequential) had relatively little effect. At the same total, cumulative oxalate loading rate, the cumu- lative amount of AI or P released following sequential additions of oxalate at low concentrations were essentially the same as the amount released following a single addition of oxalate at a much higher con- centration. This suggests that a continuous release of even small amounts of organic anions in forest soils could solublize large amounts of P and AI on an annual basis. O XALATE is a common low-molecular-weight or- ganic anion in most forest soils (Stevenson, 1967; Graustein et al., 1977). In a group of Spodosols sup- porting slash pine (Pinus elliottii Englem.) in Florida, oxalate was the dominant organic anion (Fox and Comerford, 1990). Oxalate has also been identified as a major component of the suite of organic anions in other forest soils throughout the world (Bruckert, 1970; Cromack et al., 1979; Malajczuk and Cromack, 1982; Pohlman and McColl, 1988). Oxalate can increase the solubility of both P and Al in soils through the formation of stable complexes with Al (Martell and Smith, 1977; Martell et al., 1988). Ligand-exchange reactions between oxalate and P at oxide surfaces directly release P to solution (Goldberg and Sposito, 1985; Stumm, 1986). Surface complex- ation reactions of oxalate also stimulate the dissolution of Al-oxide surfaces in soils (Huang and Schnitzer, 1986; Stumm 1986), which increases the solubility of both Al and P. The release of P and Al from soils generally increases as the concentration of organic an- ions in solution increases (Swenson et al., 1949; Na- garajah et al., 1970; Fox et al., 1990). T.R. Fox, ITT Rayonier, Southeast Forest Resources, Forest Re- search Center, P.O. Box 819, Yulee FL 32097; and N.B. Com- erford, Soil Science Dep., Univ. of Florida, Gainsville, FL 32611. Contribution of the Florida Agric. Exp. Stn. Journal no. R-01332. Received 11 Jan. 1991. * Corresponding author. Published in Soil Sci. Soc. Am. J. 56:290-294 (1992). Oxalate concentrations are generally higher in the rhizosphere (Cromack et al., 1979; Fox and Comer- ford, 1990), which may increase P availability to the plant. With current analytical techniques, however, it is difficult to accurately measure oxalate concentra- tions in situ at soil microsites such as the rhizosphere. Therefore, oxalate concentrations reported in the soil are usually based on the analysis of bulk soil extracts or displaced soil solution, which may dilute their con- centration. Oxalate also tends to be rapidly degraded in soil (McColl et al., 1990). These factors may result in an underestimation of the oxalate concentration at the soil surface. Since the actual oxalate concentration at a soil surface at any point in time is largely un- known, a reasonable approach to determine the effects of oxalate on P solubility was to investigate a range of oxalate loading rates. Therefore, the objectives of this study were to: (i) investigate the effect of increased oxalate loading rates on Pj, soluble P 0 , and Al release; and (ii) compare P release following a single oxalate addition with that following sequential additions of oxalate. MATERIALS AND METHODS Soil Material Soil from two sites in the Lower Coastal Plain of the southeastern USA supporting slash pine plantations were used in this study. The soil at the first site, in Alachua County, Florida, is a Pomona series (sandy, siliceous, hy- perthermic Ultic Haplaquod) while the soil at the second site, in Charlton County, Georgia, is a Leon series (sandy, siliceous, thermic Aerie Haplaquod). Samples of the A and Bh horizons of the two soils were collected from a single soil pit at each site. The soil from each horizon was air dried and passed through a 2-mm sieve. Selected physical and chemical characteristics of these soils, determined by standard methods (Page et al.., 1982; Klute, 1986), are pre- sented in Table 1. Experiment 1: Single Oxalate Addition The effect of a single oxalate addition at loading rates from 0.5 to 50 mmol oxalate kg- 1 soil on P and Al release was examined in the Pomona Bh horizon soil material. The oxalate solutions were made from reagent-grade sodium oxalate (Fisher Scientific, Pittsburgh, PA) dissolved in deionized water and were adjusted to pH 4.3 with HC1 to approximate the soil pH. The ionic strengths of the solu- tions were standardized to 0.023 M by adjusting the con- ductivity of each solution to 1.8 dS m -1 with NaCl. Triplicate, 10-g samples of the air-dried soil were placed in polyethylene bottles and 50 mL of oxalate solution was added. To retard microbial growth, 0.1 mL of toluene was Abbreviations: P,, inorganic P; NBS, National Bureau of Stan- dards; P 0 , organic P. 290

Influence of Oxalate Loading on Phosphorus and Aluminum Solubility in Spodosols

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DIVISION S-7-FOREST & RANGE SOILS

Influence of Oxalate Loading on Phosphorus and Aluminum Solubilityin Spodosols

T. R. Fox* and N. B. Comerford

ABSTRACTOrganic anions can increase the solubility of P and AI in soils and

may influence nutrient availability and soil weathering. In Spodosolsof the southeastern USA, oxalate has been identified as the dominantorganic anion. Therefore, the objectives of this study were to examinethe influence of oxalate on the release of inorganic P, organic P, andAI from A, and Bh horizons of two forested Spodosols from the LowerCoastal Plain. Oxalate loading rates ranging from 0 to 125 mmol kg-'soil were examined in a series of batch experiments using single, one-time additions and repeated sequential additions of oxalate. Oxalatehad a negligible effect on P and AI release from the A horizon soils.In contrast, oxalate had a large effect on both inorganic and organicP, as well as AI release from the Bh horizon soils. The effect of se-quential additions of oxalate on P and AI release from these soils wasfound to be cumulative. The amount of AI or P released was primarilycontrolled by the cumulative oxalate loading rate. In addition, themanner of oxalate loading (single vs. sequential) had relatively littleeffect. At the same total, cumulative oxalate loading rate, the cumu-lative amount of AI or P released following sequential additions ofoxalate at low concentrations were essentially the same as the amountreleased following a single addition of oxalate at a much higher con-centration. This suggests that a continuous release of even small amountsof organic anions in forest soils could solublize large amounts of P andAI on an annual basis.

OXALATE is a common low-molecular-weight or-ganic anion in most forest soils (Stevenson, 1967;

Graustein et al., 1977). In a group of Spodosols sup-porting slash pine (Pinus elliottii Englem.) in Florida,oxalate was the dominant organic anion (Fox andComerford, 1990). Oxalate has also been identified asa major component of the suite of organic anions inother forest soils throughout the world (Bruckert, 1970;Cromack et al., 1979; Malajczuk and Cromack, 1982;Pohlman and McColl, 1988).

Oxalate can increase the solubility of both P and Alin soils through the formation of stable complexeswith Al (Martell and Smith, 1977; Martell et al., 1988).Ligand-exchange reactions between oxalate and P atoxide surfaces directly release P to solution (Goldbergand Sposito, 1985; Stumm, 1986). Surface complex-ation reactions of oxalate also stimulate the dissolutionof Al-oxide surfaces in soils (Huang and Schnitzer,1986; Stumm 1986), which increases the solubility ofboth Al and P. The release of P and Al from soilsgenerally increases as the concentration of organic an-ions in solution increases (Swenson et al., 1949; Na-garajah et al., 1970; Fox et al., 1990).

T.R. Fox, ITT Rayonier, Southeast Forest Resources, Forest Re-search Center, P.O. Box 819, Yulee FL 32097; and N.B. Com-erford, Soil Science Dep., Univ. of Florida, Gainsville, FL 32611.Contribution of the Florida Agric. Exp. Stn. Journal no. R-01332.Received 11 Jan. 1991. * Corresponding author.

Published in Soil Sci. Soc. Am. J. 56:290-294 (1992).

Oxalate concentrations are generally higher in therhizosphere (Cromack et al., 1979; Fox and Comer-ford, 1990), which may increase P availability to theplant. With current analytical techniques, however, itis difficult to accurately measure oxalate concentra-tions in situ at soil microsites such as the rhizosphere.Therefore, oxalate concentrations reported in the soilare usually based on the analysis of bulk soil extractsor displaced soil solution, which may dilute their con-centration. Oxalate also tends to be rapidly degradedin soil (McColl et al., 1990). These factors may resultin an underestimation of the oxalate concentration atthe soil surface. Since the actual oxalate concentrationat a soil surface at any point in time is largely un-known, a reasonable approach to determine the effectsof oxalate on P solubility was to investigate a rangeof oxalate loading rates.

Therefore, the objectives of this study were to: (i)investigate the effect of increased oxalate loading rateson Pj, soluble P0, and Al release; and (ii) compare Prelease following a single oxalate addition with thatfollowing sequential additions of oxalate.

MATERIALS AND METHODS

Soil MaterialSoil from two sites in the Lower Coastal Plain of the

southeastern USA supporting slash pine plantations wereused in this study. The soil at the first site, in AlachuaCounty, Florida, is a Pomona series (sandy, siliceous, hy-perthermic Ultic Haplaquod) while the soil at the secondsite, in Charlton County, Georgia, is a Leon series (sandy,siliceous, thermic Aerie Haplaquod). Samples of the A andBh horizons of the two soils were collected from a singlesoil pit at each site. The soil from each horizon was airdried and passed through a 2-mm sieve. Selected physicaland chemical characteristics of these soils, determined bystandard methods (Page et al.., 1982; Klute, 1986), are pre-sented in Table 1.

Experiment 1: Single Oxalate AdditionThe effect of a single oxalate addition at loading rates

from 0.5 to 50 mmol oxalate kg-1 soil on P and Al releasewas examined in the Pomona Bh horizon soil material. Theoxalate solutions were made from reagent-grade sodiumoxalate (Fisher Scientific, Pittsburgh, PA) dissolved indeionized water and were adjusted to pH 4.3 with HC1 toapproximate the soil pH. The ionic strengths of the solu-tions were standardized to 0.023 M by adjusting the con-ductivity of each solution to 1.8 dS m-1 with NaCl.

Triplicate, 10-g samples of the air-dried soil were placedin polyethylene bottles and 50 mL of oxalate solution wasadded. To retard microbial growth, 0.1 mL of toluene wasAbbreviations: P,, inorganic P; NBS, National Bureau of Stan-dards; P0, organic P.

290

FOX & COMERFORD: OXALATE LOADING EFFECTS ON SOLUBILITY 291

Table 1. Selected physical and chemical properties of Leonand Pomona soils.

Leon series Pomona seriesProperty A horizon Bh horizon A horizon Bh horizonSand (%)Clay (%)Organic C (%)PH (H20)Phosphorus (mg kg'1)

TotalOrganicMehlich 1Water0.01MCaCl2Aluminum (mg kg ~')Acid oxalatePyrophosphateMehlich I1MKC1Water

Iron (mg kg-1)Acid oxalatePyrophosphateMehlich 1Water

Potassium (mg kg'1)Mehlich I

Calcium (mg kg-1)Mehlich I

Magnesium (mg kg-')Mehlich I

9410.984.22

18.08.53.55.44.0

113903174

153220.6

4

34

12

8360.904.38

40.734.17.90.51.0

961904153107

8

6172.09

0.1

12

1

9311.774.03

21.89.65.19.17.0

22621537152

123530.7

13

111

30

8942.194.23

65.549.27.60.42.0

1357137943215813

61920.6

3

13

4

added and the bottles were then placed on a reciprocatingshaker at 100 cycles min-1. After 12 h, the samples wereremoved and the extracting solution was filtered through a0.45-jim nylon membrane filter.

Experiment 2: Sequential Oxalate AdditionsThe effect of oxalate on the solubility of P and Al was

also examined using a sequential loading procedure. Oxa-late solutions of 0.0, 0.5, and 5.0 mM were prepared fromreagent-grade sodium oxalate and were adjusted to pH 4.3using HC1. The ionic strength of each solution was alsoadjusted to 0.023 M in the same manner as for Exp. 1.

Duplicate, 20-g samples of air-dried soil from the A andBh horizons of the two soils were placed in preweighedpolycarbonate bottles. One hundred mL of oxalate solutionwas added along with 0.1 mL of toluene. The samples wereplaced on a reciprocating shaker at 100 cycles min-1 for12 h. The extracting solution was then decanted and filteredthrough a 0.45-jjim nylon membrane filter. Soil remainingon the filter was returned to the bottle, which was thenreweighed to account for entrained extracting solution. Theextraction procedure was repeated a total of five times.

The 5:1 solution/soil ratio used in this experiment re-sulted in the following oxalate loading rates in each se-quential extract: 0.0 mmol kg-1 in the 0.0 mM oxalatesolution; 2.5 mmol kg-1 in the 0.5 mM oxalate solution;and 25 mmol kg-1 in the 5.0 mM oxalate solution. Cu-mulative oxalate loading rates were obtained by multiplyingthese individual loading rates by the number of sequentialextractions performed. Therefore, the cumulative oxalateloading rates were 0 mmol kg-1 in water and ranged from2.5 to 12.5 mmol kg-1 in the lower concentration oxalatesolution and from 25 to 125 mmol kg-1 in the higher con-centration oxalate solution.

Statistical differences in P and Al release were deter-mined using analysis of variance. The study was analyzedas a split-plot design with oxalate concentration as the mainplot and extraction number as the subplot. Least significantdifferences were calculated using weighted t values (Gomezand Gomez, 1984).

o

EI

0.0060

Oxalate Loading (mmol kg )

Fig. 1. Release of inorganic P, organic P, and Al from thespodic horizon of a Pomona soil following a single oxalateaddition.

Chemical Analysis of ExtractsThe conductivity of the filtrate was measured with a Pt

conductivity electrode (Rhoades, 1982). Solution pH wasmeasured with a combination glass electrode (McLean, 1982).Aluminum was determined by flame emission spectropho-tometry using an N20-C;,H2 flame (Barnhisel and Bertsch,1982).

Inorganic P in the filtrate was determined by the Mo blueprocedure (Murphy and Riley, 1962). This is an operationaldefinition of P, because the Mo may hydrolyze some P0compounds (Stainton, 1980; Tarapchak et al., 1982). Totalsoluble P was also determined in each samples as follows:Twenty milliliter of the filtered extract was evaporated todryness in a 50 mL Pyrex beaker. The beaker was thenplaced in a muffle furnace for 12 h at 500 °C. Ten milliliterof 4.8 M HC1 was added to each beaker and evaporated todryness, followed by another 5 mL of 12 M HC1, whichwas also evaporated to dryness. The sample was then re-dissolved in 20 mL of 0.1 M HC1, and P in solution wasmeasured again. Pine tissue standards obtained from theNBS were used to check the accuracy of the total-P anal-yses. Soluble P0 was operationally defined as the differencebetween total soluble P and the initial Pj in the extract.

RESULTS

Experiment 1. Single Oxalate AdditionsThe release of Pj from the Pomona Bh horizon soil

increased as the oxalate loading rate increased (Fig.1). At low oxalate loading rates, the amount of P0released was greater than the amount of Pj released.Above a loading rate of 10 mmol kg-1, however,release of Pj exceeded that of P0. The release of Alalso increased in a nearly linear fashion as the oxalateloading rate increased (Fig. 1). The amount of Alreleased in the presence of oxalate from the PomonaBh horizon was nearly 100 times greater than the amountof P released.

Experiment 2. Sequential Oxalate Additions

Inorganic PhosphorusAlthough large amounts of Pj were released from

the A horizon soil from both the Pomona and the Leonseries, the effect of oxalate was small (Fig. 2). The

292 SOIL SCI. SOC. AM. J., VOL. 56, JANUARY-FEBRUARY 1992

Extraction NumberFig. 2. Release of inorganic P from surface and spodic horizons

of a Pomona and a Leon soil following five sequential additionsof oxalate. Vertical bars represent LSD (0.05).

0 1 2

Extraction NumberFig. 3. Release of organic P from surface and spodic horizons

of a Pomona and a Leon soil following five sequential additionsof oxalate. Vertical bars represent LSD (0.05).

amount of Pt released in 5.0 mM oxalate was onlyslightly greater than that released in water. The Pjrelease also declined rapidly following the initial ex-traction, with negligible amounts of P; released afterthe second extraction.

Release of Pj was small in the water extracts of theBh horizons from both soils (Fig. 2). In contrast, theaddition of oxalate substantially increased the releaseof Pj, with greater amounts of Pj released in the 5.0mM oxalate solution than in 0.5 mM solution. In ad-dition, the release of relatively large amounts of Pjcontinued from the Bh horizons after the first extrac-tion.

Organic PhosphorusIn the first extraction, the addition of oxalate slightly

increased P0 release from the A horizon; however, theeffect of oxalate disappeared in the subsequent ex-tractions (Fig. 3). As with Pj, the addition of oxalatehad a large impact on P0 release from the Bh horizons.The effect of 5.0 mM oxalate on P0 release from theBh horizons was more pronounced than the effect onPC Substantial amounts of P0 continued to be releasedin all five extractions.

12.0

^ 9.0

'05 6.0

"5 3.0

§ °-°(

L

- i

I

son A Horizon

0.0 mM OxalMi* .........

0.5 mM Oxalat*A ---- A

5.0 mM Oxal*f*• —— •

) 1 2 3 4 5

12.0

9.0

6.0

3.0

Pomona A Horizon

I

V^°-°0 1 2 3 4 5

15.0

12.0

9.0

6.0

3.0

0.0,

Leon Bh Horizon15.0

12.0

9.0

e.o

3.0

Pomona Bh Horizon

2 3 4 5 6 0 1 2 3 4 5 6

Extraction NumberFig. 4. Release of A] from surface and spodic horizons of a

Pomona and a Leon soil following five sequential additionsof oxalate. Vertical bars represent LSD (0.05).

Aluminum ReleaseThe release of Al in the sequential extracts of the

Leon and Pomona series followed the same generalpattern as P release, although the absolute amount ofAl release was approximately two orders of magnitudegreater than P (Fig. 4). The release was Al in waterwas negligible in all the soils studied. Oxalate sub-stantially increased Al solubility in the Bh horizonsbut had only a small effect on Al release in the Ahorizons. The amount of Al released from the Bh ho-rizons was nearly ten times greater than that from theA horizons. In addition, in the Bh horizons, there wassubstantial release of Al in the later extractions whenoxalate was added.

Cumulative Inorganic Phosphorus, Organic Phosphorus,and Aluminum Release

The additive effect of oxalate on the cumulativerelease of Pj, P0, and Al from the Bh horizon soilsfrom the two series is presented in Fig. 5. For com-parative purposes, the release of Pf, P0, and Al fromthe Pomona Bh horizon following the one-time oxa-late additions from Exp. 1 are also plotted in Fig. 5.In the Pomona Bh horizon, at comparable cumulativeoxalate loading rates, the release of Pi5 P0, and Alwere similar with the two loading methods. It appearsthat the release of P;, P0, and AJ could be describedby continuous-release curves, primarily controlled bythe total oxalate loading rate.

The data presented in Fig. 5 also clearly shows therelative importance of soluble P0 in the Bh horizons.In the Leon Bh horizon, the cumulative amount of P0released was nearly four times greater than the amountof PJ released. The release of P0 in the Pomona Bhhorizon was also large, equaling the amount of P( re-leased at the highest oxalate loading rate.

DISCUSSIONThe increased release of P and Al from subsoils

following the addition of ojralate observed in this studyhas also been observed in a number of other soils

FOX & COMERFORD: OXALATE LOADING EFFECTS ON SOLUBILITY 293

25 50 75 100 125

0.5

0.4

Pomona "Sequential"Pomona "One Time"

Cumulative Loading (mmol kg )Fig. 5. Relationship between cumulative oxalate loading rate

and cumulative release of inorganic P, organic P, and Alfrom the spodic horizon of a Pomona and a Leon soil followingsequential and one-time oxalate additions.

(Swenson et al., 1949; Nagarajah et al., 1970; Trainaet al., 1986). Two complementary mechanisms canbe invoked to explain the increased release of P andAl from the Bh soil horizons following the additionof oxalate. The Bh horizons in both the Leon andPomona series contained large amounts of Al and or-ganic matter (Table 1). Oxalate is specifically sorbedas an inner-sphere complex at Al-oxide surfaces and,therefore, can replace P bound to these surfaces througha ligand-exchange reaction (Goldberg and Sposito,1985; Fox et al., 1990). Ligand exchange of oxalatealso stimulates the dissolution of the Al-oxide surfaces(Stumm, 1986), which releases both P and Al. Thedata indicates that the dissolution of the Al-oxide sur-faces increases as the oxalate loading rate increases.

In contrast to the Bh horizon, there was only a smalleffect of oxalate on P or Al release from the A hori-

zons. The A horizons in Spodosols of this region donot sorb P or oxalate (Humphreys and Pritchett, 1971;Ballard and Fiskell, 1974; Fox et al., 1990). This isdue to the small amount of Al present in the A hori-zons and the subsequent scarcity of ligand-exchangesites. Fox et al. (1990) demonstrated that the Pj pres-ent in A horizon soils was nearly all water soluble.Therefore, oxalate would not be expected to have alarge effect on either Al or P release in the surfacesoils.

Because organic anions are rapidly degraded in soils(Hodgkinson, 1977; Fox et al., 1990; McColl et al.,1990), the effects of organic anions on P and Al re-lease from soil tend to be ephemeral, ceasing whenthe added organic anion has been degraded (Fox etal., 1990). Given the rapid degradation of oxalate insoils, the presence of oxalate in native Spodosols ofthe southeastern USA (Fox and Comerford, 1990)suggests that oxalate must be produced more or lesscontinuously in these ecosystems. The repeated ad-ditions of oxalate in the present study were intendedto mimic this continuous production.

The nearly 1:1 relationship between the two oxalateloading methods (one-time vs. sequential) demon-strated in Fig. 5 for the Pomona Bh horizon soil sug-gests that the release of Pj, P0, and Al from both ofthese soils is controlled by the total oxalate loadingrate, and is relatively independent of the manner inwhich the oxalate was added to the soil. Both P andAl release seemed to follow a single release curvewhere the cumulative amount of P or Al released de-pended only on the total, cumulative oxalate loadingrate. The cumulative effect of oxalate indicates how,over a pedogenic time frame, large amounts of Alcould be translocated through the profile by relativelysmall, incremental organic-anion additions. The com-plexation and translocation of Al and Fe by organicanions is thought to be a principle mechanism in-volved in the podzolization process (Ponomereva, 1969;Buurman, 1984). The cumulative effect of oxalateloading on Al release helps to explain the genesis ofthe well-developed Bh horizon present in Spodosolsof the southeastern USA. The cumulative effect ofrepeated additions of oxalate on P release from thesubsoil horizons also has implications for P availabil-ity to trees in the flatwoods. In Spodosols of the lowerCoastal Plain of the southeastern USA, the Bh hori-zons contain a significant proportion of the total nu-trient capital of the soil (Neary et al., 1990). Theresults from this study clearly show that continuousadditions of oxalate, even at low concentrations, cansubstantially increase the amount of available P in thesubsoil. Additional emphasis probably needs to beplaced on subsoil fertility, particularly in forestry wherefertilizer additions are infrequent.

The release of P0 from the Bh horizons followingthe addition of oxalate was large and was similar tothe release of Pj. Since phosphatases are common inmost forest soils (Speir and Ross, 1978), the data pre-sented here suggest that soluble P0 is a source of plant-available P. In that case, P0 from the spodic horizonmay contribute to the P nutrition of trees growing onthese soils.

The relative importance of oxalate in terms of the

294 SOIL SCI. SOC. AM. J., VOL. 56, JANUARY-FEBRUARY 1992

supply of P to trees needs to be viewed in the contextof the natural loading rates of oxalate encountered inthese soils. Since tree roots and microorganisms areactive throughout most of the year in the pine eco-systems of the Southeast (Comerford et al., 1987), thecumulative amount of oxalate released into the soil onan annual basis may be quite large. Two examplesusing literature data illustrate the types of oxalate loadingrates that may occur in soil supporting pine trees. Inthe first, data on oxalate release by radiata pine (Pinusradiata D. Don) seedlings (Smith, 1969) was used.For an average seedling producing 200 g of oxalateover a 10-d period and assuming a 0.5-mm-thick zoneof influence around a 2-mm-diam. root segment 50mm long in a soil with a bulk density of 1.4 g cm-3,the oxalate loading rate was calculated to be 0.08 mmolkg-1 for a 10-d period. Continuous production of ox-alate at this rate would result in a cumulative loadingrate of ==30 mmol kg-1 yr-1. In the second example,the oxalate concentration measured in the rhizosphereof slash pine (Fox and Comerford, 1990) was used toestimate a potential oxalate loading rate. An oxalateconcentration of 2.5 mM at a gravimetric moisturecontent of 300 g kg-1 resulted in a steady-state load-ing rate of =0.75 mmol kg-1. Assuming the entirepool of oxalate is replenished on a weekly basis, whichis perhaps conservative based on the degradation dataof McColl et al. (1990) and Fox et al. (1990), theoxalate loading rate would approach 40 mmol kg-1

yr"1. These calculations demonstrate that the potentialexists for the oxalate loading rates used in this studyto occur under field conditions. Clearly, these oxalateloading rates can greatly enhance the supply of P toplants from the Bh horizons.