Phosphorus Forms and Concentrations in Leachate under Four Grassland Soil Types

Phosphorus Forms and Concentrations in Leachate under Four Grassland Soil TypesBenjamin L. Turner and Philip M. Haygarth*

ABSTRACTThe transfer of P in water draining from agricultural land can

contribute to eutrophication and the growth of toxic algae. Tradition-ally, research has focused on particulate P transfer in surface pathways,with transfer by subsurface pathways perceived as negligible. Weinvestigated this by monitoring P in leachate draining through large-scale monolith lysimeters (135 cm deep, 80 cm diam.) installed in afield site in southwest England. The lysimeters were taken from fourgrassland soil types with a range of textures (silty clay-sand) andextractable-P contents (15-75 mg kg~' NaHCO3 extractable P) andleachate was sampled over two drainage seasons. Export of total Pwas <0.5 kg ha"1 yr~' for all soil types. Concentrations of total P inthe leachate routinely exceeded 100 |ig L ' and remained relativelystable throughout the drainage season, except during the late springperiod when maximum concentrations >200 jig L~' were detectedfrom all soil types. Physically, most of the leachate P was dissolved(<0.45 urn), although 21 to 46% occurred in the particulate(>0.45 (tm) size fraction, most notably from the sandy-textured soils.Chemically, the leachate was dominated by reactive (inorganic) Pfrom all soil types (62-71%), although a large proportion was inunreactive (organic) P forms (29-38%). Reactive P occurred mainlyin the • 0.45 (xm fraction, while unreactive P was predominantly inthe >0.45 fraction. Unreactive P in the <0.45 p-m fraction was greatestduring the springtime (April-May), probably reflecting microbiologi-cal turnover and release of P in the soil. Our results indicate that (i)subsurface P transfer from soil to surface water can occur at concentra-tions that could cause eutrophication and (ii) unreactive and >0.45 jxmP forms are important in subsurface P transfer.

THE TRANSFER OF P IN WATER draining from agricul-tural land to surface waters can contribute to eutro-

phication, toxic algal blooms, and a general deteriora-tion of water quality (Foy and Withers, 1995). Concernover agricultural P pollution has been heightened re-cently, because of the risks from aquatic organisms tohuman health, notably the potential for neurologicaldamage from outbreaks of the dinoflagelatte Pfiesteriapiscidia in the Chesapeake Bay area of eastern USA(Burkholder et al., 1992). Although the amounts of Ptransferred from the land are small in agronomic terms,typically <1 kg ha"1 yr"1, low concentrations of P inexcess of 35 jxg L""1 can contribute to eutrophication(Vollenweider and Krekes, 1982). Agricultural P trans-fer has been directly linked to elevated P loading andexcess algal growth in receiving water bodies and thereis evidence that loads are increasing (Foy et al., 1995).

The transfer of P from agricultural land can occurthrough surface or subsurface pathways, although thecapacity of most subsoils to fix inorganic P has meantthat subsurface transfer has traditionally been perceived

Soil Science Group, Inst. of Grassland and Environ. Res. (IGER),North Wyke, Okehampton, Devon, UK EX20 2SB. B.L. Turner alsoat Dep. of Geography, Royal Holloway, Univ. of London, Egham,Surrey, UK TW20 OEX. Received 22 Mar. 1999. * Correspondingauthor ([email protected]).

Published in Soil Sci. Soc. Am. J. 64:1090-1099 (2000).

to be of minor importance (Baker et al., 1975; Burwellet al., 1977; Sharpley and Syers, 1979). However, it isnow recognized that P can be exported through subsur-face pathways at levels that can cause problems forwater quality (Foy and Dils, 1998). This phenomenonis not restricted to waterlogged soils (Khalid et al., 1977)or sandy-textured soils under heavy fertilization as tradi-tionally thought (Ozanne et al., 1961; Breeuwsma andSilva, 1992), but includes many soil types, especially claysoils that are susceptible to cracking and preferentialflow (Simard et al., 1998; Stamm et al., 1998). However,despite the accepted role of subsurface pathways in Ptransfer, most evidence is derived from studies on tiledrainage at the plot or field scale (e.g., Sawhney, 1978;Turtola and Jaakkola, 1995; Grant et al., 1996; Haygarthet al., 1998; Simard et al., 1998; Stamm et al., 1998). Thisapproach may not give a true reflection of subsurfaceP transfer under natural conditions, because artificialdrainage creates preferential flow pathways that mini-mize contact with the subsoil and can strongly alter theprocesses controlling P release to drainage water. Inaddition, these studies only represent heavy soils thatrequire artificial drainage and do not include lighter,more freely draining, soils.

Thus, there is little direct field information on theforms and concentrations of P in water draining throughsoil that is not artificially drained. This information isessential, to understand the impacts of subsurface drain-age on water quality and to allow the development ofstrategies for the control of agricultural P pollution.The aims of this study were to determine P forms andconcentrations in leachate water at the soil profile scaleunder field conditions in the absence of artificial drain-age. To achieve this, we used large-scale monolith lysim-eters to monitor P in leachate from four grasslandsoil types.

METHODSExperimental

The monolith lysimeters consisted of intact blocks of soil,135 cm deep and 80 cm diam., sampled to preserve the soilstructure by a method described by Belford (1979). Therewere four soil types, representing both a textural gradient anda range of NaHCO3-extractable P contents (Table 1). Thefour soil types were: a silty clay (Typic Haplaquepts), a well-drained clay loam (Dystochrepts), a sandy loam over chalk(Hapludalfs) and a sandy soil (Udipsamments). There werefour replicates of each soil type. The 16 monoliths were in-stalled in a block-grid design at a field site near Okehampton,

Abbreviations: RP, reactive phosphorus; RP (<0.45), reactive phos-phorus <0.45 (xm; RP (>0.45), reactive phosphorus >0.45 |xm; TP,total phosphorus; TP (<0.45), total phosphorus <0.45 (xm; TP (>0.45),total phosphorus >0.45 |im; UP, unreactive phosphorus; UP (<0.45),unreactive phosphorus <0.45 u.m; UP (>0.45), unreactive phosphorus>0.45 |xm.

1090

TURNER & HAYGARTH: PHOSPHORUS LEACHING UNDER GRASSLAND SOILS 1091

Devon, UK, and positioned so that the lysimeter surface waslevel with the surrounding ground surface. The lysimeterswere sown with perennial ryegrass (Lolium perenne L.) andmanaged as typical cut grassland, with fertilizer applicationand herbage cutting timed according to typical UK farm man-agement guidelines (MAFF, 1994). Annual fertilizer applica-tion rates were: 40 kg P ha"1,340 kg N ha"1,220 kg K ha"1. Thelysimeters were exposed to natural rainfall and environmentalconditions, with a mean annual rainfall of about 1100 mm.

SamplingGrab samples of leachate draining under gravity from the

base of the lysimeters were collected in 25 L vessels (after24 h of drainage) from an underground chamber. The drainageflux was determined by weighing the vessels prior to sampling.Between September 1993 and June 1994 (1993-1994 drainageyear), leachate was sampled weekly, or more frequently, andanalyzed for RP (<0.45) only. During the period betweenOctober 1994 and May 1995 (1994-1995 drainage season),leachate was sampled monthly, but analyzed for all P fractions.

AnalyticalThe leachate was analyzed for a range of operationally

defined P fractions. Total P (TP) was determined by a sulfuricacid-persulfate digest adapted from Eisenreich et al. (1975)and described by Rowland and Haygarth (1997), except thatwe used 0.15 (± 0.01) g of potassium persulfate reagent, ratherthan the incorrect amount stated in the published method.Reactive P (RP), generally considered to be inorganic ortho-phosphate, was determined using a Tecator 5020 flow injectionanalyzer with an autosampler (Method Application ASN60-03/83, Tecator Ltd, Hoganas, Sweden) using a molybdenumblue reaction at 690 nm (Haygarth et al., 1998). The differencebetween TP and RP is unreactive P (UP).

The analyses were conducted on (i) unfiltered samples and(ii) samples filtered through a 0.45 \um cellulose-nitrate mem-brane filter. The qualifier <0.45 is used to describe P formsin filtered samples (traditionally termed dissolved P), while>0.45 is used to describe the difference in P between totaland filtered P fractions (traditionally termed particulate P).Fractions with no qualifier describe P forms in unfiltered sam-ples (Haygarth and Sharpley, 2000).

StatisticalDifferences in leachate TP between soil types during the

1994 through 1995 drainage year were analyzed statisticallyusing one-way analysis of variance tests. Tests were conductedon (i) TP concentrations on individual sampling events (ii)mean TP concentrations over the entire drainage year (1994-1995) and (iii) TP loads over the entire drainage year. Signifi-cant differences between individual soil types were deter-mined by comparing the least squared difference of meansvalue (LSD), obtained from the original ANOVA test, withthe difference in mean values between any two soil types. Ifthe LSD value was less than the difference in means for anytwo soil types, this indicated that mean values for the two soiltypes were statistically significantly different.

RESULTSDrainage

Volumes of leachate were similar for three of the fourlysimeter soil types and ranged between 332 and 350 mmduring the 1993 through 1994 drainage year and between

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Aug-93 Dec-93 Apr-94 Aug-94 Dec-94 Apr-95Fig. 1. Reactive P (RP [<0.45 (xin]) in leachate for the four soil types between September 1993 and June 1995. SE, standard error of the mean;

F-W-Mean, flow weighted mean concentration.


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451 and 491 mm during the 1994 through 1995 drainageyear. The exception was the silty clay, from which drain-age was about 50% less because high rainfall causedwaterlogging of the topsoil and overspilling of the lysim-eters. This was the result of drainage being impeded bya clay layer at 50-cm depth, raising questions about thesuitability of this soil type for monolith lysimeter exper-iments.

Loads and Concentrations of Phosphorusin Leachate

Temporal Trend in Reactive PhosphorusReactive P (<0.45) concentrations were determined

across two drainage years. Mean concentrations acrossthe full sampling period between September 1993 andJune 1995 ranged between 53 (xg L"1 for the sandy loamto 121 u,g L"1 for the silty clay (Fig. 1). For each soiltype, the mean concentrations were larger than the flow-weighted mean concentrations of RP (<0.45), implyingthat the mean data were distorted by a few high concen-tration events. Concentrations of RP (<0.45) for indi-vidual sampling events frequently exceeded 100 jxgLT1, especially for the silty clay, which represents thesingle most common soil type in the UK (Haygarthet al., 1998). Concentrations were generally consistentthroughout the drainage year, although elevated con-centrations occurred in the late spring periods (April-May). At these times, maximum concentrations of>200 |j,g L""1 were recorded for all soil types, indicatingthat particular processes were affecting P in leachateirrespective of soil type (Fig. 1). The clay-textured soilshad greater RP (<0.45) concentrations in leachates thanthe sandy-textured soils. The sandy loam leachates con-sistently had the smallest concentrations of RP (<0.45).We suggest that this effect might be partly related tothe soil pH levels, which were higher in the top 30 cmof the sandy-textured soils (Table 1).

Differences in P concentrations appeared to be unre-lated to soil P fractions. For example, the silty clay soil,with the lowest Olsen-P status, gave one of the highestflow-weighted mean RP (<0.45) concentrations (15 mgOlsen-P kg'1; 103 |xg RP (<0.45) LT1). Conversely, the

sandy soil, with the highest Olsen-P status, gave one ofthe lower flow-weighted mean RP (<0.45) concentra-tions (75 mg Olsen-P kg"1; 48 jig RP (<0.45) LT1).Similar patterns were evident for water and CaCl2 ex-tractable-P fractions (Table 1).

Total Phosphorus ConcentrationsTotal P concentrations in leachate were determined

during the 1994 through 1995 drainage year. Leachatecollected during this drainage year was analyzed lessintensively, but for the full range of P fractions. Theless intensive sampling resulted in the mean concentra-tions for the sand and the clay loam being skewed bya single high concentration event (Fig. 2). As a result,flow-weighted means provide a better indication of theconcentrations of P fractions for these soils during thisdrainage year. A summary of the mean concentrationsof each fraction is shown by soil type in Table 2. TotalP concentrations during 1994 through 1995 showed asimilar temporal and between-soils trend to those forRP (<0.45) for the previous year (Table 2, Fig. 2). Flow-weighted mean concentrations of TP in leachate rangedbetween 53 u,g Lr1 for the sandy loam, to 163 |xg L"1

for the silty clay.Differences in mean TP concentrations among soil

types during the 1994 through 1995 drainage year werenot significant (P > 0.05). However, differences amongflow-weighted mean TP concentrations during the 1994through 1995 drainage year, which represented a morerealistic measure of the mean P concentrations, werestatistically significant (P < 0.01). Statistical analyses ofdifferences among soil types were also performed onTP concentrations for individual sampling events. Theseshowed significant differences on samples taken duringthe winter drainage period (January-March) (P <0.005), but at all other times the differences were notsignificant. Not all soils were significantly different fromeach other. When significant differences were detected,the silty clay was always significantly different from theother soil types. However, the sand and the clay loamwere not significantly different from each other in anyof the tests.


Table 2. Concentrations of total P, reactive P, and unreactive P in size fractions during the 1994-1995 drainage year.____

Total P Reactive P Unreactive P

Silty clayMeanStandard errorRangeFlow weighted mean

Clay loamMeanStandard errorRangeFlow weighted mean

Sandy loamMeanStandard errorRangeFlow weighted mean

SandMeanStandard errorRangeFlow weighted mean

Total

17914

136-247163

1624472-44694

651033-10953

24013156-114588

<0.45 (Jim

1301585-210

109

1172562-26173

399

10-9128

711636-16148

>0.45 fun

491210-10754

45215-184

20

265

10-5325

16911710-98439

Total

1321990-223

107

1052533-23466

391112-10834

973833-35952

<0.45 jim

—— fig L ' —

1171681-21097

952229-19159

2567-56

20

631529-15743

>0.45 (Jim

1655-53

10

105I^t46

1463-52

14

34244-2029

Total

4791-94

56

58232-211

28

2661-51

19

143930-786

35

<0.45 fun

1360-47

12

2391-71

14

1470-499

940-355

>0.45 urn

34100-94

44

35171-140

14

1241-32

10

134930-783

30

Total Phosphorus LoadsThe load of TP exported from the lysimeters was

calculated by multiplying the TP concentration at thetime of sampling with the volume of leachate precedingthe sample. This gave a very imprecise estimate of theTP loads, because of the infrequent sampling times.However, these values are useful to give an idea of themagnitude of P export and to determine the propor-tional contribution of the various P fractions to the TPload. The load values for each fraction are presentedin Table 3. During the 1994 through 1995 drainage year,the mean TP export was <0.5 kg ha"1 yr"1 for all soiltypes. Total P exports for individual lysimeters rangedbetween 202 and 594 g ha~'. These exports typicallyrepresented <1% of the annual fertilizer applied (40 kgha"1) and a minute fraction of the total reservoir of soilP. Total P loads were remarkably similar for all soiltypes except the sandy loam, which was about 60% ofthe others (Table 3). However, statistical analysis of TPloads for the 1994 and 1995 drainage year showed thatdifferences among soil types were not significant (P >0.05), even for the sandy loam soil. This was a conse-quence of the variability among lysimeters of the samesoil type. Total P export from the silty clay was similarto the clay loam and the sand, even though drainagethrough the silty clay soil was considerably less.

Forms of Phosphorus in LeachateIn general, the majority of the TP occurred in

<0.45 forms, accounting for 54 to 79% of the TP export(Tables 3 and 4). The well-drained clay loam leachedthe highest amount and proportion of P in the <0.45fraction. However, despite the dominance of P (<0.45),a substantial proportion of the leachate P from all soiltypes occurred in >0.45 forms, especially from thesandy-textured soils, where TP (>0.45) accounted foralmost half of the TP exported. Concentrations of TP(>0.45) of up to about 1000 u,g L"1 were recorded forthe sandy soil and up to 184 u,g L"1 for the clay loam(Table 2), despite TP (>0.45) representing only 21%of the total export from this soil (Table 4).

Chemical fractionation revealed that the majority ofthe leachate P from all soil types was present in reactiveforms (62-71%). The clay loam leached the greatestproportion of RP (71%) and concentrations of up to359 (Jig L"1 were recorded for the sandy soil (Table 2).However, UP forms constituted a substantial proportionof the TP from all soil types (29-38%) (Table 4). Thesandy soil leached the greatest proportion (38%) andhighest concentration of UP (up to 783 (jig L"1). How-ever, the silty clay soil leached the highest flow-weightedmean concentrations of both RP and UP.

Most of the RP occurred in the <0.45 size range

Table 3. Total drainage and export of total P, reactive P, and unreactive P in size fractions during the 1994-1995 drainage year.

Silty clayStandard errorClay loamStandard errorSandy loamStandard errorSandStandard error

Drainage

mm250

451

469

491

Total

40075

41354

24320

40845

Total P

<0.45 fun

26772

32347

13022

23129

>0.45 (Jim

133269033

11326

17735

Total

26364

28938

15726

24729

Reactive P

<0.45 urn

—— g P ha-' -23867

261439117

20529

>0.45 (Jim

264

286

6623425

Total

13630

12426867

16140

Unreactive

<0.45 (Jim

307

6219398

266

P

>0.45 urn

1072862304710

13536


Table 4. Proportions of total P, reactive P, and unreactive P in size fractions during the 1994-1995 drainage year

Soil type

SHty clayStandard errorClay loamStandard errorSandy loamStandard errorSandStandard error

Total

100

100

100

100

Total P

<0.45 urn

648

797

549

575

>0.45 fun

368

217

469

435

Total

647

715

636

627

Reactive P

<0.45 juno/

568

636

386

516

Unreactive P

>0.45 |xm

8272

268

112

Total

367

295

376

387

<0.45 n-m

81

166

17461

>0.45 jim

287

146

205

326

(38-63% of the TP) (Tables 3 and 4). Thus, RP (<0.45)dominated the P export from all soil types. The clayloam leached the greatest proportion of RP (<0.45),while the sandy loam leached the smallest proportion.Reactive P (>0.45) was mostly small and only accountedfor a significant proportion of the TP exported from thesandy loam (26%).

In contrast to RP, which occurred mostly in the <0.45fraction, UP was present mainly in the >0.45 fraction,accounting for up to one-third of the TP export. Indeed,UP (>0.45) dominated the TP (>0.45) fraction from allsoil types except the sandy loam (e.g., for the silty clay,as shown in Fig. 3). In particular, UP (>0.45) exportedfrom the silty clay and the sandy soil represented about75% of the TP (>0.45) export. Concentrations of UP(<0.45) were generally small from all soil types (flow-weighted mean concentrations between 5 and 14 u,g L"1)and only accounted for 6 to 17% of the TP exported,although this represented more than 50% of the UPexport from the clay loam (Table 4). However, a cleartemporal trend in UP (<0.45) was apparent, with maxi-mum concentrations of up to 71 |xg L"1 recorded fromall soil types in the springtime (April-May) (Fig. 4).Therefore, UP (<0.45) appears to have a pronouncedseasonal cyclicity.

DISCUSSIONConcentrations and Loads of Phosphorus

The transfer of P through subsurface pathways hastraditionally been perceived as being of minor impor-tance, because of the large capacity for P fixation in theusually P-deficient subsoil (e.g., Burwell et al., 1977).This is true in agronomic terms, with P export represent-ing <1% of the applied fertilizer and a minute fractionof the total soil P. However, from an environmentalperspective, P concentrations in subsurface drainage wa-ter are high enough to contribute to water quality prob-lems. Lake quality models predict eutrophication at TPconcentrations between 35 and 100 .g TP L"1 (Vollen-weider and Krekes, 1982). In leachate from the fourcontrasting soil types used in this study, mean concentra-tions of RP (<0.45) alone were either within or ex-ceeded this range and maximum concentrations up to>1000 jxg L"1 were recorded from certain soils. There-fore, it is clear that subsurface P transfer occurs at con-centrations that could contribute to eutrophication.

Concentrations of RP (<0.45) did not fluctuate

greatly about the mean value. This appears to indicatethat sorption processes between the solid soil and theleachate water were acting to maintain an equilibriumP concentration in the soil water. The exception to thiswas during the late spring period (April-May), whenmaximum P concentrations of >200 (jug L"1 were de-tected, which were similar across all soil types. The rea-sons for the peaks in RP (<0.45) concentrations at thesetimes are uncertain, but may represent a range of pro-cesses, including preferential flow, incidental losses ofsurface-applied fertilizer, or biological P release throughwetting and drying cycles. For example, preferentialflow during the spring would result in the rapid move-ment of high P water from the surface down through thesoil profile and, therefore, a breakdown of the chemicalequilibrium processes operating during the winter drain-age period. In addition, application of fertilizer to thesoil surface during the springtime may increase the po-tential source of P available to drainage water. A combi-nation of these two factors may have resulted in thehigh P concentrations determined in the spring period.

The sandy loam had the smallest P export. This maybe related to the presence of a chalk layer at depth, ascalcium in the soil is known to strongly fix inorganicP by precipitation (Frossard et al., 1995). Calcium insolution appeared to cause the precipitation of inorganicP in the drainage outlets and collection vessels, whichwas observed as a white insoluble precipitate. Althoughthis was not analyzed, it is likely that these were carbon-ates, given that the leachate had passed through calciumcarbonate in the soil profile. This reduction of inorganicP in solution may have resulted in an overestimation ofthe importance of other P forms in drainage water fromthe sandy loam soil and must be considered when inter-preting the data from this soil.

In addition, problems were encountered with the siltyclay soil. A clay layer at 50-cm depth caused saturationof the topsoil under high rainfall, impeded verticaldrainage and caused overspilling of the lysimeters: thiswould not happen in a true field situation. This resultedin mean drainage from the four replicate lysimeters tobe about 50% of that from the other three soil types.Therefore, the data obtained from this soil are of argu-able validity.

Despite wide variations in the extractable-P contentsof the remaining soil types, there were remarkably smalldifferences in the TP exported in leachate (about 0.4 kgha~1 yr"1) (Table 3). These differences were not statisti-


200 ^• Total P (>0.45)

3 Reactive P (>0.45)

Q Unreactive P (>0.45)

0Oct-94 Nov-94 Dec-94 Jan-95 Feb-95 Mar-95 Apr-95 May-95

Fig. 3. Total P, reactive P and unreactive P, in the >0.45-n,m size fraction, for the silty clay during the 1994-1995 drainage year.

cally significant, because of the large variations in Pconcentrations within the four replicates of each soiltype. Mean annual export of RP (<0.45) was about0.3 kg ha"1 from all soil types. Similar exports have beenreported by Turtola and Jaakkola (1995) for heavy claysoils under barley and grass (0.32 and 0.34 kg ha"1 yr"1,respectively), although much lower losses were reportedby Burwell et al. (1977) and Sharpley and Syers (1979).Garwood and Tyson (1973) recorded P export of 0.3 to0.4 kg ha~! yr"1 from lysimeters containing the samesandy loam over chalk soil used in this work. In addition,they reported P concentrations ranging between 0.1 and0.7 mg L"1, but stated that these losses were insignifi-cant, reflecting the traditional agronomic view that Pleaching was unimportant in context with the agro-nomic system.

There were no relationships between total P, Olsen-extractable P, water-extractable P, or CaCl2-extractableP in the soil and P fractions in drainage water. Indeed,lysimeter soils that might be classed as P deficient onthe basis of simple soil P tests, actually leached moreP than soils with much greater P contents. An initialinterpretation might suggest that this is in contrast tothe findings of Heckrath et al. (1995), who discoveredthat for arable soils with a history of fertilizer applica-tion, the Olsen-P status was related to the inorganic Pconcentration of drainage water, with an acceleratedchange point at 60 mg Olsen-P kg"1. However, thereare two important differences between the study ofHeckrath et al. (1995) and the current study: (i) Heck-rath et al. compared P contents in one soil type whereasour range of Olsen-P values are represented by differentsoils; and (ii) the current systems are grasslands whichdo not represent the high range of Olsen-P concentra-tions that are common in arable soils.

Therefore in grassland soils, other factors may be pro-portionately more important in determining leachateP concentrations. In particular, the hydrology of thesoils may exert a strong control on P transfer throughsubsurface pathways. At a simple level, the amount ofrainfall will determine the P export from a soil of agiven P status. This was demonstrated by Baker et al.

(1975), who found that P export varied considerablyfrom year to year depending on the rainfall and, there-fore, the amount of runoff. In addition, the responseof the soil to rainfall can determine the P transferred,especially through the extent of preferential flowthrough the soil (Simard et al., 1998). An alternativeexplanation of the lack of correlation with soil P poolsmay be the controlling influence of soil pH. The clay-textured soils (pH 5.7 and 6.5) leached greater concen-trations of P than the sandy-textured soils (pH 7.0 and7.3), suggesting that P leaching may be restricted athigher pH levels. This may be linked to the relationshipwith calcium discussed previously.

Forms of PhosphorusPhosphorus export from all soils was dominated by

reactive P in the <0.45 u.m size fraction, as found inother studies (Sharpley and Syers, 1979; Heckrath etal., 1995; Chapman et al., 1997). This fraction is oftenassumed to represent true dissolved orthophosphate,although there is a wealth of evidence that this is notnecessarily the case. Firstly, the acidic nature of the Mo-blue color reaction for the determination of RP canresult in the hydrolysis of labile sugar phosphates, al-though more recent evidence suggests that this is negligi-ble if samples are rapidly analyzed (Denison et al., 1998).Secondly, it is now clear that the <0.45 jjim fractiondoes not represent dissolved P, but actually containsa continuum of particle sizes containing P bound toinorganic and organic colloids, which contribute to RP(e.g., Haygarth et al., 1997; Sinaj et al., 1998). This hasimplications for the transport of P through the soil,because many UP and colloidal P forms are less stronglysorbed in the soil than inorganic P, which is readily fixedand prevented from leaching by precipitation with Ca,Fe, and Al, or sorption to clays and other soil particles(Frossard et al., 1995). Therefore, these forms can moveeasily through the soil and escape to waters (Rolstonet al., 1975; Frossard et al., 1989; Chardon et al., 1997).

A substantial proportion of the P export from all soiltypes was in >0.45 |j,m forms. The potential for P


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Fig. 4. Unreactive P (UP [<0.45 |jim]) for all soil types during the 1994-1995 drainage year.

(>0.45) transport through macropores was demon-strated by Simard et al. (1998), while Dils and Heath-waite (1996) detected P concentrations in macroporeflow through agricultural grassland soils of >1 mg I/"1,which was dominated by TP (>0.45). In addition, sub-stantial subsurface concentrations of TP (>0.45) havebeen recorded indirectly in drain outflow (Turtola andJaakkola, 1995; Grant et al., 1996; Haygarth et al., 1998).These were found to contribute up to about 70% of theTP exported in artificial drainage from loamy arablesoils (Grant et al., 1996) and up to 0.2 kg ha~' yr"1 infrom grassland plots (Turtola and Jaakkola, 1995). Thelarge amounts of TP (>0.45) present in subsurfacedrainage water indicate that erosion mechanisms withinmacropores may contribute to P transfer. The likelymechanism of release is simple physical detachment ofparticles from the walls of macropores and other prefer-ential drainage pathways. The importance of TP (>0.45)transport confirms that process terms such as P leaching(which is synonymous with all subsurface P transport,but actually means the release and movement of exclu-sively dissolved P), can be very misleading (Haygarthand Sharpley, 2000).

The extent to which preferential flow through thesubsoil is likely to determine the impact of subsurfaceP transfer on catchment water quality is open to ques-tion, but is likely to depend on the degree of hydrologicalconnectivity with the receiving surface drainage chan-nels. Evidence is accumulating that excellent connectiv-ity exists between subsoils and receiving streams, espe-cially for undisturbed, permanent grassland, throughwell-defined macropore and soil pipe systems. For ex-ample, Goulding and Webster (1992) noted the impor-tance of preferential flow through soil pipes in experi-mental field soils, while Nieber and Warner (1991)stated that in many instances, soil pipes contribute mostof the total subsurface stormflow from an idealized hill-slope. Preferential flow paths may be responsible forthe majority of the subsurface P transfer. For example,Jensen et al. (1998) discovered that orthophosphatetransfer only occurred through wide-aperture mac-ropores in structured soils, despite water flow not beingrestricted to the same macropores. However, this may

be due to inorganic P adsorption to the pore walls, andmight not necessarily be the case for particulate andunreactive (non-orthophosphate) P forms that are lesssusceptible to sorption. The large amounts of these non-orthophosphate forms determined in subsurface drain-age in this study indicates that much of the P is notrestricted to large diameter flow pathways and has thepotential to move through a range of water pathways.

A large proportion of the TP export was unreactiveand, therefore, likely to be mostly in an organic form.Soil scientists have traditionally neglected organic P asan algal available P source, because orthophosphate wasconsidered to be the main bioavailable form. However,UP forms are potentially available to algae, especiallyin the <0.45-(jun size range, after hydrolysis and releaseof inorganic P by the action of phosphatase enzymes(Jansson et al., 1988; Turner and Haygarth, 2000). Infor-mation on the transfer of UP forms is limited, becausethey are very difficult to separate and detect, without theavailability of time consuming and expensive analyticaltechniques (Espinosa et al., 1999). However, organicP forms vary considerably in their bioavailability andbehavior in the soil. For example, nucleic acids andnucleotides appear to be rapidly broken down, whereasinositol-bound P forms are much more recalcitrant andunavailable for biological uptake (Turner and Haygarth,2000). As a result of the variable bioavailability of or-ganic P, it is important to have some knowledge aboutthe specific forms present in drainage waters, in orderto gauge the impact of P transfer on eutrophication.Although it is currently difficult to quantify the bioavail-ability of organic P to algae, new techniques using en-zymes to characterize specific organic P forms appearto offer the best prospect of this becoming routine(Turner and Haygarth, 2000). Until this is possible, theentire organic P fraction must be considered when as-sessing the impact of P transfer from the land.

Unreactive P (>0.45) accounted for the majority ofthe TP (>0.45) export. Information on the specific formsof P constituting the UP (>0.45) fraction is scarce, butprobably represents P held within soil particles, organicP bound to soil particles, and bacterial cell debris (Han-napel et al., 1964). For example, Heathwaite et al. (1990)


stated that about 80% of the TP export from a smallmixed agricultural catchment in southwest England wasin the form of organic P bound to soil particles. Stevensand Stewart (1982) found that alkali-soluble TP (>0.45)isolated from river water draining into Lough Neagh,Northern Ireland, resembled humic acid, with P associ-ated with iron and organic matter. The P may have beenan integral part of the humic acid structure, or boundthrough P-iron-humic complexes.

Unreactive P in the <0.45-(JLm size fraction contrib-uted significant amounts of P on a seasonal basis acrossall soil types. Unreactive P (<0.45) comprises a range oforganic P forms, including inositol phosphates, nucleicacids, nucleotides, phospholipids and sugar phosphates(Turner and Haygarth, 2000), although condensed inor-ganic P forms, such as pyrophosphates may also be in-cluded (Ron Vaz et al., 1993). Maximum concentrationsof up to about 70 |xg Lr1 in the springtime (April-May)(Fig. 4), which mirrored a similar trend for RP (<0.45),were detected over several drainage seasons (Fig. 1).The release of UP (<0.45) during the springtime maybe a result of biological processes in the soil. Elevatedtemperatures and an increase in the availability of car-bon substrates in the soil results in a maximum soilmicrobial biomass size and activity during the spring-time (Patra et al., 1990; Lovell et al., 1994). The resultingrapid turnover of microbial biomass through the solu-tion pool may result in the availability of organic Pto transfer in drainage water. In addition, wetting anddrying cycles during this time may result in a flush ofmicrobial intracellular P through the lysis of desiccatedcells by rewetting (Salema et al., 1982; Kieft et al., 1987).The turnover of microbial biomass P has been estimatedto be in the region of 25 kg ha"1 yr^1 and is certainlyresponsible for the release of large amounts of P to thesoil (Brookes et al., 1984; Sarathchandra et al., 1989).The biomass released P will be in the form of mobile andlabile nucleic acids, phospholipids and sugar phosphates(Bieleski, 1973). As a result, it has a high potential toescape to surface waters if drainage is occurring, whereit will provide a bioavailable P source. However, theseprocesses are speculative and require further investi-gation.

CONCLUSIONSWe determined P forms and concentrations in water

draining under field conditions from monolith lysime-ters containing four grassland soil types. Although theexport of P represented a small loss in agronomic terms(<0.5 kg ha"1 yr"1), P concentrations in drainage wateroccurred at levels that could contribute to eutrophica-tion. There were statistically significant differences inflow-weighted mean leachate P concentrations amongsoil types. However, differences in P export among soiltypes were remarkably small and not statistically signifi-cant. In addition, P concentrations in leachate did notrelate to the soil P pools, suggesting that other factors,such as the hydrology, or soil calcium, were more impor-tant in controlling P transfer than the soil P status. Reac-tive P in the <0.45-u.m size fraction was the dominant

form of P in drainage water, although unreactive Pforms, mainly in the >0.45-|xm size fraction, representeda considerable proportion of the total P export. How-ever, concentrations of unreactive P in the <0.45-jji,msize fraction increased in the springtime from all soiltypes, indicating the release of P from the soil microbialbiomass. The large proportion of P in the >0.45 u,mfraction indicated the importance of subsurface erosionmechanisms and preferential flow in subsurface P trans-fer. The mechanisms responsible for the release of thevarious P fractions require urgent investigation, if weare to advance our understanding of P transfer fromsoils to surface waters.

ACKNOWLEDGMENTSBen Turner receives funding from the Institute of Grassland

and Environmental Research (IGER) and the Natural Envi-ronment Research Council (NERC). IGER receives grantaid from the Biotechnology and Biological Sciences ResearchCouncil (BBSRC). The authors are grateful to Liz Dixonand Elizabeth Williams for laboratory assistance with samplecollection and analysis and Professors Steve Jarvis and RogerWilkins for comments on the manuscript.


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Documents

Phosphorus Forms and Concentrations in Leachate under Four Grassland Soil Types