indicators of phosphorus status in soils: significance and ... · indicators of phosphorus status...

BASE Biotechnol. Agron. Soc. Environ.201620(S1),257-272

Indicatorsofphosphorusstatusinsoils:significanceandrelevanceforcropsoilsinsouthernBelgium.AreviewMalorieRenneson,SophieBarbieux,GillesColinetUniversityofLiège-GemblouxAgro-BioTech.TERRA.AgricultureIsLife.PassagedesDéportés,2.BE-5030Gembloux(Belgium).UniversityofLiège-GemblouxAgro-BioTech.BiosystemsEngineering.Water-Soil-PlantExchanges.PassagedesDéportés,2.BE-5030Gembloux(Belgium).E-mail:[email protected]

ReceivedonApril3,2015;acceptedonFebruary19,2016.

Introduction.Phosphorus (P) isanessentialelement forplantgrowth.Therefore, it isessential toaccuratelyevaluate itscontentinthesoil.ThisrequiresreliableindicatorsofsoilPstatus.Literature.ThispaperreviewsliteratureregardingtheindicatorsofPstatusinsoils.Manyindicatorscanbefound,includingsingle extractions (soluble, available, or total P), which are the most common indicators used worldwide. Over time,increasinglycomplexPindicatorshavebeendevelopedassequentialextractionswhichcharacterizethevariousformsofP,degreeofPsaturation,diffusivegradientsinthinfilms,biologicalextractions,isotopicmethods,ormorecomplexmodels.Tomakeachoiceamongthem,differentcriteriashouldbeapplied,includingrelevance,costandtime,easeofinterpretation,and,mostimportantly,theobjectiveoftheanalysis.Itisalsonecessarytoanalyzetheappropriatenesstosoilandclimate.Firstly,thispaperdescribesthevarioustypesofindicatorspresentintheliterature,andproposesaclassificationsystem.Secondly,allcitedindicatorsareevaluatedandcompared.Finally,thePindicatorsmetinWallonia,southernBelgiumarediscussed.Conclusions. Each P indicator presents advantages and disadvantages. This review highlights the importance of carefulconsiderationofindicatorchoice,andtheestablishmentofinterpretationthresholds.Keywords. Soil,phosphorus,indicators,fertility,soilanalysis,Belgium.

Indicateurs de l’état du phosphore : signification et pertinence dans les sols agricoles en Wallonie (synthèse bibliographique)Introduction.Lephosphore(P)estunélémentessentielpour lacroissancedesplantesdont ilestnécessairedeconnaitreprécisémentlateneurdanslesol.Celapasseinévitablementparl’utilisationd’indicateursadéquats.Littérature.CetarticleprésenteunerevuebibliographiquedesindicateursduPcontenudanslessols.Dansla littérature,il existe unemultituded’indicateurs de l’état duPdans le sol.On retrouvenotamment différentes extractions chimiques(Psoluble,disponibleoutotal)quisontlesindicateurslesplusutilisésdanslemonde.Aufildutemps,desméthodespluscomplexesontétédéveloppéestelleslesextractionsséquentiellesquicaractérisentlesdifférentesformesduPdanslesol,letauxdesaturationenP,lesrésineséchangeusesd’anions,lesextractionsbiologiques,lesméthodesisotopiquesoucertainsmodèlespluscomplexes.Pourfaireunchoixparmicesdifférentsindicateurs,différentscritèresdoiventêtreprisencomptedontlapertinence,lecoutetletempsd’analyse,lafacilitéd’interprétationmaissurtoutl’objectifdel’indicateur.Ilestégalementnécessaired’évaluerl’adéquationaveclesoletleclimat.Premièrement,lesdifférentstypesd’indicateursduPretrouvésdanslalittératureontétédécritsetuneclassificationaétéproposée.Ensuite,lesdifférentsindicateursontétéévaluésetcomparésentreeux.Enfin,lasituationenRégionwallonneaétédiscutée.Conclusions.TouslesindicateursduPprésententdesavantagesmaiségalementdesinconvénients.Cetterevuebibliographiquesoulignel’importancedebienréfléchirauchoixdesindicateursetdedisposerdeseuilsd’interprétationcorrespondant.Mots-clés.Sol,phosphore,indicateurs,fertilité,analysedesol,Belgique.

1. INTRODUCTION

Phosphorus(P)isanessentialnutrientforplantgrowth,and is therefore a critical part of the fertilizationrequirementsforcropproduction.However,excessive

bioavailablePinputscanleadtotheeutrophicationofsurfacewaters, which represents amajor concern intheworld.Increasingenvironmentalissuesandriseinfertilizerpriceshaveledtothereconsiderationofcertainagriculturalpractices.AccordingtotheNationalUnion

258 Biotechnol. Agron. Soc. Environ. 201620(S1),257-272 RennesonM.,BarbieuxS.&ColinetG.

of French Fertilizer Industries (UNIFA; www.unifa.fr), the consumption of phosphate mineral fertilizerhasdecreasedbynearly80%over thepast 30years.ThisdeclinethreatenstheavailabilityofPinsoilsoverthelongterm,andincreasestheriskofPdeficiencyinthemostsensitivesoils.Thus,theidentificationofsoildeficiencyrisk,concomitantwiththeminimizationofenvironmentallossesbyerosionandrun-off,requiresthe development of relevant indicators ofP status insoils.

Insoil,totalPcanbeseparatedintodifferentpoolsoforganicandinorganicfractions.InorganicPincludesprimary P minerals (apatite, strengite, variscite);secondaryPminerals(Ca,Mg,FeorAlphosphates);Padsorbedontotheedgesofclayminerals;Pboundtoorganicmatterthroughmetalliccations;anddissolvedP(H2PO4

-,HPO42-,PO4

3-).DissolvedPrepresentsmorethan 96% of the P taken up by plants according toBecketal.(1994)andcanbeconsideredasthePformdirectly available to plants, the quantity dependingon the time of ions exchange (Fardeau, 1993). Themobility and bioavailability are controlled by thelow levels of dissolved P in soil solution, which ismainly governed by high rates of adsorption and/orprecipitation of dissolved P with positively chargedsoilcompounds,includingmetalcations(Ca,Mg,Fe,Al)andFe-,Al-oxyhydroxide(Hinsinger,2001).So,Pionsconcentrationinsoilsolutionisthereforelargelycontrolledbycationicactivity insoil solution,whichis influenced by soil pH and environmental factorsas redox potential andmineral solubility (Pierzynskietal.,2005).However,Pestimatedavailableforplantscan highly differ from P really taken off by cropsbecauseitdependsonplantspeciesandsoilconditions.

Organic fraction is defined as P bound with C(organic matter and biological compounds such asDNAandphospholipids)(Condronetal.,2005).Theproportion of this fraction depends on factors suchas land use and pedo-climatic conditions, and canvary from 25-30% to 75-80% of total P (Fardeauetal.,1994).SoilPflowsoccurbetweenorganicandinorganicpoolsviaimmobilizationandmineralizationprocesses mediated largely by soil microorganismactivity (Oberson et al., 1996). Soil conditions,includingsoilmoisture,temperature,pH,andsurfacechemical properties are integral factors promotingthesereactions.

Duetotheprofusionofexistingindicators,itcanbedifficulttoidentifythemostappropriateindicatorforagivensetofconditions.Themanagementofagivenenvironmentrequires indicatorswhichareadaptedtolocalconditions,indicatorswhichareeasytomeasure,andindicatorswithknownthresholdsandlimitations.ItisnotthepurposeofthisarticletoprovideanexhaustivelistofexistingmethodsthatcurrentlyservetoindicateP status in soil. Rather, the goal of this study is to

evaluatethemostcommontypesofmethodsinorderto highlight the advantages and limitations of each.ThisevaluationwasalsoappliedmorespecificallyinWallonia,aregionsituatedinsouthernBelgium,thoughresultscanbeextrapolatedtofacilitatebothagronomicandenvironmentalmanagementofPinotherregions,providedthatthespecificedaphicpropertiesofagivenregionareconsidered.

2. MAIN METHOD CLASSES FOR P CHARACTERIZATION IN SOIL

Manydiagnostictoolshavebeendevelopedtoevaluatesoilfertility.Accordingtotheliterature,theavailabletoolsarebasedonthefollowingvariables:–theobjective(agronomic,environmental,orboth);–the scale of the study (e.g., a cultivated field orwatershed),or

–themeasurementprinciple.

Theseclassificationsaredescribedinthefollowingsectionsandinfigure 1.

2.1. Chemical extractions

Various chemical methods have been developed toestimatethecapacityofasoiltoprovidethelevelsofPnecessaryforplantgrowth.Currently,severaldozenmethods exist, with varying degrees of complexity.Mostofthesemethodsweredevelopedunderspecificconditions(soilorcultivationsystem),andwerefixedwithrespecttointerpretationreferences.Consequently,it is difficult to apply a uniformmethodworldwide,despite regulatory efforts within Europe for theestablishmentofastandardizedsetofmethods(Proix,2013).Therefore,definingspecificthresholdsbecomesimportant.

Single extractions of P content. Main existingindicators estimate the levels of available (orexchangeable)Pbasedon“thesumofPimmediatelyavailabletoplantsandofPthatcanbeconvertedintoanavailableformthroughphysical(desorption),chemical(dissolution), or biological (enzymatic degradation)processes in nature during a growing season”(Boström et al., 1988). Some analytical methodsutilizeextractantstomimictheactionofrootsthroughdissolution,desorption,orchelationreactions.Of theseveraldozenchemicalextractionmethodsreportedinliterature,thesedifferdependingontheextractantused,theextractiontime,andtheratioofsoiltoextractant.Someextractionmethodsarealsospecifictoacountryor region, while other methods are more universal(e.g., themethodsofOlsen,Mehlich3, orBray; seetable 1). Within the same country, several methods

Indicatorsofphosphorusinsoils 259

canalsocoexist.ThisisparticularlyevidentinFrance,where levels of available P are commonly evaluatedaccordingtothemethodsofOlsen,Joret-Hébert,andDyer(Table 1),explainedpartlybydiversityofsoils.

Rather than providing an exhaustive survey ofthe existing methods, the following discussion willbe limited to the most commonly cited methods inthe literature (Table 1). Proix (2013) proposed thatextractionmethodscanbeclassifiedintofourtypes(I–

IV),dependingonwhichsoilbindingmodeisaffectedbytheextractionperformed(Table 2).

TypeI analysismethods that detect soluble P arenotcommonlyusedasagronomicalindicators.Instead,these are routinely used as environmental indicatorsin somecountries.To formulate the riskofP loss,Pis extracted with distilled water (Sissingh, 1971).This method is used in The Netherlands, Austria,and Switzerland, among others; it estimates the

Figure 1. Classificationofthedifferentindicatorsofphosphorusstatusinsoils—Classification des différents indicateurs du phosphore dans les sols.

Diagnostic tools for the status

of soil phosphate

CHARACTERIZATION MODELIZATION

Input data

IsotopesBiological extractions

Phosphorus index for risk of P loss (PI)Phosphorus export

diagnostic tool (ODEP)

Various modelsModelisation

of phosphorusdynamic transfer

to the soil solution

Predictivemodeling

based on the function of Freundlich

Kineticisotope

exchange

Compartmentalrepresentationof P availability

Sequentialextractions

HedleyChang & Jackson

etc.

Degree ofphosphorussaturation

Singleextraction ofphosphorus

content

Water extractionOlsen

AA-EDTAAcetate-Lactate

Joret-HébertBray

Mehlich 3, etc.

Oxalate Mehlich 3

Chemical extractions

Anion exchangeresins

Table 2. ClassificationofthechemicalextractionsproposedbyProix(2013)—Classification des méthodes d’extraction chimique proposée par Proix (2013).

State of P in the soil How it works Extraction methodsTypeI Soluble Dissolutionofsolubleelementsis

performedandthisisusedasthesoilsolution

Water,CaCl2

TypeII Linkedtoclay-humiccomplexes

Pismainlyreleasedfromclayandorganicmatter

Neutralsalts,bufferingeffect(Olsen,etc.)

TypeIII Adsorbedorprecipitatedonoxyhydroxides

Pisreleasedfromironandaluminiumoxyhydroxides

Neutralsalts,bufferingeffects,chelatingmolecules(AA-EDTA,acetate-lactate,etc.)

TypeIV Totalorpseudo-totalstock TotalPconcentrationisdeterminedinordertoperformapedologicalcharacterizationofagivensoil

Triacid,Aquaregia

260 Biotechnol. Agron. Soc. Environ. 201620(S1),257-272 RennesonM.,BarbieuxS.&ColinetG.Ta

ble 1

.Characteristicsofsingleextractionsofphosphoruscontentmostcom

monlycitedintheliterature(com

pletedfrom

Jordan-M

eilleetal.,2012)—

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aceticacid+0.025MEDTA

,pH4.65

1:10(w:v),

30minsh

aking

Belgium

(Wallonia),

Switzerland

Acidictoneutralsoils

ALakanenetal.,

1971

Acetatelactate

0.1Mammoniumlactate+0.4M

aceticacid,pH3.75

1:20(w

:v),

2hshaking

Belgium

(Flanders),

Sweden,N

orway

Allsoils

AEgneretal.,

1960

Bray1

0.03M

NH

4F+0.025M

HCl,pH

3.75

1:10(w

:v),

5minsh

aking

USA

,Spain,Italy,

Greece,India

Acidicsoilsrichiniron,not

inneutraltocalcareousso

ils,

notinclayeyso

ilswithhigh

degreeofexchangeablebases

ABrayetal.,

1945

CaCl 2Soluble

0.01MCaCl 2

1:10(w

:v),

2hshaking

TheNetherlands

A,E

Houbaetal.,

2000

CAL

0.05M

lactatedeCa+0.05M

calcium

acetate+0.3Maceticacid,pH4.1

1:20(w

:v),

2hshaking

Germany

Allsoils

ASchüller,1969

Dyer

Citricacid2%

,pH2

1:5(w:v),

2hshaking

France,B

elgium

(forest

soils)


ADyer,1894

NF31-160,

1999

Joret-H

ébert

0.2Moxalated’NH

4,pH

71:20(w:v),

2hshaking

France

Neutraltocalcareousso

ilsA

Joretetal.,

1955

NFX31-161,

1999

Mehlich1

0.0125M

H2SO

4+0.05MHCl

1:4(w:v),

5minsh

aking

USA


AMehlich,1953

Mehlich3

0.015MNH

4F+0.2M

CH

3COOH+

0.25M

NH

4NO

3+0.013M

HNO

3+

0.001MEDTA

1:10(w

:v),

5minsh

aking

Canada,USA

Allsoils

AMehlich,1984

Morgan

0.72M

sodium

acetate+0.52Macetic

acid,pH4.8

1:5(v:v),

15minsh

aking

USA

AMorgan,1941

Morgan

modified

0.62M

NH

4OH+1.25MCH

3COOH

1:5(v:v),

15minsh

aking

Ireland,USA

AcidicsoilswithaCEC

below

20cmol

+ . kg-

1A

McIntosh,1969

Olsen

0.5MNaH

CO

3,pH

8.5

1:20(w

:v),

30minsh

aking,

at20°C

France,England,U

SA,

Spain,Tunisia,South

Africa,Greece,India,

Iran,etc.

Calcareoussoilsextendedto

allsoilsexceptedhighlyacidic

soils

AOlsenetal.,

1954

ISO

11263:1994

./..


amount of P present in the soil solution, similar tothe extraction of Pwith 0.01MCaCl2. The latter isgenerallycharacterizedbyvalueswhicharetypically2to3timessmallerthantheformer.Generally,calciumisthefirstcationtoformcomplexesinsoil,andthus,0.01M CaCl2 typically represents the average ionicstrengthofasoilsolution(Houbaetal.,2000).LevelsofsolublePmayalsovaryaccordingtoseason,withlevelsof extracted solublePbeinghigherduring thewetterperiodsthanthedryerperiods,whichisduetotheextentofmineralisation.

TypeII and III indicators correspond with thelevels of available (or exchangeable) P. These areagricultural indicators which are commonly usedworldwide.Themethodsvary,andcanbecustomizedfordifferentactionprocesses.Forexample,theOlsenmethod extracts P by ligand exchange, whereas theBraymethod extracts P by forming complexes withcalciumoraluminiumphosphates(Honsetal.,1990).Therefore,somemethodsaremoresuitableforacidicsoils, while others are more suitable for calcareoussoils(Table 1).

The properties of a soil can influence the resultsobtained from chemical extraction methods, inparticular, clay, organic matter, or pH (Tran et al.,1985). Moreover, the presence of carbonates caninfluencetheextractioncapacityoftheBraymethod,andthepresenceofclaycandisruptthefiltrationphaseof awater extraction.Extractionmethodswhich useammoniumacetateandethylenediaminetetraaceticacid(EDTA)arealsoaffectedbypH(Lakanenetal.,1971).FewcorrelationswithyieldhavebeenobservedfortheextractionofPviathelattermethodincalcareoussoils,sincethemechanismsofPextractionarenotsuitableinthesesoils.Incontrast,thesedifferentpropertieshaveonlyanegligibleinfluenceontheuptakeofPbyplants(Beaudin,2006).

Although it is difficult to compare the variousmethodsofPextraction,manyauthorshaveattemptedto do so (Homsy, 1992; Pote et al., 1999; Hoodaet al., 2000; Maguire et al., 2002; Neyroud et al.,2003). Consequently, some authors have proposed aclassification of extractants based on the amount ofPthatisextracted(Honsetal.,1990;Neyroudetal.,2003).However,theseclassificationsarenotuniversalforalltypesofsoil.Thus,itisessentialtoaccountforthespecificsofananalyticalmethodpriortoselectingitasanindicator.Inaddition,itisoccasionallydifficulttochooseoneuniquemethodduetodifferencesintheedaphicpropertiesofacountryorregion.Amorerobustmethodcanpotentiallybeappliedtoawiderangeofsoils, such asMehlich3 in Quebec (Beaudin, 2006)or theOlsenmethod. Interestingly, the lattermethodwasdevelopedincalcareoussoils,whiletheresultsofthismethodology are also satisfactory in acidic soils(Moreletal.,2000).Ta

ble

1 (c

ontin

ued)

.Characteristicsofsingleextractionsofphosphoruscontentm

ostcommonlycitedintheliterature(completedfromJordan-Meilleetal.,

2012)—

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acté

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ique

s des

pri

ncip

ales

mét

hode

s chi

miq

ues d

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ion

du p

hosp

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renc

ontr

ées d

ans l

a lit

téra

ture

(com

plét

é de

Jor

dan-

Mei

lle e

t al.,

201

2).

Met

hod

Che

mic

al e

xtra

ctan

tsEx

trac

tion

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and

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o ex

trac

tant

*

Cou

ntri

esSo

il ty

pes

Indi

cato

r ty

pes

Ref

eren

ces

Triacid

HCl+HF+HClO

4Hotextraction

Allsoils

TNFX31-147,

1996

Truog

0.002MH

2SO

4+(N

H4)2SO

4,pH

3.0

1:200(w:v),

30minsh

aking

Neutraltocalcareousso

ilsA

Truog,1930

WaterSoluble

H2O

1:60(w

:v),

22hincubation

and1hshaking

TheNetherlands,

Germany

Allsoils

A,E

Sissingh,1971

1:10(w

:v),

5minsh

aking

Allsoils

Pansuetal.,

2003

*w:v:w

eight/volum

eratio—

rapp

ort p

oids

/vol

ume;v:v:volum

e/volumeratio—

rapp

ort v

olum

e/vo

lum

e;indicatortypes—

type

s d’in

dica

teur

s:A:agronom

ic—

agr

onom

ique,E:

environm

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env

ironn

emen

tal,T:total—

tota

l.


Finally, total P content (typeIV) is occasionallyused as an indicator of theP reserve present in soil.VariousacidmethodsexistandconsistofusingacidssuchasHF,HClO4,HNO3,HCl,orH2SO4separatelyorincombination,withvariableefficiency.ThisindicatormaybeusefulinevaluatingthePcontentofsedimentlost by erosion, or in assessing the pedologicalcharacterization of a given soil. Recently, total PcontentwasusedtoestimatePreservesboundtoparentmaterialsinWallonia(Rennesonetal.,2013).

Sequential extractions. Complementary to theapproachofestimatingavailableP,someauthorshavedevelopedsequentialextractionmethodstoidentifyPpoolsofvaryingsolubilityinsoil.Differentextractionmethodsarecombinedsequentiallyinordertodepletea soil of its content from decreasingly availablefractions.ResidualPisdeterminedbythedifferenceofPformscomparedtototalP.

Manyprotocolsforsequentialextractionhavebeenproposed (Table 3), and two major types have beendistinguished.Onemajor type involves classificationon the basis of the type of bonding,while the otherinvolves classifications on the basis of the degreeof availability to plants. The most commonly usedprotocols were originally developed by Chang et al.(1957)andHedleyetal.(1982).TheoriginalpurposeoftheformerworkwastodistinguishdifferentformsofPdependingonthetypeofbondingtosoilminerals(e.g.,Pboundtoironvsaluminiumvscalcium,etc.).However, the selectivity of the extractant in thisprotocolwasoftenquestioned.Fractionationschemesareunabletoisolatediscretemineral,astheysolubilizegroupsofmineralsusuallydefinedasPassociatedwithAl,Fe,Ca,orresidualforms(Pierzynskietal.,2005).Nevertheless, thisprotocol ledto thedevelopmentofasecondtypeofprotocols.Inparticular,Hedleyetal.(1982)incorporatedtheconsiderationofPavailability

for plants, without specifying the forms of bondingwithin each fraction, thereby assuming the samechemicalformispresentindifferentfractions.

Manystudieshaveattemptedtocompareavarietyofextractionmethodsusingthesamesetofsoilsamples(Levyetal.,1999;Taoufiketal.,2004).However,theresultsaredifficult togeneralizedue to the influenceofsoiltypes.Williamsetal.(1967)demonstratedthatthemethod outlined byChang et al. (1957)was notapplicableforcalcareoussoilsorsediments.Accordingto Tiessen et al. (1993), the fractionation of Hedleyet al. (1982) is the only method that can be usedwithmoderatesuccessfortheevaluationofavailableorganicP.Inacidsoils,somepoolsarenotcompletelyseparated (bicarbonate and hydroxide-extractable Pi)and represent a continuum of Fe- andAl-associatedP extractable (Tiessen et al., 1993). The Hedleyfractionation is also used in tropical soils, followingprotocol modifications (use of resin P fraction andgenerallylessPfractions)(Negassaetal.,2009).

Overall,whilethesemethodsprovideanextensivecharacterization of the different forms of P presentin a soil, they are expensive and time consuming toimplement,therebypreventingtheirroutineuse.

Degree of P saturation.Giventheimportanceoftheenvironmental issues related to P, and the influenceof edaphic properties on the results of chemicalextractions,anenvironmentalindicatorwhichaccountsforthedistinguishingcharacteristicsofaparticularsoilwas developed. This indicator evaluates the degreeof soil P saturation by measuring the proportion ofpotential binding sites in soil which are actuallyoccupiedbyP,withmainbindingsites involving theoxide and hydroxide groups of iron and aluminium(vanderZeeetal.,1988).Thus,theindicatoraccountsforboth thebindingcapacityand thefixedPcontentofthesoil.Leinweberetal.(1999)havedemonstrated

Table 3. SequentialP extractionsmost commonlycited in the literature—Principales méthodes de fractionnement du phosphore retrouvées dans la littérature.Method Chemical extractants ReferenceChang&Jackson NH4Cl,NH4F,NaOH,H2SO4,Na2S2O4-citrate Changetal.,1957Williamsetal. NH4Cl,NH4F,NaOH,Na2S2O4-citrate,NaOH,HCl Williamsetal.,1967Hieltjes&Lijklema NH4Cl,NaOH,HCl Hieltjesetal.,1980Hedley Anionexchangeresins,NaHCO3,NaOH,dissolvedHCl,concentratedHCl,

H2SO4

Hedleyetal.,1982

Bozongoetal. HydrogenperoxideandmethodofChang&Jackson Bozongoetal.,1989Ruttenberg MgCl2,CDB,acetate,HCl,hotHCl Ruttenberg,1992Paludan&Jensen H2O,bicarbonate-dithionite,NaOH,HCl Paludanetal.,1995Golterman Distilledwater,Ca-EDTA/dithionite,Na-EDTA,H2SO4,H2SO4inautoclave Golterman,1995Rydin&Welch NH4Cl,Na2S2O4/NaHCO2,NaOH,HCl Rydinetal.,1998


thatthisparametercanbecalculatedaccordingtothefollowingequation:

Degree of P saturation = Poxα Alox +Feox( )

×100 (Eq .1)

where the degree of P saturation is expressed asa percentage, and Pox, Alox, and Feox represent theamounts of P, aluminium (Al), and iron (Fe) thatare extracted with ammonium oxalate (mmol.kg-1),respectively, while α represents a scaling factor(generally0.5).

Thismethodwasoriginallydevelopedintheacidicsandy soils ofTheNetherlands, and since has beenappliedtootherregionsandcountries(e.g.,Flanders[northern Belgium], Quebec, etc.). In Quebec, thisindicatorhasbeendefinedastheratiobetweenPandAl,asdeterminedbytheMehlich3extraction(Khiariet al., 2000). In 1999, Beauchemin et al. identifiedthevarious formulas thathadbeenused tocalculatethedegreeofPsaturationintheliterature.InastudyconductedinWallonia(Rennesonetal.,2015),itwasdemonstratedthattheequationhasfirsttobeadaptedto thepedologicalandgeologicalcontextof thesoilunderinvestigation.

Moreover, it was shown that the degree of Psaturation is correlatedwith the concentrations of Ppresentinrun-off(Poteetal.,1999)andindrainagewater(Leinweberetal.,1999),therebyindicatingthatit represents a relevant environmental indicator. InTheNetherlandsandinFlanders,thisindexhasbeenincorporated into legislation,whereby adegreeofPsaturationthatexceeds25%isdefinedasunacceptabledue to the riskofP transfer to the soil solution andwater(Breeuwsmaetal.,1995).

Agronomic and environmental thresholds. To berelevant as an agronomic indicator, the amount ofextractablePmustbecloselyrelatedtocropresponse,suchasplantgrowthoruptakeofP (Figure 2).Thecorrelationbetweensoilandplantscanbeconductedingreenhouse,agrowthchamber,oronthefield.Testcalibrationsmustbeconductedoverabroadrangeofsoils in order to define fertility classes according tosoil properties, such as texture or pHvalues (Genotetal.,2011;Jordan-Meilleetal.,2012).

Similarly,environmentalthresholdcanbedefinedifextractablePiscorrelatedwithPlostbyrun-offandleaching.Thethresholdtypicallycorrespondsto:–the content of P which is tolerated in water uponlegislation,or

–the“changepoint”ofthecurverelatingsoillosstosoilPcontent,whichisthepointofthecurvewheretheslopeincreases(Poteetal.,1999)(Figure 2).

Generally, environmental threshold is higher thanagronomicthreshold,providingacontrolleverforthemanagementofP.

2.2. Anion exchange resins and diffusive gradients in thin films (DGT)

Anionexchangeresinsweredevelopedforwaterandsedimentsamplesinthe1930stoassessthepresenceof labile P in soil samples or the presence of P insoil solutions.Upon contactwithwater and soil, theanionexchangeresinsactasasinkforP.Sincethen,analytical protocols designed tomimic the effects ofrootshaveevolved(Qianetal.,2002).

In the1990s, theuseofDGTwaspreferredoveranionexchangeresinsforestimatesofPavailabilityinsoilstoobviatethedisadvantagesofthelatter(Chardonetal.,1996).Thesedisadvantagesincluded:–modificationof thephysicochemicalbalanceof thesoilexamined;

–theabsenceofan infinitebindingcapacity, therebyresultinginthepotentialfornon-maximaldesorption;

–difficultyinseparatingtheresinfromthesoil;–lackofspecificityintheadsorptionanddesorptionofcertainanions;

–theinfluenceofsulphateornitrateconcentrationsonthequantitiesofPextracted.

Diffusivegradients inthinfilms(DGT)areapassivesampling technique which has been successfullyapplied to aquatic systems for measuring P (Zhanget al., 1998), and more recently, for predicting cropresponse to applied P in soil (Mason et al., 2010).Basedonthesameprincipleasanionexchangeresin,DGTiscomposedofalayerofferrihydritebindinggelwithastrongaffinityforPbehindadiffusivehydrogellayer and an overlying protective filter membrane(Six et al., 2012). Diffusive gradients in thin films

Figure 2. Representationof agronomicandenvironmentalthresholds determination — Représentation de la détermination des seuils agronomique et environnemental.

P lo

st b

y ru

noff

Cro

p y

ield

s

Low Medium High

Environmental classes

Agronomicthreshold

Environmentalthreshold


can be placed directly onto a saturated soil paste,allowingforfieldmeasurements.Diffusivegradientsin thin films provide a better estimate of P uptakeby plants compared to certain chemical extractionmethods(Zhangetal.,2014),andarelessvulnerableto potential chemical constraints, such as anionicinterferencesorpH(Masonetal.,2008).Moreover,alowcoefficientofdeterminationfortheregressionfitbetweenDGTandresinmeasurementwasobservedbyMasonetal.(2008).Despitetheresultsappearinghopeful for tropical soils, additional studies arenecessary to expand the types of plants, soil, andclimatic conditions that can be tested (especiallyin European soils), and to correctly interpret DGT-derivedresults(Zhangetal.,2014).

2.3. Biological extractions

To overcome the disadvantages of chemicalextractants,ithasbeenproposedthatplantscouldbeusedtoevaluatethebioavailabilityofP.

In bioassays, algae are grown inwater inwhichP is a limiting factor, for which the only source isthesoilsample.Unicellularalgaearegenerallyusedin these tests, includingSelesnatrum capricornutumor Scenedesmus quadricauda. The experiments aregenerally performed aerobically for 2 to 4weeks,and the algae are renewed weekly (Ekholm et al.,2003). Phosphorus availability is calculated onthe basis of the algae biomass present, and resultscan be extrapolated for longer periods of time todetermine the long-term availability of P. Severalstudies,includingthatofBoströmetal.(1988),haveshownthattheresultsobtainedusingsuchbioassaysare consistent with results obtained using chemicalextractions. However, bioassay results are onlyrelevant for the experimental conditions tested, andcannot be extrapolated to the natural environment.Furthermore, numerous species of algae exist innaturalenvironments,and theycanadapt tovariousformsofP.

Other biological methods have been used toestimatethecapacityofasoiltosupplyPtoaplant,including theuseofmicro-cultures (Stanford et al.,1957). In these cultures, plant growth is used toanalyze the amount of absorbedP.Unlike chemicalmethods,thistechniquemoreaccuratelyaccountsforallofthefactorsaffectingplantfood.

However,bioassaysaredifficulttoestablish,moretime-consumingtoperform,andtheexperimentsmustberepeatedtoachievetherepeatabilityandaccuracyofresults.Therefore,thesebioassaysdonotrepresenta substitute for chemical extraction methods, butshouldratherbeamethodperformedtocomplementchemicalextractionmethods.

2.4. Isotopic methods

TimeplaysasignificantroleindeterminingPavailability(Fardeau, 1993). Methods involving radioactiveisotopes of P were developed in France by Fardeau(1993),andmorerecentlybyMoreletal.(2000)andMoreletal.(2014).Isotopes,unlikechemicalreactions,enabletheobservationofsoilbehaviorwithregardtoPwithoutaffectingthebalancebetweentheformsofPpresentinsoil.Themainisotopesused,32Pand33P,haveahalf-lifeof14.3daysand23days,respectively.

Ingeneral,isotopicmethodsconsistofaninjectionwithadefinedamountofradioactivityintoasolution,followed by subsequent measurements of theradioactivityremaininginthesolutionafteradefinedperiodoftime.TheabilitytodetectPisbasedonthreeprinciples:–the concentration of phosphate ions in solution isconstantovertime;

–the isotopic tracer is instantly and uniformlydistributedinthesoilsolution;

–the flow of labelled P ions is equal to the flow ofunlabelledPions(Némery,2003).

The kinetics of the isotopic exchange methodalso enables the development of a compartmentalrepresentation of soil P reserves, as shown infigure 3 (Fardeau,1993).Thisdiagramillustrates theheterogeneityofphosphateionsthathavethepotentialto reach the soil solution over variable periods oftime.Moreover,thisrepresentationcanalsobelinkedto chemicalmethodsused todetermine the availableand total reservesofP (Figure 3).Theshapesof theextracted P vary depending on the extractant used(Figure 3).

Additional studies have demonstrated a range ofpotential applications for P isotopes. For example,in 2000, Morel used isotopic methods to model thedynamics ofP ion transfer between soil and the soilsolutionasafunctionofthedurationofPtransferandtheconcentrationofPinsolution.Morerecently,Moreletal. (2014)determined the relationshipbetween thephosphatebalancesheetandPexchangesusinglong-term test parcel data. Finally, P isotopes have beenusedinthestudyofplantgrowthandtomeasuretheefficiency of plant growth following the addition ofisotope-labelledfertilizers(Frossardetal.,1996).

However, there are disadvantages associatedwith the use of isotopes. In addition to the lack ofinformation regarding the organic fraction of P, thismethodcannotberoutinelyused,andthemanipulationof radioelements is extremely delicate. Furthermore,it is difficult togeneralize the results obtainedunderexperimentalconditionstoeventsoccurringnaturallyin soils. To address the latter point, scientists haveattemptedtorelatetheparametersofkineticequations


to the physicochemical characteristics of soil (Moreletal.,2014).

2.5. Complex models

P indices for risk of P loss. Knowledge of a soil’sstatus is not sufficient for estimating the risk of Pexporttosurfaceorsubsurfacewatersincethelossesthatoccurareinfluencedbybothsourceandtransportfactors.IndicesforriskofPlossrepresentmanagementtoolsatthescaleofaparcelwhichareusedtoidentifycritical source areas of P loss and farming practicesthat increase the risk.Critical source areas of P losscanbedefinedby theircoincidingsource (soil,crop,andmanagement inducinghighP loss) and transport(runoff, erosion, and proximity to water course orbody)factors(Sharpleyetal.,2014).ThisindicewasdevelopedtosolveproblemsoflocalizedexcessofP.

Source factors represent theamountofP thatcanpotentially be mobilized, as well as the conditionspredisposingit toaccumulation(e.g.,soil testP,rate,method,timingofapplicationofbiologicalormineralfertilizers,andleachingofPfromplantresidues),whiletransportfactorsareessentiallyhydrologicalinnatureand affect the transport of P to rivers (e.g., erosion,surface run-off, subsurface drainage, connectivity).Phosphorus indices (PI) are simple models derivedfrom the results of more complex experiments ormodels(Buczkoetal.,2007).

AnumberofP indices,allofwhicharebasedontheoriginalPIofLemunyonetal.(1993)(Equation2),have been developed according to the regionalcharacteristicsofagivenstateand/orcountry.SeveralP indices use the Pennsylvania PI. Currently, the PIapproach is routinely used in 47U.S. states, someCanadian provinces, and was adopted by severalEuropeancountries,includingFinland(2001),Ireland

(2003), Sweden (2005), Norway (2005), Denmark(2006),andGermany(currentlyinprogress).

Site vulnerability = 1.5*soil erosion + 1.5*irrigationerosion + 0.5*runoff class + 1*soil p test +0.75*P fertilizer application rate + 0.5*P fertilizerapplicationmethod+1*organicP source applicationrate+1*organicPsourceapplicationmethod(Eq.2)

wherethevaluesforeachcharacteristicarespecifiedinLemunyonetal.(1993),accordingtolevel.

TheprimaryadvantageofusingaPIisinitsspeedand ease of use. However, some parameters, such asmode of farm management, are difficult to map. Inaddition, a PI can accommodate correctivemeasures.Therefore,aPIisnotonlyanindicatorthatcanprovideanintegrativeapproach,butitcanalsobeeasilyadaptedtolocalconditions(Buczkoetal.,2007).Correspondingly,Pindiceshavebecomehighlypopulartoolsfrombothscientificandpoliticalstandpoints.Phosphorusindicesrequirereadilyavailabledata,andtheycanbeautomatedusingcomputersoftwarewhichlinksthemtoadatabaseofinterest.

Modelling of P loss.Hydrologicalmodelsofincreasingcomplexityhavebeengeneratedtoquantifyvolumesofrun-offwater,aswellassedimentlossandPloss,whilealsoaccountingforlocalspecificities(e.g.,connectivityofparcelswithariverandtopography,amongothers).Withdifferentmodelsavailable,itmaybecomedifficulttoselectthemostappropriatemodel.Itisimportanttoconsiderthescaleofstudyforeachofthemodelsbeingconsidered,aswellastheirdisadvantages.Hydrologicalmodels are very time-consuming and difficult toimplement. In particular, they require extensivecalibration,andare thereforenot typicallysuitableforroutineuse.Moreover,thesemodelscanoftenprovetobe

Figure 3.Compartmental representationof the available formsof phosphorus (P) extracted according to isotopicmethod(adaptedfromFardeau,1993)—Représentation compartimentale des formes du phosphore (P) disponible extraites selon la méthode isotopique (adapté de Fardeau, 1993).

Liquid phase Solid phase

Ions in a soil

solution = CP

Capacity = PL/CP

Instant ion exchange = PL

Ion exchange between 1 min and 1 day

Ion exchange between 1 day and 3 months

Ion exchange between 3 months and 1 year

Ion exchange over 1 year


inadequateduetogroundconditionswhicharedifficulttoforeseeandtransferprocesseswhicharenotalwayscomplete.ThesemodelsarebasedonthehypothesisthatPtransferisaresultofrunoffanderosion.However,inpractice,thesituationismorecomplex.

Nevertheless, some regions have developed thesemodelsandusethemtoevaluatetheexportofPfromagricultural parcels. In particular, Quebec employsa Phosphorus Export Diagnostic Tool (ODEP) thatisbasedon theSWATmodel, and is able to integratedataregardingthetopography,soiltype,drainage,andagronomicmanagementofaparcel.Consequently,thissoftwaretoolcanbeusedtoquantifyPlosses,toidentifythe factors responsible for the losses, and to simulatedifferent scenarios of P management. In Wallonia,Dautrebandeetal.(2006)usedtheEPICgridmodeltoestimatePexportfromparcels.

3. EVALUATION OF DIFFERENT INDICATORS

Many indicatorsofP levelsexistworldwide,and it isoftendifficult to select themostappropriate indicator.Moreover,many criteria exist to evaluate indicators,and eachof the existingmethodspresent advantagesanddisadvantages(Table 4).Forthisstudy,indicatorswere evaluated according to the framework of the“Sustainability Assessments of Farming and theEnvironment” (SAFE) hierarchical frameworkdeveloped by Van Cauwenbergh et al. (2007). Thisframework has defined the following six criteria forenvironmentalindicators:–abilitytodiscriminateintimeandspace,–analyticalaccuracy,–costandtimeofanalysis,–easeofinterpretation,–relevancetoregulations,–theabilitytoextrapolatetheresultsobtainedusingagivenindicator.

Themain indicators were evaluated according tothesecriteria(Table 5).

One of the most important considerations is theobjectiveoftheanalysistobeperformed,andtherefore,the ability to discriminate in time and space. Threetypes of indicators exist: agronomic, environmental,and agro-environmental (Table 1). An indicator canbe considered as having an agronomic interest ifthe relationshipwithplant uptakeor yieldshasbeenproven. Similarly, a parameter can be considered asan environmental indicator when it presents a goodrelationship with P transferred to aquatic systems.Environmental indicators generally require mildextractants such aswater or calciumchloride,whichsimulate desorption or solubilisation of P from soilto solution. Naturally, there is a link between these

two objectives.When P in soil solution increases, Pyield can increase, though P loss is also susceptibleto increase. That is why some indicators as thedegreeofPsaturationpresentbothanagronomicandenvironmentalinterest.

Secondly, according to SAFE criteria, indicatorsmustbeeasytoanalyzewithinareasonablecost,andanalysisshouldbeaccurate.Someanalysesare time-consuming and expensive to implement, or requirespecific conditions (Table 5). For example, isotopicmethods, while being extremely accurate can onlybe performed by specialized laboratories. Moreover,although main extraction methods consist of a soil-extract suspension using a ratio which is seldomrepresentative of soil conditions, ratios are oftenmorerepresentativeoftheaquaticenvironment.EachmethodextractsdifferentformsofPandthechemicalextractants often mobilize significant amounts ofunavailableformsalongwithplant-availableP(Frossardetal.,2004).Additionally,thedeterminationofPcanbecomeunreliableinsomesoilsifPconcentrationsintheextractareclosetodetectionlimits,asobservedinwaterorCaCl2extractions.

Thirdly,Tiessenetal.(1993)statedthatavailablePmeasures“apoolofsoilPthatissomehowrelatedtothatportionofsoilPwhichisplantavailable”.Therefore,results should not be interpreted independently, butaccording to regional standards, which are definedaccordingtoexperimentationwithplants.Tothisend,Jordan-Meilleetal.(2012)comparedcurrentmethodsusedforrecommendingPfertilizersinEurope,despitethe large number of analytical methods employed.Twodifferentsoilsweretested,andtherecommendedPdosagevariedbetween0 and89kgP.ha-1 forbothsoils,dependingonthecountryandmethodused.

Lastly, extrapolatingmethods and results toothersoils isnotasimpletask.Indeed,nooneindicator issuitableforallsoils.Forexample,levelsofavailablePcanbedeterminedusing themethodof Joretet al.(1955). However, this method is more suitable forcalcareous soils, whereas the Dyer method providessatisfactory results in soils ranging from acidic toneutral(Table 1).

An ideal indicator should be scientificallyvalidated, be relevant in relation to the stakeholdersandobjectives,discriminateagainstexpectedchanges,haveaninterest/costratiogreaterthan1,bebasedonreadilyavailabledata,andbeconsistentwithexistingregulations. However, in practice, indicators rarelyfulfillallof thesecriteria.Therefore, theselectionofanindicatorisbasedonacompromisebetweenfixedobjectivesandcostinordertocharacterizeacomplexand often problematic phenomenon. Based on thecriteriaestablishedbyVanCauwenberghetal.(2007),itisevidentthatwaterextractionofPwasoneofthemost advantageous indicators available (Table 5).


In contrast, other methods, such as sequentialfractionation,areoflittletonointerest.

Inpractice, indicatorsareoftencorrelatedtoeachother.PhosphorusavailabilityisdeterminedbytotalPcontentandbufferingcapacity.Currently,theselectionof indicatorsforagivencountryorregionisaresultof historical origin. It is very difficult to introducechangesintothesemethodologies,primarilyduetothenecessityofensuringthestandardizationandcontinuityofdatabases,andtheuseofspecificthresholds.

4. WHICH INDICATORS CAN BE USED IN WALLONIA?

In Wallonia (Belgium), there are no measurescurrently scheduled to assess themanagement statusof P. Incidentally, according to the Organisation forEconomic Co-operation and Development (OECD),Belgium is considered as the first European countryto possess a surplus of P delivered to agriculturalparcels.Thissurplus,amountingto21kgP.ha-1.yr-1,is

Table 4. Advantagesanddisadvantagesofvarioustypesofmethodsofphosphoruscharacterizationinsoil—Avantages et inconvénients des différents types de méthodes de caractérisation du phosphore du sol.Indicators Methods Countries using

these indicatorsAdvantages Disadvantages

Chemicalextractions AvailableP(Olsen,AA-EDTA,Dyer,etc.)

Allcountries Quick,inexpensive,easytouse

Specifictocertaintypesofsoil;specificthresholdsforeachcountry;modifiesthephysicochemicalconditionsofthemedium

WatersolubleP TheNetherlands Quick,inexpensive,easytouse

Mayunderestimatetheavailablephosphorus

DegreeofPsaturation(ammoniumoxalate,Mehlich3,etc.)

TheNetherlands,Belgium(Flanders),Quebec

Indicatorisbothagronomicandenvironmental;presenceofthresholds

Sometimesdifficulttointerpret;theanalysisismoretime-consumingthanothers

Biologicalextractions Bioassays None Reproducesthebehaviourofplants

Time-consumingandimpossibletouseroutinely

Anionexchangeresins Brazil Mimicstheeffectofroots;suitableforallsoils

Affectsthephysico-chemicalequilibria;doesnotprovideaninfinitebindingcapacity

Isotopes None ProvidesamodelofPiontransferkinetics;suitableforawiderangeofsoils;itsresultscanbeextrapolatedtolongerperiodsoftime;itpermitsobservationwithoutchangingequilibria

Cannotbeusedroutinely;manipulationofradioelementsisextremelydelicate;alackofinformationabouttheorganicfraction;experimentaldilutionconditionsdonotrepresentnaturalconditions

Morecomplexmodels PI,ODEP,etc. Quebec,USA,Norway,Finland,Denmark

Comprehensive;takesintoaccountbothsourceandtransportfactors;predictstheamountsandformsofP

Greatcomplexity;significantamountofrequireddata;oftenimperfect;moderatelyoperational

268 Biotechnol. Agron. Soc. Environ. 201620(S1),257-272 RennesonM.,BarbieuxS.&ColinetG.Ta

ble

5. AnevaluationofdifferentindicatorsaccordingtoSAFE

requirementsthatagoodindicatorshouldmeet—

Éva

luat

ion

des

diffé

rent

indi

cate

urs

sur

base

des

ex

igen

ces S

AFE

que

doit

rem

plir

un

bon

indi

cate

ur(V

anCauwenberghetal.,2007)

Indi

cato

rsA

bilit

y to

di

scri

min

ate

in ti

me

and

spac

e

Ana

lytic

al a

ccur

acy

Cos

t and

tim

e of

ana

lysis

Tran

spar

ency

an

d ea

se o

f in

terp

reta

tion

Rel

evan

ce to

re

gula

tions

Tran

sfer

and

ex

trap

olat

ion

of

resu

ltsIndicatorsofP

bioavailability

AvailableP

accordingto

Lakanen-Erviö

+++

++Fast

++Agronom

ic

thresholds

-Suitablefor

Walloonso

ilsbutnot

forcarbonatedsoils

AvailableP

byothermethods

++Ingeneral

+Av

erage

+Thresholds

+Regional

thresholds

-Notsu

itableforall

soils

SolubleP

inwater

++M

ayvary

dependingonthe

season

+++

Veryfast,

lowcost

+Thresholds

+Regulations

withinso

me

countries

+Suitableforall

soils,notsensitiveto

soiltexture

DegreeofPsaturation

++Takesinto

accountsoil

properties

++Fast

-DependsonP

andsoilbinding

capacity

+Environm

ental

thresholdof25%

-Developedfor

acidicsandysoils

Sequentialextractions

+Dependson

agriculturalpractices

-Canbedifficultto

achieverepeatability

-Time-

consum

ing

andnotused

routinely

-Diffi

cultdueto

themanyforms

ofP

~None

~

Anionexchangeresins

+~Dependsonthe

presenceofN

O3and

SO4

+Simple

+~

+Suitableforall

soils

Biologicalextractions

~-P

oorrepeatability

-Time-

consum

ingand

notroutinely

used

~~None

+Suitableforall

soils

Isotopes

+DependsonP

inputs

+Cannotbeused

anyw

here

-Delicate

manipulations

needtobe

performedbyan

accreditedlab

+Kinetic

modelling

~None

+Notboundtoso

il

Indiceso

friskofPloss

++Takesinto

accountsoil

properties

-Calibrationrequired

~Rapidonce

calibrated,but

time-consum

ing

andcostly

++Provideslosses

andrisks,possible

simulations

+InNordic

countries

-Calibratedforeach

soil

++:verygood—

très

bon;+:good—b

on;~:m

ixed—

moy

en;-:bad—

mau

vais.


largelytheresultofpigfarmsbeingprimarilylocatedinFlanders.However,Walloniacontainslowersurplus,despite significant regional disparities (Genot et al.,2009;Rennesonetal.,2015).

InWallonia, the soils predominantly range fromneutral toacidic,andPextractionhasbeenroutinelyperformedbysoilanalysislaboratoriessince1990usingammonium acetate and EDTA (pH 4.65) (Lakanenetal.,1971).Ammoniumacetateisusedtodissolveanyaluminiumphosphatepresent,while theacidity leadstoareversiblereleaseofPfromiron,aluminium,andpartlycalcium(Honsetal.,1990;Woodardetal.,1994).Thechelatingagent,EDTA,aidsinthepreventionofnewly released P from binding to iron (Dao, 2004).Thus, ammonium acetate facilitates the dissolutionofasubsetofPassociatedwithironoxidemoleculesthatareeithernotcrystallizedorslightlycrystallized,correspondingwith the different forms of P that areavailable to plants. However, ammonium acetate islargely unable to dissolve calcium phosphates, thusrenderingitunsuitedtocarbonatedsoils.InWallonia,carbonatedsoilsrepresentlessthanonepercentoftheterritory.

According to table 5, available P from Lakanen-Erviö (PAA-EDTA) presents different advantages.However, to be an adequate agronomic indicator, itshould be related to P uptake or yields. Hons et al.(1990), Homsy (1992) and Woodard et al. (1994)studiedtherelationshipbetweenPexportedbyplantsand yields. PAA-EDTA was correlated with P fertilizerlevel P (R²= 0.89), primarily in slightly acidic soils(Hons et al., 1990). In a study by Woodard et al.(1994), PAA-EDTA predicted yields and P concentrationresponses in plants more accurately than Olsen P.ThresholdshavebeendefinedforWalloniaaccordingtoPplantlevelsandsoilpropertyresults(Genotetal.,2011).

Significant correlations between PAA-EDTA andotherPextractionmethodshavebeenobserved,moststronglywiththemethodsofBray,Olsen,andSissingh(Ryser et al., 2001). Hons et al. (1990) and Homsy(1992) foundR2 values (between ammonium acetateand EDTA extraction and others) varying between0.66and0.91andbetween0.64and0.90,respectively,depending on the soil.Therefore, theLakanen-Erviömethod is well correlated with other predominantmethods used worldwide. Neyroud et al. (2003)classified the amount of extractable P according to16methods.TheLakanen-Erviömethodextracts lessPthanMehlich3orBraymethods,butmorethantheOlsenmethod.

The northern region of Belgium consists of soils(loamy and sandy texture)which exhibit a relativelylimited P binding capacity and relatively high levelsof available P. Extraction of P by the method ofLakanenetal.(1971)hasbeenperformedinWallonia

inanagriculturalcontextwithoutconsiderationforitsimpactontheenvironment.Celardin(2003)discovereda significant relationship between Lakanen-Erviö Pcontentandwater-extractableP,whichisrepresentativeofPlossrisk(R²of0.625and0.47forpH4.6-6.5andpH 6.6-8.6, respectively). Houben et al. (2011) andRennesonetal.(2015)haveevaluatedthepotentialforusing thedegreeofP saturationasanenvironmentalindicator in the Walloon region. The latter mayrepresentapromisingindicatorofPstatusinWallonia,provided that the existing equations are adapted tothe soil characteristics beforehand (Renneson et al.,2015).Moreover,Rennesonetal.(2015)showedthatextraction of P using the method of Lakanen et al.(1971)correlated(R=0.78)with theextractionofPperformedwithammoniumoxalate,whichisusedforcalculatingthedegreeofPsaturation.

OtherindicatorssuchasPindexforriskofPlosscouldbedeveloped.However,relativelyfewdataarecurrently available regarding the amount of P lossoccurringintheagriculturalparcelsofWallonia.

5. CONCLUSIONS

AprofusionofPindicatorstypescanbefoundintheliterature.ThemajorityofindicatorscurrentlyusedaredesignedtocharacterizethestatusofthesoilbasedontheirPcontent,whereasother,morecomplexindicatorsaredesignedtomodelPflowtotheenvironmentandestimate the riskofP loss.The latter assessesP losswhilesimulatingcontributionsto,ormanagementof,Pcontent.Thisapproachhasacertainadvantage,yetthesemethodscanbetime-consumingdifficulttoimplement.Consequently,mostcountriesusesimplermeasurementindicatorsofsoilP,involvingvarioustypesofanalysesrangingfromsingleextractionmethodstotheuseofPisotopes.Severalmethodsalsouseionexchangeresinsorbiologicalextractions.Moreover,someauthorshavecombined single extractions tomeasureP forms intothesoil(sequentialextraction).

Correlations can often be found between thedifferentindicatorsreportedintheliterature.However,theseresultscanrarelybeappliedgenerallyanddependfromonesoiltoanother.SomeauthorshaveproposedaclassificationsystembasedontheamountofPthatisextracted.However, thisisnotuniversalforall typesofsoil.

Eachofthemethodsavailablehascertainadvantagesand disadvantages, and no indicator is suitable forevery soil.Theappropriatenessof all indicator typeshasbeenevaluatedaccording tovariouscriteria.Theselection of an indicator is generally a compromisebetweenfixedobjectiveandothercriteria,suchasthecostandtimeofanalysis.However,tomakeachoiceinexistingindicators,aglobalanalysismustbemade.


Someindicators,suchasisotopicmethodshavegreatpotential as P indicators, though their routine use isdifficult(necessityofspecializedlaboratories).Currentindicators are generally explained by their historicaluse. Some authors suggest that extraction methodsshouldbeharmonizedthroughoutEurope.However,itisdifficulttochangeamethodduetotheinadequacyinsomesoils,lackofthresholdineachcountry,andthenecessitytomaintainsoilqualitymonitoring.

At present, in Wallonia (southern Belgium), Pextraction has been performed using ammoniumacetateandEDTA(pH4.65).Thisextractionmethodiswell correlatedwith other extractionmethods andcropyields.AreflectionismadetostudytheinterestofotherindicatorsasthedegreeofPsaturationwhichisanenvironmentalindicator.

This review highlights the importance of carefulconsiderationofindicatorchoice,andtheestablishmentof interpretation thresholds. This review creates acomparisonofappropriateandregionalizedreferencevalues.

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