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Weathering Intensity and Presence of Vegetation Are Key Controls on Soil Phosphorus Concentrations: Implications for Past and Future Terrestrial Ecosystems

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Phosphorus (P) is an essential limiting nutrient in marine and terrestrial ecosystems. Understanding the natural and anthropogenic influence on P concentration in soils is critical for predicting how its distribution in soils may shift as climate changes. While it is known that P is sourced from bedrock weathering, relationships between weathering, P, and other soil-forming factors have not been quantified at continental scales, limiting our ability to predict large-scale changes in P concentrations. Additionally, while we know that Fe oxide-associated P is an important P phase in terrestrial environments, the range in and controls on soil Fe concentrations and species (e.g., Fe in oxides, labile Fe) are poorly constrained. Here, we explore the relationships between soil P and Fe concentrations, soil order, climate, and vegetation in over 5000 soils, and Fe speciation in ca. 400 soils. Weathering intensity has a nuanced control on P concentrations in soils, with P concentrations peaking at intermediate weathering intensities (Chemical Index of Alteration, CIA~60). The presence of vegetation (but not plant functional types) affected soils' ability to accumulate P. Contrary to expectations, P was not more strongly associated with Fe in oxides than other Fe phases. These results are useful both for predicting changes in potential P fluxes from soils to rivers under climate change and for reconstructing changes in terrestrial nutrient limitations in Earth's past. In particular, soils' tendency to accumulate more P with the presence of vegetation suggests that biogeochemical models invoking the evolution and spread of land plants as a driver for increased P fluxes in the geological record may need to be revisited.
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SoilSyst.2020,4,73;doi:10.3390/soilsystems4040073www.mdpi.com/journal/soilsystems
Article
WeatheringIntensityandPresenceofVegetationAre
KeyControlsonSoilPhosphorusConcentrations:
ImplicationsforPastandFuture
TerrestrialEcosystems
RebeccaM.DzombakandNathanD.Sheldon*
DepartmentofEarth&EnvironmentalSciences,UniversityofMichigan,AnnArbor,MI48109,USA;
rdzombak@umich.edu
*Correspondence:nsheldon@umich.edu
Received:2September2020;Accepted:13December2020;Published:15December2020
Abstract:Phosphorus(P)isanessentiallimitingnutrientinmarineandterrestrialecosystems.
UnderstandingthenaturalandanthropogenicinfluenceonPconcentrationinsoilsiscriticalfor
predictinghowitsdistributioninsoilsmayshiftasclimatechanges.WhileitisknownthatPis
sourcedfrombedrockweathering,relationshipsbetweenweathering,P,andothersoilforming
factorshavenotbeenquantifiedatcontinentalscales,limitingourabilitytopredictlargescale
changesinPconcentrations.Additionally,whileweknowthatFeoxideassociatedPisanimportant
Pphaseinterrestrialenvironments,therangeinandcontrolsonsoilFeconcentrationsandspecies
(e.g.,Feinoxides,labileFe)arepoorlyconstrained.Here,weexploretherelationshipsbetweensoil
PandFeconcentrations,soilorder,climate,andvegetationinover5000soils,andFespeciationin
ca.400soils.WeatheringintensityhasanuancedcontrolonPconcentrationsinsoils,withP
concentrationspeakingatintermediateweatheringintensities(ChemicalIndexofAlteration,
CIA~60).Thepresenceofvegetation(butnotplantfunctionaltypes)affectedsoils’abilityto
accumulateP.Contrarytoexpectations,PwasnotmorestronglyassociatedwithFeinoxidesthan
otherFephases.TheseresultsareusefulbothforpredictingchangesinpotentialPfluxesfromsoils
toriversunderclimatechangeandforreconstructingchangesinterrestrialnutrientlimitationsin
Earth’spast.Inparticular,soils’tendencytoaccumulatemorePwiththepresenceofvegetation
suggeststhatbiogeochemicalmodelsinvokingtheevolutionandspreadoflandplantsasadriver
forincreasedPfluxesinthegeologicalrecordmayneedtoberevisited.
Keywords:soilfertility;phosphoruscycling;weathering;ironspeciation;biogeochemistry
1.Introduction
Phosphorus(P)isanessential,oftenlimitingnutrientinecosystemsacrosstheglobe[1,2].Phas
beenstudiedinplants,soils,rivers,lakes,andoceans—bothmodernandancient—andisatthecenter
ofcriticalquestionsaboutEarth’spastandfuture.Today,concernsaroundPinterrestrialsettings
focusonsoilfertilityandcropyields,aswellasonagriculturalrunoffanditseffectoneutrophication
[3–5].Climate,vegetation,andweatheringanderosionratesareinextricablylinked,soinorderto
improvequantitativeconstraintsonpotentialPfluxesfromsoils,eachofthesefactorsmustbe
considered.Theredoxsensitivemetaliron(Fe)isoftenassociatedwithbothPandorganic
matter/carbon(C)insoils(e.g.,[6–9]).ThePFeoxidepathwayiscriticalforterrestrialPtransport;
however,itiscurrentlylesswellunderstoodthanotheraspectsofterrestrialPcycling,suchasPC
associations.Additionally,whilemuchresearchlinkingP,weathering,andsoilagehasbeendone,
SoilSyst.2020,4,732of37
suchworkhastypicallydoneonrelativelysmallscales(e.g.,chronosequencesinHawaii)relativeto
theglobalscaleofthequestionsthatweaskhere.Therefore,constrainingclimate,vegetation,and
weatheringcontrolsonsoilPandFeiscriticalforpredictingchangestofluxesofPfromsoilstorivers
andbeyond,andtoimprovingourunderstandingofhowregionalsoilfertilitymayevolvein
responsetoachangingclimate.
Priortotheevolutionandexpansionoflandplants,changesinthefluxofPfromcontinentsto
oceansarethoughttohavebeenaprimarycontrolonmarineproductivity,andconsequently,onthe
concentrationofoxygenintheatmosphere[10,11].Itremainsdebatedwhetherlandplants’
colonizationofcontinentswouldhaveincreased[12]ordecreased[13]thatPflux.Thebetterwe
understandthemoderndistributionofandcontrolsonPinsoils,thebetterwecanmodelglobal
biogeochemicalchangesinEarth’spast.
1.1.P,Fe,andWeatheringandErosion
PisreleasedfrombedrockviatheweatheringofPbearingminerals(primarilyapatites,which
areCamineralsrelativelyresistanttoweathering;[4]).Weatheringdependsonallsoilforming
factors(climate,biology,topography,time,parentmaterial),butisprimarilycontrolledbyclimate
(e.g.,precipitation,temperature,seasonality)andtime(surfaceage,lengthofexposure;[14]).Organic
matteralsoservesasanimportantpoolofPinsoils([4,5]),andvegetation,fungi,andmicrobiallife
playanimportantroleinbothPandFeliberationandmobilizationthroughsymbiotic(mycorrhizal)
relationships[15,16].Thepresenceofplants,then,changeshowbothPandFearedistributedin
soils—butthelinksbetweenlandscapescalesoilchemistry,plantfunctionaltype,climate,and
weatheringintensityhavenotbeenquantified.
Pcanbedepletedfromasoilprofilewithinthousandstotensofthousandsofyears[4,17–20].In
terrestrialvegetation(biomass),P’sresidencetimeis~13years,andresidenceinsoilis~600years
withoutPreplenishmentbydust[4].SoilPcanalsobereplenishedthroughdustdeposition(e.g.,
[17,21,22]).Thenaturalprogressionofasoil,withaslowaccumulationofFeandAloxidesandaloss
ofmobilecationsandnutrients,meansthatPistypicallyverylowinoldand/orintenselyweathered
soils[19].Othersoilproperties,suchasclaycontent,arealsolinkedtoweatheringandvegetation,
andcouldaffecttheconcentrationsofbothPandFe.Claymineralsandclaygrainsizefractionscan
sorbconsiderableamountsofPandFe,andclaycontenttypicallyincreaseswithweatheringtime
and/orintensity[23,24].Weatheringintensityisexpectedtodecreasewithlatitudeduetocolder
temperatures,drierprecipitationregimes,andlessvegetationandshallowerrootingdepths[23,24].
Additionally,becausethepotentialvolumeoferodiblematerialgenerallyincreasesasweathering
intensityincreases(thoughvarieswithfactorssuchasslopeandupliftrate,e.g.,[25–29],weathering
islinkedtoPfluxesnotonlythroughdirectapatitedissolution,butalsothrougherosionrates,e.g.,
[30,31]).UnderstandinghowweatheringintensityisrelatedtosoilP(andFe)concentrationsis
importantforpredictinghowtheirerosionalfluxesandtransportmaychangeinresponsetoclimate
change(e.g.,[30,31]).
Afterweathering,dustdepositionisanimportantsecondarysourceofP,particularlyforold
soilswhosePhasbeendepletedthroughweathering,erosion,andbiologicaluse.Forexample,
chronosequencestudiesinHawaiihaveshownthatforoldersoils,dustreplenishmentofPiscritical
tomaintainingfertilityoncebedrocksourceshavebeendepleted[17,32,33].Dustisalsoacritical
sourceofPintheAmazonbasin,whichreceivessignificantdustfluxesfromAfrica[18,34–36],
compensatingforPlossindeeplyweatheredtropicalsoils.DustsourcedPislesssignificantforareas
thatreceivelittledustdeposition,foryoungersoilsthatstillhavearegularinputofPfrombedrock
weathering,orforshortertimescalesthaneitherforsignificantdustaccumulationortimescalesof
soilslosingtheirbedrocksourcedP(i.e.,anthropogenictimescales).Arvinetal.[21]foundthatdust
depositionisanimportantPsourceinmontanesoils,despitetheirrelatively“fresh”bedrock.The
balancebetweenweatheringrateanddustaccumulationisalsorelevantfordetermininghow
importantthelatterisinsoilP[22].DustisalsocommonlyasourceofFeoxides,increasingsoil
fertility[35,37,38].
SoilSyst.2020,4,733of37
1.2.PandFeKineticsinSoils
PandFearechemicallyassociatedinsoilsinpartbecauseofP’saffinitytosorbtoFe
(oxyhydr)oxides(e.g.,[4,9,39–42]).Orthophosphate,thecommonionformoffreePinsoils,isreactive
towardsthoseoxidesandbecomesimmobile(notreadilybioavailable)whentheyassociateinto
neoformedmineralsoraresorbedontoexistingmineralsurfaces.FeboundPmaybeacriticalmode
ofPtransportamongterrestrialecosystemsandfromterrestrial/aquatictomarineenvironments,as
oxyhydroxideboundPisoneofthemajorbioavailable/exchangeableformsofPinfluvialsystems
[5].
WhileweknowthatFeoxideadsorptionofPiskineticallyfavorableinsoilsandother
environmentswherePisbeingexchanged(e.g.,[43–45]),ourunderstandingoffirstordercontrols
onFeconcentrationandspeciation(i.e.,Fe3+inoxidesversuslabileFe2+)insoilscouldbeimproved.
ExaminingafullsuiteofextractableFe(ratherthanonlydithionitecitrateand/orammoniumoxalate)
allowsamorenuancedpictureofFephasesinsoils,whichcanultimatelybelinkedtovariableP
mobilityindifferentsoilredoxconditions[43,46,47].Itispossiblethatregionalclimatefactors
(precipitation,temperature)exertcontrolonFespeciation(i.e.,reducedvs.oxidized)insoilsviasoil
moisture.Controlsonsoilmoisturearedebatedandlikelyvaryfromprofiletoprofile[48–53],and
pH(relatedtosoilporespacesaturation;oftenmicrobiallymediated)isaprimarycontrolon
reduction/oxidationofFephasesinsoils(e.g.,[54]).Additionally,soilporewaterscanserveasan
intermediatestepbetweenrecalcitranceandfluvialsuspensionfordissolvedconstituents(e.g.,
[54,55]).ExploringtheserelationshipsiskeyforunderstandingthelikelihoodofPmobilizationunder
differentmoisturesituations.
1.3.PandFeintheGeologicRecord
Geologistsareinterestedinconstrainingmanyoftheprocessesdescribedaboveinancientsoils
andecosystems.WeatheredP,transportedtotheoceansviafluvialnetworksandcontinental
drainage,istypicallyinvokedasafirstordercontrolonmarineprimaryproductivity,whichis
associatedwithCsedimentation,atmosphericCO2drawdown,andoxygenproduction(e.g.,
[10,11,56]).SimilartoP,Fecanstimulatemarineproductivityonshorttimescales(e.g.,[57–60]),but
itisofadditionalinterestbecauseinthepast,itservedasaPsorptionsiteinmarinesystems,
effectivelysequesteringPsothatitisnotbioavailable(thesocalled“FePtrap”)andlimitingmarine
productivity[10].
Withtheseprocessesinmind,geologistsareinterestedincontinenttooceanfluxesandmobility
ofFeandPthroughtime.However,thecontinenttooceantransportofP,althoughcentraltomany
biogeochemicalmodels,isusuallyprescribedandnotbasedonactualchangesinfossilsoil(paleosol)
Pvaluesorweatheringintensitydata.Rather,ithasbeengeneralizedbasedoncrustalPvaluesand
fluxescontrolledbylargemagnitudechanges,suchasglobalglaciationsorlimitedmodern
observations(e.g.,[61]).ThevalueinprovidingrobustconstraintsontherangeanddistributionofP
insoils,aswellasclimaticandenvironmentalcontrolsonitsmobilityandbioavailability,willvastly
improvetheparametersforthesemodels.
Inadditiontobiogeochemicalmodels,geologistsareinterestedinhowtheevolutionof
terrestrialvegetationandmycorrhizae(fungalrootsymbionts)affectedPbioavailability,terrestrial
mobility,andfluxestotheoceans.Researchershavearguedforbothanincrease[12,56,62,63]and
decrease[13,15]inPfluxestotheoceansfollowingthespreadoflandplants.Constraininghow
modernvegetationrelatestosoilPmobilityanddistributionswillhelptoanswerthisquestion.
Finally,thegeochemicalcompositionoffossilsoilsisusedtoreconstructarangeofclimaticand
environmentalchanges;however,thesetoolsareprimarilybasedonsmalldatasets.Thiswork
providesrangesofreasonablevaluesforsoilchemistriesunderknownenvironmentaland
atmosphericconditions,providingcriticalbackgroundinformationtoimproveourpaleoclimateand
paleoenvironmentreconstructions.
SoilSyst.2020,4,734of37
1.4.ThisWork
Inthecontextoftheseconcerns,severalbigpicturequestionsremain.Despitemuchresearchon
Pinmodernsoils,ecosystem(vegetation,anthropogenicimpact)andclimate(precipitation,
weathering)controlsonitsconcentrationhavenotbeenquantifiedonlandscapetocontinentalscales
forgeneralizableconclusions.Additionally,althoughPisknowntohaveastrongaffinitytowardsFe
(oxyhydr)oxidesinsoils,Fespeciesandsoilredoxconditionshavenotbeenconsideredaspotential
firstordercontrolsonlandscapescalePbioavailability.Questionsabouthowclimate(precipitation,
temperature),soilfactors(drainage,soilmoisture,claycontent),andoverallweatheringintensity
affectsoilFePassociationsneedstobeinterrogated,toseeifFespeciesultimatelyservesasa
mediatorforPbioavailability.Moreover,oncethedistributionofandcontrolsonsoilPhavebeen
quantified,howdothosenewconstraintsaffectorconstrainourunderstandingofbiogeochemicalP
modelsinEarth’spast?
Weaddressthesequestions,alongwithhypothesesaboutPinthegeologicrecord,usingalarge
geochemicaldatabaseofmodernsoils(n>5000).Theresultscaninformmodelsofbothpastand
futureterrestrialbiogeochemicalcycles.
2.MaterialsandMethods
2.1.Sampling
Soilsampleswerecollectedinthefieldbytheauthors,andreceivedfromtheUSDAKSSLsoil
repository,andfromcontributors(totaln=404).Amajority(n=247)ofthesesamplesareBhorizons,
andtherefore,reflectlongerterm,stablepoolsofP(e.g.,PsorbedtoclaysorFe(oxyhydr)oxides,
whichwetestforhere,orrecalcitrantorganicmatter)andFe.ThesesoilsincludeEntisols,Inceptisols,
Alfisols,Ultisols,Histosols,Mollisols,andAridisolsandprovidecoveragefromlow(Hawaii)tohigh
(Iceland)northernlatitudes(SupplementalFigureS1).Soilscollectedinthefieldorfromrepositories
werescreenedforanthropogenicinfluence(i.e.,ondevelopedordisturbedland),with
anthropogenicallyalteredsoilsnotedassuchandbinnedseparatelyinourtreatmentsofthedata
here.Tocomplementthesesoils,weusedaUnitedStatesGeologicalSurvey(USGS)soilgeochemistry
interactivereport[64]andcollatedgeochemicaldatafortheTop5cm,Ahorizons,andChorizonsof
allthesoilsincludedinthatwork(n=4857).SeeSupplementalTableS1forsummarydescriptionsof
thesetwodatasets,andSupplementalTableS2forcompletesampleinformationforthenewdataset
presentedhere.GeochemicaldatafortheUSGSdataset[64]areavailableat
https://doi.org/10.3133/sir20175118.
Combiningthesesoildatabasesyieldsexcellentspatial,climatological,profiledepth,drainage,
vegetation(asplantfunctionaltype/generalizedbiome),andsoilordercoverage(Supplemental
FigureS1).WhilethesamplesaremostlyfromtheUnitedStates,variationinsoilformingfactors
acrossthecontinentreflectmostofthepotentialglobalvariation.Thetropicsaresomewhatunder
sampled,butdatafromHawaiiarebroadlyreflectiveofthoselatitudes.Whiletherearespecies
specificrelationshipsbetweenplantsandsoilgeochemistry,generalizedbiomeisacceptableforthe
continentalscaleofquestionsexaminedinthiswork,aswellasthecomparativenatureofour
inquiries(e.g.,[65–68]).Differencesbetweenplantfunctionaltypestypicallyswampdifferences
withinoneplantfunctionaltype.Additionally,becausebiomesonsubcontinentalscalesaretypical
forglobalbiogeochemicalmodels,andbecausemorespecificdataarenotavailableforthelatitudinal
scaleofthiswork,generalizingbybiomescale/plantfunctionaltypeisappropriate.
2.2.GeochemicalAnalyses
AllanalyseswereperformedondrysoilswithintheSheldonlab,asweresamplesduringthe
USGSanalyses.SoilsweredriedperUSDAAPHISregulationsuponimportandgroundto<70μm
forhomogeneityduringanalyses;sampleswerestoreddryinoxic(ambientroom)conditions(during
theexperimentaldesign,testsofpre‐andpostdrying,withdryingtemperaturesvaryingbetween
25,50,and100C,showednosignificantgeochemicaldifferences).Bulkelementalgeochemistrywas
SoilSyst.2020,4,735of37
performedatALSLaboratoriesinVancouver,BC,Canada,usingafouraciddigestionand
InductivelyCoupledPlasmaMassSpectrometer(ICPMS)analysis.Externalprecisionwas0.01%for
allmajorelementsand10ppmforP2O5,analyzedatALS.ForsamplesfromtheUSGSdataset,1σ
sampleerrorswere488ppmand1.39wt%forP2O5andtotalFe,respectively)[64].The1σsample
errorsfortotalCandorganicCwere4.6%(n.d.forinorganicC[64]).
ToanalyzeFespeciationinsoils,weusedafourstepsequentialFeextractionfollowingPoulton
andCanfield[69]andRaiswelletal.[70].Thisextractionprotocolseparatesoperationallydefined
poolsasascorbicacid(labileFe;Feasc),dithionite‐(crystallineFeoxyhydroxides;Fedith),ammonium
oxalate‐ (highlycrystallineFeoxyhydroxides;Feox),andsodiumacetateextractableFe(Fein
carbonates,analyzedseparatelyfromthesequentialextraction;Feacet).Initially,weincludeda
chromiumreduciblesulfur(CRS;[71])extractionseparatelytoanalyzeforFesulfides(e.g.,pyrite),
butthoseconcentrationswerebelowdetectionlimitforallnonwetlandsoils.CRSwaslimitedto
wetlandsoilsaftertheinitialexploratoryextractions,andrawvalueswerenormalizedtoaveragesoil
orderdensitiestoaccountforthelargedifferenceindensitybetweenHistosolsanddenser,more
mineralrichsoilorders[72].Samplesupernatantswerecentrifugedanddiluted50×,andFewas
measuredviaICPOpticalEmissionSpectroscopyandICPMSattheUniversityofMichigan.
Aninternalstandardofalocalsoil,driedatthreedifferenttemperatures,wasincludedinall
runsforinterrunconsistencychecksandtodeterminereproducibility.Theabsolutestandard
deviationforasample’sreproducibilitydependsontheconcentrationofeachFespecies,sothe
relativepercenterror((σ/μ)×100)isusedinstead.RepeatedanalysesshowthatFeascisreliablewithin
3.7%ofthemean,Fedith,within1.7%ofthemean,andFeox,within7.0%ofthemean.Reproducibility
ontheFeacetsampleswaslarger,at15%ofthemean,becausethemeansinmanyofthesampleswe
analyzedwerenearmachinedetectionlimits(~1000ppm),whereadifferenceofseveralhundred
ppmyieldsfarlargerstandarddeviationsthanspecieswithhigherconcentrations.TheFeascpoolis
consideredtobealowconstraintbecausealargerproportionoflabileFemaybeinasoil’sporewater
(ratherthansolids).Becausehereweareprimarilyinterestedinrelativedifferencesbetweenoxide
andlabileFespecies,andbecausespecies’concentrationstendtodifferbyordersofmagnitude,we
havedeemedthisapproachacceptable.Forfurtherreadingonnuancesinsamplepreparationand
extractionprotocols,seetworecentreviewsbyRaiswelletal.[73]andAlgeoandLiu[74].
FespeciationdatawerefilteredforoutliersbyremovinganysampleswhereFespecweight
percent>totalFeweightpercent(i.e.,theconcentrationofameasuredFepoolwashigherthanthe
measuredtotalFe).Thesediscrepancies(n=107,95,139,106,forFeasc,Feacet,Fedith,andFeox,
respectively)likelyarosefromthe50×dilutionbeinginsufficient,withhighFeconcentrations
overwhelmingtheICPOES,leadingtoerroneousmeasurements.FutureworkusingFespeciation
onsoilsshouldconsiderusingstrongerdilutionfactorsforhighFesoilstocircumventinstrument
limitations.
2.3.OtherDataCollated
GeochemicaldatafortheTop5cm,Ahorizons,andChorizonsfrom4857soilsaroundtheU.S.
areavailableonaninteractiveportal[64].Weatheringrelevantcations(Al,Ca,Na,K)andthe
nutrientsofinterest(P,Fe)alongwithlatitudeandtotalclaycontent(clay:AandChorizonsonly)
werecompiled.FortheUSGSdataset,claycontentwasdeterminedbyXRaydiffraction(for10and
14Åclaysonly).Organic,inorganic,andtotalcarbondatawereavailableforamajorityofsoilsinA
andChorizonsintheUSGSdataset(notallhadinorganiccarbon).Itshouldbenotedthatadrysoil
samplewillhaveahigherrelativeweightpercentorganiccarbonduetowaterlossduringdrying.
Soilordersanddrainagecapabilityarenotgivenforsoilsinthisdatabase,butbecausetheyspanthe
conterminousU.S.,itisareasonableassumptionthattheyincludeavarietyofsoilorders.Clay
contentasgrainsizefraction,parentmaterial,andsoilmoistureregimewerealsogatheredforthe
newlycollecteddatasetfromUSDASoilSeriesreportswhenavailable(SupplementalTableS2).
ClimatedataweregatheredfromSoilSeriespages;whenthesewerenotavailable,thePRISM
30yearaverages(1981–2010)wereused.IfneitherSoilSeriesnorPRISMclimatedatawereavailable
SoilSyst.2020,4,736of37
(i.e.,fornonUSsamples),countryspecificgovernmentalmeteorologicaldataorjournalarticleswere
used(seeSupplementalTableS2fordetails).
TheChemicalIndexofAlteration(CIA)[75]wascalculatedtoassessthedegreeofweathering
foreachsoil,whichreflectsallofthesoilformingfactorstosomedegree,butpredominantlysoilage
andclimate[14].TheCIAisthoughttoreflecttheweatheringoffeldsparstoformclaymineralswhere
highvaluesrepresentmoreintenseweathering.Molaroxideratiosareused(Equation(1)).TheCIA
ofanindividualsoilmaynotcorresponddirectlywiththeCIAvaluesofsedimentsinrivers(e.g.,
[76]).However,becauseoursoils’averageCIAvalues(59±15,42±32,and57±21forA,B,andC
horizons,respectively)arewithin1σoftheNorthAmericanrivers’averagefromthatstudy(n=7;
meanCIA=66±8.5,range53–73),thoughwithlargerranges(0.5to99forallthreehorizons),we
proceedwithitsusehere.
CIA=(Al2O3/(Al2O3+CaO+NaO+K2O))×100(1)
2.4.StatisticalAnalyses
ToexaminehowthetotalFeandPcorrelatewithCIA(weatheringintensity),arunningaverage
witha10CIAunitwindowwasused.Linearleastsquaresregressionswereusedtotestfor
significantlinearcorrelationswhereappropriate,basedonthehypothesesthatwerebeingtested.
DuetothelargesizeoftheUSGSdataset,pvaluesareextremelylow(<1016;essentially0),meaning
itishighlylikelythatthetwovariablesarenotrandomlycorrelated.Strengthofcorrelationshould
beprimarilyconsideredbeforetakinganyrelationshipstobepredictive,butforsimplicity’ssake,
coefficientsofdetermination(R2values)arereported.PrincipleComponentsAnalysis(PCA)was
performedforUSGSdatabasesoilsbyhorizon(Top5cm,A,andC).Theseanalysesareusedtotest
theexpectedrelationships(e.g.,thatFeandPwillbepositivelyandlinearlyrelated),ratherthanto
buildpredictivemodels.AllofthestatisticalanalyseswereperformedinMatlab.
3.Results
CompletesampleinformationandallgeochemicalresultsarefoundinSupplementalTablesS2–
S5andareavailableintheonlineversionofthispaper.ForthelargerUSGSdataset[64],wehave
geochemical,drainage,andvegetation(generalizedplantfunctionaltype)data,butnosoilorderor
Fespeciationinformation.Resultsforsoilorder,drainage,parentmaterial,andFespeciationanalyses
arebasedonlyonthenewlycollecteddataset(n=404).
3.1.FeandP:Concentrations,SoilOrder,andVegetation
3.1.1.P
TheaveragetotalPconcentration(referredtosimplyas“P”hereonwards)inthecontinental
crustisca.870ppmP,andthereisrelativelylittlevariationbetweenbedrocklithologies[77].The
maximumsoilPconcentrationinthiscompilationwas>20,000ppm(>2wt%;Chorizonofan
anthropogenicallymodifiedsoil),anorderofmagnitudegreaterthanthecrustalaverage.MeanP
concentrationsforeachhorizonstudiedweredepletedrelativetothecrustalaverage:Top5cm,A,
andChorizonsmeanswere660,632,and508ppmP,respectively(USGSdata),andBhorizonsfrom
thenewdatasethadameanof937ppmP.Pconcentrationsvaryspatiallythroughoutcontinental
U.S.(SupplementalFigureS2),withhotspotsinavarietyofdifferentgeologic,climatic,andbiologic
provinces.Pconcentrationsvariedamongsoilorders(Figure1),withthehighestconcentrationsin
Inceptisols(Figure1A),arelativelyweaklydevelopedsoiltype.
SoilSyst.2020,4,737of37
Figure1.PandFeconcentrationsinmodernsoils’Bhorizons,binnedbysoilorder.Redlinesare
medianvalues,bluebinsare25thand75thquartile,andredcrossesarebeyondthe75thquartile;the
horizontaldashedgraylineisthecrustalaverage(870ppmPand3.5%Fe)[77].(A)Pconcentrations,
showingthatyoungersoils(Entisols,Inceptisols)tendtohavehigherPthanoldersoils(Alfisols,
Ultisols).OxisolshavehighrangesofP,butamuchsmallerbinsizethanotherorders(n=8).Andisols
havehighinheritedPfromvolcanicparentmaterials.(B)Feconcentrations,showingconsistentFe
concentrationsbetweenca.1and10wt%withtheexceptionofOxisols,whicharedefinedbytheir
highratesofFeoxideaccumulation.Binsizes:Entisols(44),Inceptisols(75),Alfisols(42),Ultisols(6),
Oxisols(8),Aridisols(76),Mollisols(60),Andisols(15),Spodosols(9),Gelisols(30),Unknown(39).
ThevegetationgroupsareBarren(lackingvegetation),Altered(i.e.,affectedbyhumanactivity),
Forest(nodifferentiationbetweendeciduousandconiferous),Herbaceous/Grassland,
Developed/Cultivated(i.e.,occupiedormanicured),Shrubland,andOther(mostlyincludingearly
successionplantsormicrobialearths).ForUSGSsamples,protocolsdictatedthatroads,buildings,
andindustrialsitesbeavoided[64].Amongvegetationtypes,thereislittlevariabilitybetweenP
concentrationsandvegetationtypes(Figure2);whileBarrenlandscapes’Bhorizonsshowhigher
meanP,thisislikelyduetosmallbinsize(n=5;Figure2C).Overall,variabilityinconcentrationsin
differentvegetationtypesandsoilorderswasminimal,withmostsoilsbeingdepletedinPrelative
tothecrustalaverage.Vegetatedsoils(bothnaturalandanthropogenicallyaltered)hadhigher
rangesofPthanunvegetated(Barren).Therewasaweakpositivecorrelationwithmeanannual
precipitation(SupplementalFigureS3A),buttherangeofPvaluesatagivenprecipitationamountis
SoilSyst.2020,4,738of37
typicallytoolarge(~1500ppm)tobeofpredictiveuse.Thistrendcouldalsobeexplainedbyrelated
factors,suchasvegetationcoverage(coverversusbareground)andweatheringintensity,whichare
associatedwithprecipitation.
Figure2.
P
concentrationsinallhorizons,binnedbyvegetationtype.Dashedgraylineinallisthe
crustalaverageP(870ppm)[77].Arrowsindicateoffplotvalues.(A)
P
intheTop5cm,whereBarren
(novegetation)andAlteredsoilshavethelowestrangeofPconcentrations.(B)
P
inAhorizons,with
similardistributionstoTop5cm.(C)
P
inBhorizons,withmuchsmallerbinsizesthanotherhorizons.
(D)P
inChorizons.Binsizesfor(A,B,D):Barren(68),Altered(209),Forest(1252),Herbaceous(815),
Developed(1568),Shrubland(945).Binsizesfor(C):Forest(43),Grassland/Herbaceous(19),
Shrubland(50),Barren(5),Cultivated(59),Unknown(74).
3.1.2.Fe
ThecrustalcompositionoftotalFe(Fe
tot
)ismorevariablethanthatofPbecausewhilePis
sourcedprimarilybyapatitegroupminerals,Fe
tot
canbepresentinawiderangeofmineralsand
lithologies.ThePhanerozoicuppercontinentalcrustisestimatedtohave3.5wt%Fe[77].Fe
tot
averagesinthesoilsstudiedherewere2.1wt%forTop5cm,1.6wt%forAhorizon,2.6wt%forC
horizon,and4wt%forourdataset(primarilyBhorizons).DensitynormalizedFe
tot
variedslightly
bysoilorder(SupplementalTableS3),showingtheexpectedtrendofmodestlossshiftingtomodest
accumulationduringtheAlfisolUltisoltransition(Figure1B),butotherwisewasrelatively
consistent.OxisolshadthehighestvaluesandrangeinFe,withsomevariabilityamongtheothersoil
ordersbutgenerallywithinconsistentranges(Figure1B).
SoilSyst.2020,4,739of37
Fe
tot
isgenerallyconsistentbothwithinagivenhorizonandbetweendifferentvegetationcover
types(Figure3).Whilemostsoilordersandmosthorizonshad<3wt%Fe
tot
,valuesrangedupto
>15%(Figure3;SupplementalFigureS2).TheonlynotablevariabilityinFeconcentrationsamong
vegetationisinCultivatedsoilBhorizons(Figure4C),whicharedepleted,and‘Other’soilBhorizons
thataresubstantiallyenrichedrelativetotheothervegetationcovergroups.‘Other’vegetation
containsmostlybasaltparentedsoilsfromalimitedgeographicalarea(primarilyIceland),sothis
resultislikelyanartifactofoursampling.TherewasnostrongcorrelationbetweenFe
tot
inasoiland
meanannualprecipitation(R
2
:0.2;SupplementalFigureS3B).
Figure3.Feconcentrationsinallhorizons,binnedbyvegetationtype.Dashedgraylineinallisthe
crustalaverage(3.5%)[77].Arrowsindicateoffplotvalues.(A)FeintheTop5cm,whereBarrenand
AlteredsoilshavethelowestmaximumFeconcentrations.(B)FeinAhorizons,withsimilartrends
toTop5cm.(C)FeinBhorizons.TherewerenodataforBarrenBhorizons’Fe.(D)FeinChorizons.
Binsizesfor(A,B,D):Barren(68),Altered(209),Forest(1252),Herbaceous(815),Developed(1568),
Shrubland(945).Binsizesfor(C):Forest(43),Grassland/Herbaceous(19),Shrubland(50),Barren(0),
Cultivated(59),Unknown(74).
3.2.FeandP:Latitude,Weathering,SoilOrder,andClayContent
Latitude,weatheringintensity,andclaycontentshouldeachbeassociatedwitheachotheras
wellaswithFe(e.g.,[24]).Someofthetrendsdescribedherecouldbeinfluencedbyalatitudinal
samplingbias,withmostofthesamplescomingfromthemidlatitudes(SupplementalFigureS1;
SoilSyst.2020,4,7310of37
Figure4A).Inthesoilsanalyzedhere,maximumweathering(asmeasuredbyCIA)decreasesas
latitudeincreases(Figure4B),whileclaycontent(aweatheringproduct)doesnotshowastrongtrend
withlatitude(Figure4C),withtheexceptionofaweakmidlatitude(35–40°N)peakinclaycontent
insomeChorizons.Weatheringtrendsbetweensoilordersbehaveasexpected,withCIAvalues
increasingfromEntisolstoUltisols/Oxisols(Figure5).Claycontentandweatheringshowawell
behavedandexpectedpatternofincreasethatmatchesthetheoreticalunderstandingofthatmetric
[24,75,78,79],withBhorizonsspecificallyshowingthehighestclaycontentuntilthehighestCIA
valuesarereached(Figure4D,greenpoints).Claycontentandgeochemicaldatawerenotavailable
fortheTop5cmsubsetoftheUSGSdataset,sothathorizonisexcludedfromthosecomparisons.
Figure4.(A)Latitudinaldistributionofsoilsusedinthisstudy,wheresmallerbinsrepresentthe
USGSdataset[64]andthewiderbinsareoursoils.Therangeoflatitudescoveredhereisto73°N,
butmostofthesamplesfallbetween20and50°N.(B)LatitudinaltrendsinChemicalIndexof
Alteration(CIA).(C)Latitudinaltrendsinclaycontent.(D)CorrelationbetweenCIAandclaycontent
(R
2
:0.05,<0.01,and0.19forA,B,andChorizons,respectively).Overall,theseresultssupportcommon
assumptionsabouttherelationshipsbetweenlatitudeandweathering.
SoilSyst.2020,4,7311of37
Figure5.ChemicalIndexofAlteration(CIA)binnedbysoilorder.CIAincreasesthroughtheEntisol
Ultisolsoildevelopmentprogression,asexpected,andOxisolshavethehighestCIAwithamedian
near100.Aridisols’medianCIAfallsbetweenEntisolsandInceptisols.Binsizes:Entisols(44),
Inceptisols(75),Alfisols(42),Ultisols(6),Oxisols(8),Aridisols(76),Mollisols(60),Andisols(15),
Spodosols(9),Gelisols(30),Unknown(39).
Fe
tot
contentincreasesmoderatelywithhigherlatitudes(Figure6A),higherweathering(Figure
6C),andclaycontent(Figure6E).Pinsoilsbehaveslesslinearlywithrespecttothesevariables:P
increasesmoderatelywithlatitude(Figure6B),butratherthandecreasingwithweathering,asmight
beexpectedbasedonsoilagePrelationships,averagePconcentrationsinallhorizonsvarylittle,
thoughthereisageneralincreaseinrangeatmoderateweatheringintensities(CIA~40–70)(Figure
6D).Pconcentrationsareweaklynegativelycorrelatedwithclaycontent(Figure6F).Fe
tot
andP
showedpositivecorrelationswithoneanotherinallhorizons,asexpected(SupplementalFigureS4).
SoilSyst.2020,4,7312of37
Figure6.Trendsbetweenlatitude,weathering,andclaycontentandtotalFeandPconcentrations.
Solidlinesin(C)and(D)arerunningaveragesforTop5(yellow),A(red),B(blue),andC(black)
horizons.Solidlinesin(E)and(F)arelinearleastsquaresregressionsforA(red),B(blue)andC
(black)horizons.(A)Feinsoilsbylatitude.(B)Pinsoilsbylatitude.(C)Fe
tot
andweatheringintensity.
Solidlinesaretherunningaverages(10CIAunitwindow)foreachhorizon;colorscorrespondto
datacolors.TheBhorizonaverage(blueline)ishighestatveryhighCIAsdueinparttolowersample
densityatveryhighCIAvalues,especiallybetween98–100.(D)Pconcentrationsandweathering
intensity.(E)Feandclaycontent(R
2
is0.34and0.37forAandChorizons;0.01forBhorizons,much
smallerdataset;p=0.7forBhorizon,<<0.01forotherhorizons).Solidlinesarelinearleastsquares
regressions;theBhorizon(blueline)isskewedduetohighFe,lowclaypointsoffplot.(F)Pandclay
content.Solidlinesarelinearleastsquaresregressions(R
2
is0.11,0.08,and0.01forA,B,andC
horizons,respectively;p<<0.01forallhorizons).
SoilSyst.2020,4,7313of37
3.3.Weathering,ClayContent,andVegetation
Betweenvegetationtypes,thereislowvariabilityinclaycontent,withForestsshowingslightly
lowervaluesinBhorizonsthantheothergroups(SupplementalFigureS5).Forestshavethehighest
weatheringinnatural(unaltered/notdeveloped)soils,followedbyGrasslandsandShrublands
(Figure7).
Figure7.ChemicalIndexofAlteration(CIA)binnedbyvegetationtype,forhorizons(AC)(bulk
chemistrydataforCIAwerenotavailablefortheTop5cmhorizon).(A)Alteredsoilshavethehighest
CIAvaluesoverall,andForestsoilshavethehighestCIAfornonanthropogenicallyaffectedsoils,in
Ahorizons.(B)ForestshavethehighestCIAvaluesinBhorizons,followedbyGrasslandsand
SoilSyst.2020,4,7314of37
Shrublands(Barrensoils’smallbinsize,n=5,precludesitfromanalysishere).(C)Alteredsoilsalso
havethehighestCIAinChorizons,followedbyForests.
3.4.FeandP:DrainageandSoilMoisture
ModeratelypoorlydrainedsoilshadhigherFeandPconcentrationsthantheotherdegreesof
drainage.PerudicmoistureregimeshadthehighestFeandPvalues(Figure8);however,these
samplesweredominatedbybasaltparentedsoilsinIceland,whichmayskewtheresults.Asidefrom
that,AquicandXericmoistureregimeshadhighFe,andUsticandUdicsoilshadhighP(Figure8).
SometrendsemergewithrelativelyhighFe/lowP(Aquic)andviceversa(Udic).
Figure8.VariabilityinFeconcentrations(A,C)and
P
concentrations(B,D)indifferentsoildrainage
andmoisturesregimes.DashedgraylinesrepresentcrustalaveragesforFeandPinrespectiveplots.
(A)Febinnedbydrainage.(B)
P
binnedbydrainage.(C)Febinnedbymoistureregime.(D)
P
binned
bymoistureregime.SomepatternsemergewhencomparingFeandPbetweensoilmoistureregimes,
e.g.,UdicsoilshavinglowFe/highPandAquicsoilshavinghighFe/lowP.Perudicsoilsare
dominatedbybasaltparentedsoils,influencingtheirFeandPconcentrationsandhighlightingthe
importanceofbedrockparentmaterial.NodataforPermafrostFeorPconcentrations.Binsizesfor
(A,B):Poorlydrained(166),Moderatelypoorlydrained(25),Moderatelywelldrained(16),Well
drained(185),Excessivelywelldrained(12).Binsizesfor(C,D):Unknown(111),Aridic(73),Aquic
(15),Udic(82),Perudic(27),Ustic(54),Xeric(21),Permafrost(N.D.).
SoilSyst.2020,4,7315of37
3.5.FeSpeciation
3.5.1.FeSpeciationandP
Pyrite/Feinsulfides,normalizedtoaveragesoilorderdensity,wereabovedetectionbytitration
inonly22testednonwetlandsoilsoutof63fromaroundthecontinentalU.S.(SupplementalTable
S5),sothattestwasexcludedfromsubsequentanalyses.Densitynormalizedpyriteyieldsfromall
wetlandsedimentsanalyzed(n=15)weremeasurable/abovedetectionlimit(>0.1wt%),asopposed
tononwetlandsoils(Alfisols,Aridisols,andMollisols),whichtypicallyfellbelowthislimitafter
beingdensitynormalized.Inanonperenniallywaterloggedsoil(e.g.,notwetlands),itisreasonable
toassumethatFecontentsinsulfidesarenegligible.
Contrarytoexpectations(seeSection4.3below),Fe
dith
andFe
ox
poolsdidnotdisplaystrongeror
morerobustcorrelationswithP(Figure9)thantheotherFepools.Therewerenoclearcorrelations
betweenPandanyFespecies(forall,R
2
<0.01).SeeSupplementalTableS4forallFespeciesresults
andFe
3+
/Fe
2+
ratios.
Figure9.RelationshipsbetweenFespeciesandPconcentrations,allwithR
2
values<0.1.Arrows
indicateoffplotvalues.(A)NostrongcorrelationbetweenFe
asc
andP(p=10
6
).(B)Nosignificant
correlationbetweenFe
acet
andP(p=0.2).Notethedifferentxaxisscale.(C)Nostrongcorrelation
betweenFe
dith
andP(p=0.003),whereapositivecorrelationhadbeenexpectedduetoP’saffinityto
sorbtoFe(oxyhydr)oxides.(D)NostrongcorrelationispresentbetweenFe
ox
andP(p=10
9
),again
whereastrongpositivecorrelationmighthavebeenexpected.
SoilSyst.2020,4,7316of37
3.5.2.FeSpeciation,Precipitation,SoilMoisture,andDrainage
Contrarytoexpectations(seeSection4.2.2below),Fespeciationpoolsshowednostrong
predictiverelationships(allR
2
<0.2)withmeanannualprecipitation(Figure10),buttherewere
differencesbetweensoilmoistureregimes.PermafrostsoilsfromtheNorthSlopesofAlaska,highin
organicmatterandFe,weredominatedbylabileFe(Fe
asc
).AquicandUdicsoilshadhighFein
carbonates(Fe
acet
),andPerudicsoils(primarilyfromIceland,withbasaltparentmaterial)hadhigh
Fe
ox
(Figure11).MeanFe
dith
wasconsistentbetweensoilmoistureregimesbuthasanextremelyhigh
rangeinAridicsoils.
Figure10.Relationshipsbetweenmeanannualprecipitation(MAP;mmyr
1
)andFespecies,colored
bysoildrainage.Arrowsindicateoffplotvalues.(A)NocorrelationbetweenMAPandFe
asc
(R
2
<0.1,
p=10
9
).(B)NostrongcorrelationbetweenMAPandFe
acet
(R
2
<0.01;p=0.0005);therecouldbea
maximumMAPvaluebeyondwhichFe
acet
islesskineticallyfavorable.Notethedifferentyaxisscale.
(C)NosignificantcorrelationbetweenMAPandFe
dith
ispresent(R
2
<0.01,p=0.7).(D)Modest
significantcorrelationbetweenMAPandFe
ox,
(R
2
=0.16,p=10
17
).ForallFespecies,thereareno
strongpredictivetrendsbetweenFespeciesconcentrationandMAP,andnopatternsindrainage.
SoilSyst.2020,4,7317of37
Figure11.Fespeciesbinnedbysoilmoistureregime.(A)Udic,Perudic,andPermafrostsoilshavethe
highestmedianFe
asc
.(B)Fe
acet
isquitelowinmostsoils.Notethedifferentyaxisscale.(C)Aridicsoils
havehighFe
dith
,butothermoistureregimesshowlittlevariability.(D)Littlevariabilitycanbeseenin
Fe
ox
acrosssoilmoistureregimes.Binsizes(slightvariabilitybetweenFespeciesisduetoextraction
yields):(A)Unknown(106),Aridic(73),Aquic(15),Udic(82),Perudic(26),Ustic(53),Xeric(21),
Permafrost(21).(B)Unknown(111),Aridic(73),Aquic(15),Udic(82),Perudic(27),Ustic(54),Xeric
(21),Permafrost(18).(C)Unknown(111),Aridic(61),Aquic(14),Udic(78),Perudic(26),Ustic(48),
Xeric(15),Permafrost(21).(D)Unknown(106),Aridic(73),Aquic(15),Udic(82),Perudic(27),Ustic
(54),Xeric(21),Permafrost(21).
PoorlydrainedsoilshadahighrangeofFe
asc
andFe
dith
,whilewelldrainedsoilshadhigh
amountsofFe
acet
(Figure12A).TheotherFespeciesanddrainageswereconsistent.Theroleofslope
wasconsidered,butlocal,smallscaletopographicreliefwasnotreadilydeterminableformost
samples.
SoilSyst.2020,4,7318of37
Figure12.Fespeciesbinnedbysoildrainage.(A)ExcessivelywelldrainedsoilshaveverylowFe
asc
.
(B)Notethedifferentyaxisscale.Fe
acet
ishighestinwelldrainedsoils,althoughpoorlydrainedsoils
showanunexpectedlyhighrangeaswell.(C)ExcessivesoilsshowthelowestmaximumFe
dith
,with
theotherdrainagesshowingsimilarmediansandranges.(D)Fe
ox
hassimilarpatternstoFe
dith
,with
threeveryhighvaluesformoderatedrainages.Binsizes:(A)Poorlydrained(104),Moderatelypoorly
drained(16),Moderatelywelldrained(11),Welldrained(140),Excessivelywelldrained(3).(B)
Poorlydrained(109),Moderatelypoorlydrained(16),Moderatelywelldrained(11),Welldrained
(142),Excessivelywelldrained(3).(C)Poorlydrained(101),Moderatelypoorlydrained(15),
Moderatelywelldrained(9),Welldrained(123),Excessivelywelldrained(3).(D)Poorlydrained
(109),Moderatelypoorlydrained(16),Moderatelywelldrained(11),Welldrained(142),Excessively
welldrained(3).
3.5.3.FeSpeciation,Vegetation,andSoilOrder
ForestsoilshadhigherFe
acet
(Figure13B)thantheothervegetationgroups,butFespecies
concentrationswereconsistentotherwise.OxisolshadhigherFe
dith
andFe
ox
thanothersoilorders
(Figure14C),butFespecieshadlittlevariabilitybetweensoilordersotherwise.
SoilSyst.2020,4,7319of37
Figure13.Fespeciesbinnedbyvegetationtype.(A)ForestsandGrasslandshavehigherFe
asc
than
othervegetations(exceptforUnknown).(B)ForestshavethehighestFe
acet
values(medianandrange).
(C)NopatternseeninFe
dith
andvegetation,exceptthatBarrensoilshavelowFe
dith
.(D)Nopatterns
inFe
ox
andvegetation.
SoilSyst.2020,4,7320of37
Figure14.BoxplotsofFespeciesbinnedbysoilorders,wherecenteredredcirclesarethemedians,
thedarkblueboxesarethe25thinnerquartiles,thewhiskersaretheouter25thquartiles,andthered
circlesarethestatisticaloutliers(outside3σ).(A)NodifferencesinFe
asc
betweensoilorders.(B)
UltisolshavethehighestFe
acet
,althoughAlfisolshaveahighrangeofFe
acet
aswell.(C)Oxisolshave
thehighestFe
dith
,asexpected.(D)OxisolshavethehighestFe
ox
,asexpected.
3.6.OrganicCarbon
Organiccarbon(C
org
)inAhorizonsrangedfrom0to61wt%,andinChorizons,rangedfrom0
to43wt%[64].TheaverageC
org
:PratioforAhorizonswas17.9:1,andforChorizons,18.3:1.
RelationshipsbetweenC
org
andCIA,P,Fe,andclaycontentwereweak(SupplementalFiguresS6and
S7).TherewerenostrongcorrelationsbetweeninorganicCandFe
tot
,orbetweenC
tot
andFe
tot
(SupplementalFigureS7).
3.7.ParentMaterial
ThereisslightvariabilitybetweenFe
tot
andPconcentrationsandparentmaterial,withigneous
bedrockandash/volcanicshavingthehighestmedianvaluesforFe,withhighPvaluesaswell
(SupplementalFigureS8).WhilelimestoneparentmaterialsshowhighFe,thereareonlytwosamples
inthisgroupandtheyarenotincludedinthisdiscussion.Theparentmaterialdoesnotappeartobe
apredictivecontrolineitherFe
tot
orP(SupplementalFigureS8).However,thiscouldbeduetothe
mixedprovenanceofmanymodernsoils’parentmaterial(i.e.,alluvium/colluvium,glacialdeposits,
etc.asopposedtocompositionallyhomogeneousbedrock).Amoretargetedexplorationinto
bedrockparentedsoils,Fe
tot
,andPcouldwellshowmorewellbehavedrelationships.
SoilSyst.2020,4,7321of37
3.8.PrincipleComponentsAnalysis
Resultsfromtheprincipalcomponentsanalyses(PCA)fortheUSGSdataareshownin
SupplementalFigureS10.Principlecomponents1and2explain~60%ofthevarianceatamaximum.
However,alleigenvaluesareverylow(<0.6)anddataareessentiallyclusteredaroundtheorigin,so
whilethereareassociations,theyareveryweakandshouldbeinterpretedconservatively.Latitude,
P,Fe,andAlwereassociatedwithPC2intheTop5cmandAhorizons,andvegetationandCIAwere
associatedwithPC1.GroupingschangedintheChorizons,withFeandAlassociatedwithPC2,but
PandlatitudeassociatedwithCorgalongPC1.Inallhorizons,CIAandvegetationvectorsareopposite
toeachother,andwereorthogonaltosubparalleltomostothervectors.
4.Discussion
Inthissection,weexploreourresultswithaseriesofquestionsfocusedonconstrainingcontrols
onFeandtotalPinsoils,definingtheirrelationshipsoncontinentalscales,andmakinginferences
abouthowthoserelationshipsmayimpactterrestrialPfluxes.Theyarecenteredaroundkeysoil
formingfactors,suchasclimate,vegetation,andweatheringintensity,aswellassoilredoxspecific
factors(i.e.,precipitation,moistureanddrainage,andFespecies).Becauseamajorityofsamples
comefrombetween20and50°N,theinterpretationsmadeherearemostrobustforthoselatitudes.
4.1.HowdoLatitude,Weathering,andClayContentAssociatewithPandFeConcentrations?
Mostoftheexpectedrelationshipsbetweensoilorder,FetotandP,weathering,andlatitudewere
supported.Asexpected,weatheringgenerallydecreaseswithlatitude,droppingfromamaximum
CIAof>95at30–35°NtoaCIA~75at50°N(thenorthernlimitofthelargeUSGSdataset).Farther
north,someBhorizonsdeviatefromthatpattern,withCIAvalueshigherthanmightbeexpected
basedontheirclimaticregime[80,81].Interpretingthelatitudinaltrendshereshouldincludeacaveat
forlatitudinalsamplingbiasandlimitationsofthedatasetto,mostly,between20°and50°N.While
thePCAresults(SupplementalFigureS10)donotsupportarelationshipbetweenPandweathering,
weinterpretthisdiscrepancyasbeingduetothecomplexitieswithinthePweatheringrelationship
ratherthannegatingtheobservationsmadebetweenlatitude,weathering,andPconcentrations
becausethePCAeigenvaluesaresolow(most<±0.2),andessentially,areclusteredontheorigin,
whichindicatesanotpredictivevalue.P’srelationshipwithweatheringiscomplexandduetoa
varietyoffactors(climate,time,slope/erosionrate,etc.),andbylookingonlyattheendproduct—
whichiswhatisleftinthegeologicrecordforanalysis—wemustinherentlyworkaroundthose
limitationsanddrawthemostrobustconclusionspossiblefromlimiteddataontheselargescales.
Fetotbehavesasexpected,accumulatinginBhorizonsassoilweatheringincreases,whileP
accumulationinallhorizonspeaksatmoderatelyweatheredsoils(CIA~60).Thelatterrelationship
isexpectedbecauseasoilthathasexperiencedmoderateweatheringandhasdevelopedsomewhat
(e.g.,Inceptisol,Alfisol)hashadsufficienttimetohavePmobilizedfromthebedrock/substrateand
bioticallycycled,butnotsolongorwithsuchintenseweatheringthatPisdepletedfromthesubstrate
andremovedviaerosionand/orlossofbiomass.Anolderand/ormoreintenselyweatheredsoil(e.g.,
Ultisol)willhaveasubstratemoredepletedofPandwillhavelostmoreP.IntermsofterrestrialP
transport,amoderatelyweatheredsoilisthemostlikelytohavealargepoolofpotentialPto
transport.Aninterestingnextstep,buildingoffthiswork,couldbetotesttheseexpectedcorrelations
bymappingsoilPwithsoilageandcollectingfluvialPloads.
Pconcentrationsare,onaverage,farbelowthecrustalaverageof870ppmP.Evenaccounting
fordensitydifferencesbetweenatypicalsoil(soil~1.62±0.2gcm3;n=659)[72]andcontinental
crust(e.g.,granite;crust~2.7gcm3),allofthesoilhorizonsaredepletedinPrelativetothecrustal
abundance.Thereissomevariabilitybetweensoilorders,butallbutasmallsubsetwerebelowthe
crustalabundance(i.e.,recentbasaltparentedsoils).Whenconsideringbulkdensity,thetotalFe
concentrationsshowtheopposite,withthesoilFeaverageof4.4%beinggreaterthanthecrustal
averageof3.5%,againwithsomevariabilitybetweensoilordersbutoverallgreaterthanthecrustal
average(SupplementalTableS3).Thishasimplicationsforbiogeochemicalmodeling(seeSection
SoilSyst.2020,4,7322of37
4.7.3).WhilePconcentrationscouldbeexpectedtogenerallydecreasewithincreasedweathering
intensity,amoderatelyweatheredsoilismorelikelytobeatamiddevelopmentalstage(e.g.,
Inceptisol,Alfisol)andsupportingvegetationthatcancyclePwithoutbeingPdepleted(e.g.,[19]).
ThismidCIAaccumulationcouldbereflectiveofthevegetation’simpactonsoilnutrients(e.g.,
mycorrhizalPmobilization,Precyclingthroughbiomass/organicmatterdecomposition),pointingto
akeybalancingpointinasoil’slifespanwherethebedrockisbeingweatheredenoughtomaximize
fertilitywithoutyetbeingdepleted.ForestandAltered/CultivatedsoilshavethehighestCIAvalues
(Figure7),butPconcentrationswererelativelyinvariantbetweentypesofvegetation,suggesting
thatweatheringintensityexertsagreatercontrolonsoilPthanthetypeofvegetation.
Claycontent(bothpercentofgrainsizefractionandabundanceofclayminerals)should
increasewiththedegreeofweatheringasoilhasundergone,andthatisreflectedintheresults(Figure
4D).Consequently,therearesecondarytrendsassociatedwithclaycontent,i.e.,claycontentpeaks
atlowerlatitudeswhereCIAvaluesarethehighest.Fetotandclaycontentshowastrongpositive
correlation,asexpected(Figure6E),butPshowsonlyaweakpotentialdecreaseasclaycontent
increases,supportingtraditionalthinkingthatPisdepletedasasoilisincreasinglyweatheredand/or
older(Figure6F).WhileitiskineticallyfavorableforPtosorbtoFeandAloxides(whichaccumulate
inBhorizonsasasoilages),clays(oftenwithsurficialFeAloxides)maybeequallyasimportantfor
immobilizingPinsoils[82].
Interestingly,ourresultsindicatenopositivecorrelationsbetweenFeoxides(FedithandFeox)and
claycontentgrainsizefraction(SupplementalFigureS9),aswouldbeexpectedbasedonsoil
developmentpatterns(i.e.,higherweatheringintensityleadstoaccumulationofclaymineralsand
Feoxides),suggestingthatwhileparticlesizemaymediateFespeciationduetovaryingdegreesof
reactivity,itisnotaprimarycontrol.WhiletheUSGSdatabase’sclaycontentisbasedonmineralogy
(determinedbyXRD),soilsusedintheFespeciesworkhadclaycontentdeterminedbysettling,
whichwouldincludebothclay‐ andnonclaymineralsthatwereclaysizedparticles.Thiscould
explainatleastpartiallythelackoftheexpectedtrend.Furtherexplorationofclaymineralogy
specificcorrelations(orlackthereof)withFespeciationpools,inthecontextofsoilredoxstate,is