Belowground effects of porous pavements—Soil moisture and chemical

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EcologicalEngineering51 (2013) 221–228

ContentslistsavailableatSciVerseScienceDirect

Ecological

Engineering

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / e c o l e n g

Belowground

effects

of

porous

pavements—Soil

moisture

and

chemical

properties

Justin

Morgenroth

a,∗

,

Graeme

Buchan

b

,

Bryant

C.

Scharenbroch

c

a_New_Zealand_School_of_Forestry,_University_of_Canterbury,_Private_Bag_4800,_Christchurch_8140,_New_Zealand b_Faculty_of_Agriculture_and_Life_Sciences,_Lincoln_University,_New_Zealand

c_Research_Department,_The_Morton_Arboretum,₄₁₀₀_Illinois_Route_53,_Lisle,_IL_60532,_USA

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received19July2012

Receivedinrevisedform23October2012 Accepted3December2012

Keywords:

Urbanforest Permeablepavement Water

Stormwatermanagement pH

Greeninfrastructure

a

b

s

t

r

a

c

t

Impermeablepavementscoveraconsiderablelandareaincities.Theireffectonthehydrologicalcycleis clear;asabarrierinthesoil–atmospherecontinuumtheyminimiserainfallinfiltrationandevaporation. Porouspavementsarebeginningtoreplaceimpermeablealternativesbecauseofperceivedhydrologic benefits.Theimpactofporouspavementsonsoilmoistureandchemistryastheyrelatetourban vegeta-tionwasinvestigatedinChristchurch,NewZealand.Anexperimentwasestablishedcomprising25plots evenlydistributedamongstcontrols(nopavement,exposedsoil)andfourdifferentpavementtreatments: afactorialcombinationofpavementtype(porous,impervious)andpavementprofiledesign(including orexcludingagreywackegravelbase).ResultsindicatethatpavementsalteredsoilpHfrom moder-atelyacidic(pH=5.75)tomoreneutrallevels(pH=6.3).TheeffectonpHwasgreaterbeneathporous pavements,andalsowhenagravelbasewasincluded.ConcentrationofsoilAl,Fe,andMgdecreased, whileNaincreasedbeneathpavements.Soilmoisturewasconsistentlyhigherbeneathpavementsthan controlplots,exceptfollowingperiodsofheavyrainfallwherehighsoilmoisturemutedalltreatment effects.Throughoutmostofthestudyperiod,soilmoisturecontentwaslowerbeneathpavementprofiles designedwiththegravelbase,presumablyduetothegravelactingasacapillarybreaktoadistillation process,wherebysoilmoisturemigratesupwardstothesoilsurface.Inearlyautumn,whensoilmoisture contentwaslowestforalltreatments,precipitationrechargedsoilmoistureincontrolplotsandbeneath porouspavements.Butimperviouspavementspreventedinfiltrationresultinginsignificantlylowersoil moisturecontentbeneaththesepavements.Pavementscanaltersoilmoistureandchemical characteris-tics,buttheeffectsdifferdependingonpavementporosityandprofiledesign.Implicationsoftheresults pertaintostressphysiologyofurbanvegetation,inparticulardroughtstressavoidance.

1. Introduction

Soil physical and chemical conditions are critical for plant growth, determining the availability of water and nutrients. Despite anthropogenicdisturbance,highly modified urbansoils supportextensiveanddiverseplantlife,includingtrees. Ofthe anthropogenicartefacts,oneofthemostpervasiveareimpervious pavements, which are known to modify physical and chemi-cal properties of soil including moisture content (Wagar and

Abbreviations: IP,imperviousconcretepavement;IP+,imperviousconcrete pavementwithgravelbase;PP,porousconcretepavement;PP+,porousconcrete pavementwithgravelbase.

∗_{Corresponding}_author._Tel.:₊₆₄₃₃₆₄_2128.

E-mail addresses: justin.morgenroth@canterbury.ac.nz (J. Morgenroth), bscharenbroch@mortonarb.org(B.C.Scharenbroch).

Franklin, 1994), temperature(Celestian and Martin, 2004),and pH(Messenger,1986).Bydoingso,pavementsimpactthegrowth and survival of urban vegetation. Though the vast majority of pavements are impervious, porous pavements (also known as permeable,pervious,orno-finespavements)havebeeninstalled morefrequentlyforstormwatermanagementorasaskid-resistant surfacecourse(Ferguson,2005).Theliteraturesupportsincreased infiltration(Beanetal.,2007)andevaporation(Starkeetal.,2010) ratesfor porouspavements, sowe knowthat urbanhydrology isaltered. Butthe literaturefailsto addresshow porous pave-mentsaffectsoilcharacteristicsthatinfluenceplantgrowth,like water content, pH, and nutrient concentrations. Are these soil conditions modified by porous pavements, and moreover can porouspavements affect adjacent vegetation? The second part ofthisquestionhasbeeninvestigated,thoughresultsare incon-sistent. Porous pavements increased root and shoot extension andbiomassofseedlingsrelativetoimperviouspavementsunder

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222 J.Morgenrothetal./EcologicalEngineering51 (2013) 221–228

Fig.1. Planandcross-sectionalviewofplotdesignsforpavementtreatmentswithandwithoutagravelbase.Soilmoisturesensorsarepositionedat5cm,10cm,or20cm beneaththepavement,orthegraveldependingonthetreatment.

specific conditions (Morgenroth, 2011; Morgenroth and Visser, 2011),butmaturetreeswereunaffected(Volderetal.,2009).It isreasonabletoexpectthatanytreegrowthdifferences related toporous pavementsare caused by changes in underlyingsoil conditions.This paperteststhehypotheses thatsoil water and chemicalcharacteristicsarealteredbypavements,andalsothat theydifferbeneathporousandimperviouspavements.Indoing so,weexplorehowpavementporosityanddesignaffectthesoil conditionsknowntoinfluencethegrowthofurbanvegetation.

2. Methods

2.1. Studysite

The site for thestudy was a council-ownedtree nursery in Christchurch,NewZealand(latitude:−43.493,longitude:172.437). Christchurchhasatemperateclimate,characterizedbymeandaily maximumtemperaturesrangingfromc.10◦_C_in_July_to₂₁◦_C_in January(McGann,1983).Drynorth-westerlywindsoccurduring springandsummer,whentemperaturescanreach30◦_C_and rela-tivehumiditycandropto20–40%(McGann,1983).Rainfallranges from600to700mmannuallyandisgenerallyevenlydistributed throughouttheyear,withatendencyforslightlyhigherearly win-ter precipitation (McGann,1983). The top meter of soil at the experimentsiteisafinesandyloam(Raeside,1974)overlyinga depositofgreywackesandandgravel,aremnantofalluvial out-wash(BrownandWeeber,1992).Greywackeisasedimentaryrock consistingofangularfragmentsofquartzandfeldspar.

2.2. Sitepreparationandexperimentaldesign

In July 2007,prior toinstalling pavements onsite, soil was cultivatedtoremoveexistingturfandensureuniformphysical con-ditionsto30cmdepth.Resultingmeansampledbulkdensityof thisupperlayerwas1.26mgm−3₍_n₌_5,_s.e.₌_0.07)._Following_site

preparation,25plotswereestablishedina5×5patternwith inter-plotdistancemeasuring50cminalldirections.Plotsizeandlayout

wererestrictedbyavailablespace.Plottreatmentswereinstalled inanaugmentedfactorialdesignconsistingofcontrols(no pave-ment,exposedsoil)andfourdifferentpavementtreatmentssplit evenlyamongstplots,suchthatfivereplicatesexistedper treat-ment.Treatmentswererandomlyassignedtoplots.Herbicidewas appliedtocontrolplotsasnecessarytokeepthesurfacefreeof veg-etation.Thepavementtreatments,measuring2.3m×2.3m,were basedonthefactorial combinationof pavement type(2 levels: porous,impervious)andpavementprofiledesign(2levels:withor withoutgravelbase)(Fig.1).Theresultingfourtreatmentswere impervious concrete pavement (IP), impervious concrete pave-mentwithgravelbase(IP+),porousconcretepavement(PP),and porousconcretepavementwithgravelbase(PP+).Itisimportant tonotethataspartofalargerresearchprogram(seeMorgenroth, 2011;MorgenrothandVisser,2011),treeswereplantedinthe cen-treofeachplot,witha30cmdiametercircularcutoutmadeinthe pavementtofacilitatetreeplanting(Fig.1).

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J.Morgenrothetal./EcologicalEngineering51 (2013) 221–228 223

were892kPa(s.e.=111),874kPa(s.e.=125),808kPa(s.e.=112), 2458kPa(s.e.=163),and2363kPa(s.e.=162)forcontrol,IP,PP,IP+ andPP+respectively.

BothIPandIP+plotswereoverlaidby100mmimpervious con-cretecomprisedof1260kgof13mmroundedaggregate,700kg sand,250kgPortlandcement,and160kgwaterpercubicmeter. BothPPandPP+plotswereoverlaidby100mmporousconcrete comprised of 1523kg of 6mm angular aggregate, 243kg Port-landcement,and50kgwater percubicmeter(FirthIndustries, Christchurch).Portlandcementiscomprisedofcalciumsilicates andsulphates,andoxidesofaluminium,iron,sodium,potassium, andmagnesium.Theporousmixdesignwasspecifiedtoachieve 30% porosity, however tested core samples achieved only 11% porosity(ASTM,2008).Thoughunderspecification,theporosityis unlikelytohaveimpededprecipitationinfiltration(deSolominihac etal., 2007).Measuredhydraulicconductivity of1.04cms−1 _in

thecoresamplesexceededlocalstandardsforporouspavement permeability(ACC,2003).

2.3. Datacollection

2.3.1. Soilmoisture

To measure soil moisture beneath pavements, three probes (ECH2O EC-20,DecagonDevices, Inc.)wereburied 5cm, 10cm,

and20cmbeneaththesoilsurfacehalfwaybetweenplotcentre andedge(75probesintotal)ofthe25plots.Sensorswereinserted paralleltothesoilsurface,withtheirflatsurfaceverticalto min-imisehydrologicaldisturbances.Soilvolumetricmoisturecontent (soil)wasmeasuredevery5min(thenaveragedbythehour)for

42weeksspanningfromJune2008toMarch2009.Following pre-viousauthors(e.g.Baumhardtetal.,2000;LaneandMackenzie, 2001)soilmoistureprobeswerecalibratedforuseatthestudysite usingthefollowingcalibration:

soil=1.2447·probe+3.5422

Heresoil(%)isthecalibration-adjusted,volumetricsoilwater

con-tent,andprobe(%)istheECH2Oprobepredictedvolumetricsoil

moisturecontent fromthemanufacturer’s calibration.By post-processingthedatawiththiscalibration, theaccuracyofsoilis

reportedly±2%(DecagonDevicesInc.,2006).

Asapointofreference,thepermanentwiltingpoint(PWPat 1.5MPa)andfieldcapacity(FCat0.01MPa)ofthesoilwere mea-suredfromsamplesextractedfrom5to10cmdepth.ThePWPof thesoilis11.1%(v/v)andwasmeasuredbypressureplate(Model 1500CeramicPlateExtractor,SoilMoistureEquipmentCorp.,Santa Barbara,CA),whilethefieldcapacitymeasuredviatensiontables is27.9%(v/v).Itisimportanttonotethatbecauseofthesampling depth,thesevaluesmaynotberepresentativeofalltreatments.In particularbothIP+andPP+treatmentswheretheupper20cmof soilwasreplacedwithagravelbase.

2.3.2. Soilchemistry

InMarch2009,pavements andgravel layers wereremoved, allowingunderlyingsoiltobeaccessed.Foursoilsub-samplesper plotwerecollectedfromtheuppermost10cm soilbya16mm internaldiametersoilprobe(AMS,Inc.,AmericanFalls,USA)and bulked together suchthat a single compositesample could be analysed for each plot.Analysisundertaken by Lincoln Univer-sitylaboratoriesincludeddetermination of pH,as wellastotal availableconcentrationsofcalcium(Ca),potassium (K), magne-sium(Mg),sodium(Na),iron(Fe),andaluminium(Al)viaICP-OES (InductivelyCoupledPlasma-OpticalEmissionSpectroscopy).Soil samples(0.7g)werefirstdigestedin5mlofconcentratednitric

acid(HNO3)(70%,15.7M)and5mlofhydrogenperoxide(H2O2)

(30%,9.8M),followingsimilarmethodstoSahandMiller(1992).

2.4. Statisticalanalysis

Thefactorialcombinationofpavementtypeandpavement pro-file design, plus a true control (bare soil) required that mean volumetric soil moisture, pH, and soil nutrient concentrations be compared via one-way analysis of variance (ANOVA) using orthogonal,apriori,singledegree-of-freedomcontraststo exam-inetreatmenteffects,andinteractionsofinterest(Marini,2003). Contrastswereasfollows:

1.Controlversusallpavementtreatments.

2.Maineffect(pavementprofiledesign): ±compactedsubgrade andgravelbase.

3.Maineffect(pavementtype):porousorimpervious. 4.Interactioneffect:pavementprofiledesign×pavementtype.

Allsignificantdifferencesarereportedforp<0.05,unless oth-erwisespecified.AnalyseswereperformedusingtheRstatistical package,version2.8.1(RDevelopmentCoreTeam,2008).

3. Results

3.1. Effectofpavementonunderlyingsoilmoisture

Soilmoisturedifferencesresultingfromoverlyingpavements occurredthroughout the majority of the studyperiod, but the durationoftreatment-relatedstatisticallysignificantdifferences dependedon pavement type, profile design, and measurement depth.Duetotheseasonalvariability ofthesoilmoisturedata, genericinferenceabouttheeffectsofpavementwasimpossible.So, dataweresplitinto4timeperiods:earlywinter(weeks1–9),late winter(weeks10–15),spring–summer(weeks16–37),andearly autumn(weeks38–42).

3.1.1. Earlywinter(weeks1–9)

Duringearlywinter,twopatternswereapparent.Inthe upper-most10cmofsoil,soilwaslowerbeneathpavementwhoseprofile

includedagravelbasethaninthoseplotswithoutthegravelbase (Fig.2).Pavementporosityonlybecameasignificantfactordeeper inthesoilprofile.Soilmoisturewasgreaterbeneathporousrather thanimperviouspavementsfortheentireearlywinterperiodat 20cmdepth,andformostoftheearlywinterperiodat10cmdepth.

3.1.2. Latewinter(weeks10–15)

Followingpersistentandrelativelyheavyrainfallduringearly winter,pavementeffectsonsoilweremutedduringlatewinter.

Soilmoisturereachedmaximumvaluesforalltreatmentsandwas consistentlyabovefieldcapacityatalldepths(Fig.2).Insuch condi-tions,therewasnoimpactofpavementinstallation,type,orprofile designonsoil.

3.1.3. Springandsummer(weeks16–37)

Fromearly spring onwards rainfall declinedand sotoo did soilmoisturevalues (Fig.2).By theend of summer,soil in all

plotshaddeclinedtotheirlowestlevels,thoughratesofdecline differedamongsttreatmentsanddepths.Rapiddeclineswere par-ticularlyprevalentatshallowdepths(5cm)incontrolplots.Soil moistureinpavedplotsdecreasedmoregradually,causing con-trolplotstohavesignificantlylowersoilthanpavedplotsfor20

ofthe22weekscomprisingthespring–summerperiod(at5cm depth).Withincreasingdepthsoilincontrolplotsdecreasedmore

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were drierthan paved plots. At 10cm depth, the averagesoil

betweenpavedandunpavedplotsdifferedfor10weeks,whileat 20cmdepth,differencesonlylasted2weeks.Differences result-ingfrompavementprofiledesignwereconsistentthroughoutthe spring–summerperiod.Theeffectwasindependentofdepthand resultedinpavedplotswithoutagravelbasehavinggreater under-lyingsoilthanthosewithagravelbase(Fig.2).Duringthisperiod

ofrelativelylowrainfall,thedurationofdifferencesattributedto porousversusimperviouspavementswasveryshort.

3.1.4. Earlyautumn(weeks38–42)

Thefinaltimeperiod,earlyautumn,beganwithafewweeksof consistentrainfall.Inresponsetothis,soilincontrolplotsandboth

porouspavement treatmentsincreasedappreciably(Fig.2).The resultisasignificantpavementtypeeffectatalldepths,whereby plotspavedwithimperviouspavementshadlowersoilmoisture contentsthanthosewithporouspavementsandcontrolplots.

3.2. Soilmoistureresponsetorainfallevents

Themagnitude of day-to-day _soil fluctuations dependedon treatment.Unpavedsoilsexhibitedhighlyvariablesoil,whereas

fluctuationsbeneathpavementswerelesspronounced.An illus-trativeexampleispresentedforweeks37–39(Fig.3),wheresoil moistureincontrolplotsincreasedsharplyinresponseto precip-itationevents;meansoil increased11%forcontrolplotsduring

these3weeks.Soilmoisturebeneathporouspavingalso exhib-itedanacuteresponsetoprecipitationevents.Increasesof7.8% and6.5%weremeasuredforporouspavementswithandwithouta gravelbase,respectively.Ontheotherhand,soilinplotscoveredby

imperviouspavementsdidnotappeartobeaffecteddirectlyby pre-cipitation.Soilmoistureincreasedbyonly1.1%beneathimpervious pavementswithagravelbase,andactuallydecreasedby0.9%for imperviouspavementswithoutagravelbase.Itappearsthat imper-viouspavementcoverbuffersunderlyingsoilfromacuteincreases ofsoilmoistureresultingfromprecipitation.

3.3. Soilchemistry

MeansoilpHrangedfrommoderatelyacidicincontrolplots toneutralinsomepavedplots(Table1).Allpavementtreatments significantlyalkalinizedsoilpHrelativetocontrolplots,with pro-filedesignandtypeaffectingthescaleofchange.Withinpavement treatments,plotswithagravelbasehadhighermeanpHthanplots wherethesewerenotincorporatedinprofiledesign.Meanwhile, porouspavements,regardlessofprofiledesign,increasedpH rela-tivetoimperviouspavements(Table1).

Relativetocontrolplots,pavementtreatmentsdidnotaffectCa orKconcentrations,butdidaffectconcentrationsofNa,Mg,Fe,and Al(Table1).ConcentrationsofAl,Fe,andMgwerelowerbeneath pavedplotsthancontrolplots,whereasNawasgreaterbeneath pavements.Differencesinsoilnutrientconcentrationswerealso evidentamongstthefourpavementtreatments.Concentrationsof Ca,Fe,Mg,andKwerelowerbeneathpavementsincorporatinga gravelbase.Pavementtypealsoaffectedsoilnutrient concentra-tions.PorouspavementsdecreasedAlandFeconcentrations,but increasedKandNa.

4. Discussion

Porouspavementsdonotguaranteeimprovedtreegrowth rela-tivetoimperviouspavements(Volderetal.,2009).However,trees grownintheseplotsduringthesamestudyperiod(aspartofa largerresearchprogram)grewupto50%largerwhensurrounded byporouspavements(Morgenroth,2011;MorgenrothandVisser,

2011).Theresultsofthisexperimentsuggestthatthecontrasting soil moistureand chemical characteristics beneath porous and imperviouspavementsmaybecontributingfactorstoobserved treegrowthratedifferences.Aconceptualmodelispresentedin

Table2anddiscussedbelow.

4.1. Soilmoisture

Forthedurationofthisexperiment,soilmoisturewasgenerally greaterbeneathpavements,supportingsimilarfindingsbyother researchers(WagarandFranklin,1994).Itissuggestedthattwo compoundingmechanismsareresponsibleforthisresult.Thefirst isadistillationprocess,wherebywatervapourdiffusestowards, thencondenseson,acoolsurface.Distillationiscausedbydiurnal fluctuationinsoiltemperature,duetosoilsgainingthenreleasing heatduringasingleday.Soilsnotonlygainheatenergyandreach theirmaximumtemperaturelaterthanmaximumairtemperature, butalsodecreaseintemperaturelaterthanairdoes(Buchan,2001; CelestianandMartin,2004).Intheearlyevening,asairtemperature dropsandthesoilsurfacecools,watervapourisdrawnupwards andcondensesontheundersideofthepavement,thendrainsback intotheuppermostlayerofsoil.Thoughdistillationmayalsooccur inunpavedsoils,thereisnopavementtoblockmoisturemigration (andhenceretainwaterinthesoil).Also,thediurnaltemperature rangeofpavedsoilsexceedsthatofunpavedsoils(AsaedaandCa, 2000).Thus,distillationisamplifiedbeneathpavedsurfaces.

Thesecondreasonforhighersoilmoisturebeneathpavements isthattheybufferthesoilfromatmosphericdemandforwater, thusminimisingevaporationloss.Duetothelargeinterconnected poresthatcharacterizeporouspavements,itwasinitiallybelieved thatthispavementtypewouldenablecomparableratesof evapora-tiontocontrolplots.However,inpracticethelargeporespreclude capillaryupflowofwaterthroughthepavement(Andersenetal., 1999).Aswaterislimitedtothesoil/pavementboundaryandnot thepavement/atmosphereboundary, evaporationfrombeneath porouspavement,likethatfrombeneathimperviouspavement,is negligible.Together,distillationandevaporationprocesseslikely drivethedifferences in soilmoisture dynamics beneath paved andunpavedsurfaces.Incontrolplots,thecombinationofweaker distillationand a drying front causedby evaporationresultsin adepth-dependentsoilmoistureprofile,wherebyrelativelylow soilmoisturecontentoccursatshallowsoildepths,andrelatively highersoilmoistureoccursatdeepersoildepths. Thisexplains whytheincidenceanddurationofsignificantdifferencesbetween controlandpavedplotsdiminishedwithincreasingdepth.

Thesignificantdifferencebetweenpavementprofiledesignscan likelyberelatedtotheeffectofthegravelbaseonsoilmoisture movement.Itisbelievedthatthegravelbaseactedasacapillary break,therebylimitingdistillation.Therelativeeffectofdistillation betweenplotswithandwithoutagravelbaseisillustratedbythe soilmoisturedynamicsduringthespring–summerperiod. Follow-ingthewinterrainsmeansoilinallplotswerestatisticallysimilar.

Bytheendofsummer,soilinplotswithgravelbasesdeclinedon

average9.4%,whileinplotswithoutagravelbase,decreases aver-agedonly2.2%.Weinferthatthefasterrateofsoildeclineisrelated

totheinclusionofagravelbaseandthatthislimiteddistillation. Withoutdistillationtoreplenishwaterinthesurfacesoil,soilwas

lowerinplotsdesignedwithagravelbase.

Thoughsoilmoisturebeneathporousandimperviouspavement treatmentsweregenerallysimilar,differencesdidoccurwhereby soilbeneathporouspavementsexceededthatbeneathimpervious

pavements.Naturally,porouspavementsallowedformorerapid infiltrationof precipitation,thereby ensuring greatersoil.Why

thenwouldsoilbeneathporouspavementsnothavebeengreater

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Fig.3.Dailyresponseofsoilmoisture(averageof5cm,10cm,and20cmvalues)toprecipitationduringweeks37–39.Theshadedregionrepresentstheplant-available waterrangebetweenthefieldcapacityandthepermanentwiltingpoint.

Table1

TheeffectofpavementtypeandprofiledesignonsoilpHandsoilnutrientconcentrations(uppermost10cm).Valuesshownrepresentmeans(1standarderror).Thebottom halfofthetableshowsp-valuesforsingledegree-of-freedomcontrasts.*p<0.05.

Treatment Reaction Soilnutrientconcentration(mgkg−1₎

pH Aluminium Calcium Iron Magnesium Potassium Sodium

Control 5.75(0.03) 5.90(0.40) 891(32) 5.35(0.37) 81.1(2.6) 164(9.3) 28.7(2.0)

Impervious 6.00(0.07) 3.91(0.58) 918(30) 4.28(0.26) 80.9(2.0) 184(5.8) 35.9(3.1)

Porous 6.35(0.05) 1.99(0.38) 912(28) 2.44(0.33) 74.4(1.7) 225(8.8) 40.9(2.7)

Impervious+gravelbase 6.26(0.03) 3.39(0.35) 812(43) 2.00(0.24) 60.2(2.5) 121(5.5) 25.7(1.4) Porous+gravelbase 6.58(0.06) 1.80(0.52) 880(34) 1.35(0.26) 58.1(3.1) 147(5.0) 44.8(3.3)

Controlversusallpavements <0.0001* <0.0001* 0.77 <0.0001* <0.0001* 0.54 0.01*

Maineffect(profiledesign) 0.001* 0.45 0.04* <0.0001* <0.0001* <0.0001* 0.25

Maineffect(pavementtype) <0.0001* <0.0001* 0.36 <0.0001* 0.08 <0.0001* <0.0001*

moisturecontentsbelowbothporousandimperviouspavingfor mostofthemeasurementperiodmayhaveprecludedany appre-ciableeffect ofinfiltration. Thisis becausewetsoilscan retain relativelylessadditionalwaterthandrysoil,thuspartlynegating anyimpactofincreasedinfiltrationviaporouspavements.Thedata supportthis,assignificantsoilmoisturedifferences,resultingfrom pavementtype,occurredonlywhenpre-rainfallsoilmoisturewas relativelylow,orfollowingaperiodofsubstantialsoilmoisture decline.Soilmoistureinearlyautumniscrucialforillustratingthis point.Followingasummeroflowandintermittentprecipitation, soilmoisturehaddroppedtotheirminimumsforalltreatments. Then,severalweeksofconsistentrainfallsawanacuteincreasein

soilforcontrolplots andbothporoustreatments.Ontheother hand,soilin plotscovered byimperviouspavementsincreased byonlyasmallmarginbeneathIP+plotsandactuallydecreased beneathIPplots.

The knowledge that soil beneath pavement was generally wetter than unpaved soil and that porous pavement allowed infiltration of rainfall more so than impervious pavement has implications for urban design. In areas where droughts are expectedtoincreaseasaconsequenceofclimatechange,porous pavementsmaybeavaluableformofwatersensitiveurbandesign (analogoustolow-impactdesigninNorthAmericaandSustainable UrbanDrainageSystemsintheUnitedKingdom).Comparedwith

Table2

Aconceptualmodelshowingtherelativeimpactsofporousandimperviouspavementsandtheirdesignonsoilmoistureandchemistry.

Process Treatment

Control Impervious Porous Impervious+gravelbase Porous+gravelbase

Infiltration High Low High Low High

Distillation Low High High Low Low

Alkalinization Low Moderate High Moderate High

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imperviouspavement,thereispotentialforporouspavementto mitigatedroughtstressforurbanvegetation.

Theresultsofthisstudymustbeconsideredincontext.Due tospacerestrictionsatthestudysite,plotsizeandinter-plot dis-tancewaslimited.Oneconcernwasthatpavementsmeasuring 230cm×230cm may not besufficiently largeto influence soil moisturedynamics.However,significantdifferencesamongstthe treatmentsshowedthatsoilmoisturewasaffectedbytreatments, thusdispellingthatconcern.Anotherconcernwasthatwithinplot soilmoisturevaluesmaynotonlybeentirelyreflectiveof treat-ment,butalsothetreatmentofadjacentplotsduetorainfallrunoff. Insandyloams,likethoseatthestudysite,lateralwaterflowis limitedduetohighhydraulicconductivity(SiyalandSkaggs,2009). Moreover,therandomizedplotdesignwillhaveminimisedthe sta-tisticalconsequence.Giventhearealimitationsofthestudysite,we believethatthechosenexperimentaldesignminimisestheimpact ofedgeeffectsonresponsestotreatments.

4.2. Soilchemistry

Relativetocontrolplots,allpavementtreatmentsincreasedthe meanpH,withfurtherdifferencesrelatedtopavementtypeand profiledesignmaineffects.Thisresultissupportedbyother stud-ieswhichhavefoundsimilarincreasesinsoilpHnearorbeneath roads(Messenger,1986;Parketal.,2010).Butourresultswent fur-therthansimplyshowingthatpavementscanalkalinizeunderlying soil.Likeprevious,recentresearch(KuangandSansalone,2011), weshowed thatsoil wasmore alkalinebeneath porous,rather thanimperviouspavement.Thereasonsforthisarethatporous pavementscontainagreaterproportionofcementthan impervi-ouspavements(Ferguson,2005)andtheirhydraulicconductivity isrelativelyhigh(Sansaloneetal.,2008).So,asrainfallinfiltrates throughthetortuousporesoftheporouspavement,moreCa(OH)2

(inthecementpaste)isexposedtowater,resultingingreaterCa2+

inputstotheunderlyingsoil.Thisisincontrasttoimpervious pave-mentswherewaterisintentionallychannelledoffthepavementto preventitsinfiltrationintothesoil.

OurresultsalsoshowedhighersoilpHinplotswithagravel basethan in those without. Thegravel base wascomprised of unwashed greywacke stone coated by clay microfine particles (<75␮m)(Mu ˜nozetal.,2010).Itisbelievedthatthe alkaliniza-tionintheseplotsisattributabletoweatheringandcationrelease ofthegreywackematerial(Guimarães,2010).

Theimportanceofpavement’seffectonsoilpHisthatpHaffects mineralsolubilityinbothorganic(LucasandDavis,1961)and min-eralsoils(Truog,1948).Thoughmineralsolubilityvariesdrastically withchangingpH,phosphorus,iron,manganeseandother micro-nutrientsmaybeunavailabletoplantsinalkalinesoils(Larcher, 2003),whileinacidicsoils,aluminiumandironmaybepresent attoxiclevels(Sparks,2003).Allnutrientshavesolubility thresh-oldsand,ofthoserequiredforplantfunction,manyareknownto decreasewithincreasingalkalinity,includingboron,copper,iron, zinc,magnesium,potassium,andphosphorus(Larcher,2003;Lucas andDavis,1961).So,ifpavement(inparticularporouspavement) increasesthepHofsoilstovalueswhichlimitnutrientsolubility, thereispotentialtoaffectplantfunctionandgrowth.

Observedincreases in nonacidcations in this researcharise fromaccumulationsexceedingremovalsvialeachingfromthe0to 10cmsoillayer.ThegreatestsignificantdifferencesforK,Mg,Ca, andFeoccurredbetweenthepavedtreatments.Onaverage,paved treatmentswithagravelbasehad53%,31%,8%,and84%lower con-centrationsofK,Mg,Ca,andFecomparedtopavementtreatments without.Thegravelbaseappearedtocauseacapillarybreak,limit infiltrationanddistillationprocesses,andoverallreducesoil mois-turecontentsofthe0–10cmlayer.Thereductionsinthesecations

arelikelyaresultoflesswatermovementandmineralweathering andalsoreduceddistillationandilluviationinthe0–10cmlayer.

Regardlessofdesigntreatment,bothNaandKinthe0–10cm soillayerweregreaterunderporouspavementcomparedto imper-vious pavement. The possible reasons for this are: (1) porous pavementscontaingreaterproportionsoftheseelementsandhave greaterhydraulicconductivity,resultingingreaterinputsofthese basestotheunderlyingsoil,and(2)increasedsoilwaterunder porouspavementsledtoincreasedweatheringandreleaseof min-eralKandNa.ItispuzzlingthatMgandCaalsodidnotincreasein soilunderporouscomparedtoimperviouspavement.Ofthemetals studied,Mgis themostmobile (Schaetzland Anderson,2005). CoulditbethatincreasedMginputsassociatedwithporous pave-mentswerequicklydissipatedthroughrelativelyfasterleaching outofthe0–10cmlayer?Couldthenaturallyhigherbackground levelsofnativeCainthesoilbemaskingpossibletreatmentinduced differencesassociatedwithinputsfromporouspavement?

5. Conclusion

These results expand on previous research by the authors (MorgenrothandBuchan,2009)byextendingthetemporal com-ponentofsoilmoisturemeasurementtonearlyacompleteyear, therebytaking intoaccount importantseasonal climaticcycles. During this experiment, soil moisture was consistently higher beneathpavements,exceptfollowingthewinterrainswherehigh soilmoisturemutedalltreatmenteffects.Pavementsalsobuffered soilsfromlargedailysoilmoisturefluctuations.Theinclusionof agravelbaseinthepavementdesignactedasacapillarybreak, limiting the distillation effect that brought water to the sur-faceunderpavements.Throughoutmostofthestudyperiod,this resultedinlowersoilmoisturecontentbeneathpavementprofiles designedwiththegravelbase.Thoughsoilmoisturedifferences beneathporousandimperviouspavementsdidnotexistforthe majorityofthemeasurementperiod,animportanteffectwasseen duringearlyautumnprecipitationevents.Imperviouspavements preventedtherapidinfiltrationofwaterfollowingsmallrainfall events,whereasporouspavementsallowedinfiltration.Soil mois-turewasrechargedbeneaththeseporouspavements,butremained atlowlevelsbeneathimperviouspavements.Thisispotentially themostimportantfindingofthisresearch,butfurtherworkis required to determine whethersuch late season soil moisture rechargescanminimisedroughtstressforurbanvegetation.

Following19monthscoveredbypavements,soilpHwasaltered frommoderately acidic toneutral. Effects on pHwere greater beneathporousratherthanimperviouspavement,andalsowhen agravelbasewasincludedinthepavementprofiledesign.This affectedmineralsolubility,reducingthesoilconcentrationofAl,Fe, andMg,whileincreasingtheNaconcentrationbeneathpavements. Thisresultisspecifictothisexperiment,butcouldpotentiallyoccur inotheracidicsoils.Wherenativesoilisneutraloralkaline,the effectofpavementsonsoilpH(andhencemineralsolubility)may differ.

Inconclusion,theinstallationofpavementscanaltersoil phys-icalandchemicalcharacteristicsnecessaryforplantgrowthand survival. Porous pavements canincrease water availability and altersoilpHandmicronutrientavailability.Futureresearchcould extendorcomplementthisworkbygeneralizingresultsacrossa varietyofsoiltypes.

Acknowledgements

(9)

TheauthorswishtothankLachlanKirk,NigelPink,JoeCartman, NeilSmith,AlwynWilliams,andLisaKulczyckiforassistancewith fieldwork.

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