physical and chemical characteristics of ash-influenced ... · volcanic ash-cap textures are...

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In: Page-Dumroese, Deborah; Miller, Richard; Mital, Jim; McDaniel, Paul; Miller, Dan, tech. eds. 2007. Volcanic-Ash-Derived Forest Soils of the Inland Northwest: Properties and Implications for Management and Restoration. 9-10 November 2005; Coeur d’Alene, ID. Proceedings RMRS-P-44; Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

P. A. McDaniel is a Professor of Soil Science, Soil & Land Resources Division, University of Idaho, Moscow, ID. M. A. Wilson is a Research Soil Scientist, U.S. Department of Agriculture, Natural Resources Conservation Service Soil Survey Center, Lincoln, NE.

Abstract HoloceneashfromthecataclysmiceruptionofMountMazama(nowCraterLake)insoutheasternOregonisamajorcomponentofmanyforestsoilsthatlietotheeastofCascadeMountainsinthePacificNorthwestregion.Therelativelyhighproductivityoftheregion’secosystemsiscloselylinkedtothisvolcanicashcomponent.Thispaperreportsontheecologicallyimportantpropertiesoftheseash-influencedsoilsbyexaminingsoilcharacterizationdatafromover500soilhorizonsoftheregioncontainedintheNationalSoilSurveydatabase.Volcanicash-captexturesaregenerallysiltyinareasdistaltotheeruption.Volumetricwater-holdingcapacityofash-caphorizonsisasmuchastwicethatofunderlyinghorizonsandunderscorestheimportanceofashcapsinseasonallydry,forestedecosystemsoftheInlandNorthwestregion.Cationexchangecapacity(CEC)determinedatfieldpH(ECEC)averages8.0cmolckg–1andislessthanone-thirdtheCECdeterminedatpH8.2,indicatingconsiderablevariablecharge.Thesedatasuggestthatash-influencedsoilshavelimitedabilitytostoreandexchangenutrientcationssuchasCa,Mg,andK.Inaddition,strongsorptionofSO4andPO4significantlylimitsthebioavailabilityofthesenutrients.Erosionandcompactionofashcapsaremajormanagementconcerns,butrelationshipsbetweentheseprocessesandash-capperformanceremainunclear.

Physical and Chemical Characteristics of Ash-influenced Soils of Inland Northwest Forests

P. A. McDaniel and M. A. Wilson

Introduction

Soilsformedentirelyorpartiallyinvolcanicejectasuchasash(glassyparticles<2mminsize),cinders (glassyparticles>2mminsize),andpumice (highlyvesicular fragments)arefundamentallydifferentfromothersoilsintermsoftheirphysical,chemical,andmineralogi-calproperties.Becauseofthis,Andisolswereestablishedasthe11thsoilorderinU.S.SoilTaxonomyin1989.TheWorldReferenceBaseofSoilResources(WRB)alsorecognizesthesesoilsasAndosols,oneofthe30soilreferencegroups(FAO/ISRIC/ISSS1998).Soilsformedinvolcanicash,suchastheash-capsoilsfoundinthePacificNorthwestregion,arecharacter-izedbywhatarereferredtoasandic propertiesinSoilTaxonomy(SoilSurveyStaff2003).Andicpropertiesrefertoacombinationofcharacteristics,includingthepresenceofglass,short-range-orderorpoorlycrystallineweatheringproductswithhighsurfacereactivity,andlowbulkdensity.

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McDaniel and Wilson Physical and Chemical Characteristics of Ash-influenced Soils of Inland Northwest Forests

VolcanicashfromtheeruptionofMt.Mazama(nowCraterLake,OR)~7,700yearsBP(Zdanowiczandothers1999)hasinfluencedmanymid-tohigh-elevationfor-estsoilsinthePacificNorthwestregion.Theuniquepropertiesoftheseash-capsareintricatelylinkedtoforestproductivityacrosstheregion(GeistandCochran1991;Meurisseandothers1991).Assuch,thesesoilsrepresentavaluableregionalresourcefrombothaneconomicandecologicperspective.Moreover,ash-capsoilsresponddifferentlytouseandmanagementthandosoilsformedinotherparentmaterials,anditisthereforeimportanttorecognizeandunderstandthebasicdifferencesinproperties.Inthispaper,wedescribesomeoftheimportantcharacteristicsofvolcanicash-influencedsoils.

WeusedtheNaturalResourcesConservationService(NRCS)—SoilSurveyLaboratorycharacterizationdatabaseforWashington,Oregon,Idaho,andMontanatoprovideanoverviewofphysicalandchemicalcharacteristicsofvolcanicash-capsoilsoftheregion.Thesoilsinthisdatabasearerepresentativeofthedomi-nantmappingunitcomponentsinsoilsurveyareas,andhavepropertiesthatareconsideredtypicalofsoilsacrosstheregion.Forinclusioninthedatabase,weidentifiedallhorizonsmeetingeitherthecriteriaforandicsoilpropertiesorandicsubgroupsinSoilTaxonomy(SoilSurveyStaff2003).Allcharacterizationdatawereobtainedusingstandardmethods(Burt2004)andhavebeensummarizedinMcDanielandothers(2005).

Soil Morphology ________________________________________________ThethicknessofashcapsacrosstheInlandNorthwestregionisvariable,but

followsageneraltrendofthinningwithincreasingdistancefromCraterLake(fig.1a).DatapresentedbyMcDanielandothers(2005)showedarangeinthicknessof2to152cmacrosstheregion,withanaveragethicknessof38cm.Itshouldbenotedthatthesereportedthicknessesincludeashcapsthinnedbyerosionaswellasthoseinwhichothersoilparentmaterialshavebeenmixedwiththevolcanicashafterinitialdeposition.Inthelattercase,thereportedthicknessesrepresentthoseofash-influencedmantlesratherthanpureashmantles.Itisimportanttorealizethattheashcapsfoundacrosstheregionhavebeenmixedtovaryingdegreesbypost-depositionalprocessesandrepresentarangeincomposition.Forpurposesofthispaperandotherscontainedintheseproceedings,thetermash capwillbeusedtoincludeallashmantles,regardlessofdegreeofmixing.

Mostoftheofficialsoilseriesdescriptions(SoilSurveyDivision2005)forash-capsoilsoftheregionincludeforestlitterlayersthatareunderlainbyAhorizonshavingthicknessestypicallyrangingfrom2.5-7.5cm(1-3inches).ThedevelopmentofAhorizonsformedinvolcanicashintheInlandNorthwest isusuallyweakdespitethefactthatash-capsoilsgenerallycontainrelativelylargeamountsoforganicmattercomparedtoothermineralsoils(Dahlgrenandothers2004).Thisweakdevelopmentisprobablyduetothefactthatmostash-influencedsoilsareforestsoils,wherethemajorityofcarboniscontainedinlitterlayersandrelativelysmallquantitiesareaddedtoAhorizonsintheformofroots.Thisfactorresultsinrelativelythin,light-coloredAhorizons.Verydark-coloredAhorizonscanforminvolcanic-influencedsoils thatsupportgrassesorunderstoryvegetationwithextensiverootsystems.Deep,darkAhorizonshavebeendescribedunderbrackenfern(Pteridium aquilinum)communitiesinnorthernIdaho(Johnson-Maynardandothers1997).Thesecommunitiesmayhaveasmuchas3,500g/m2ofbelowgroundbiomassintheformofrhizomesandfineroots(Jimenez2005).

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Physical and Chemical Characteristics of Ash-influenced Soils of Inland Northwest Forests McDaniel and Wilson

Figure 1—Change in (a) ash-cap thickness and (b) particle-size distribution of Mazama tephra as a function of distance from Crater Lake, OR. Particle-size data represent averages for the ash caps of Lapine, Angelpeak, Threebear, and Jimlake soils and are adapted from McDaniel and others (200�).

UnderlyingtheAhorizonsarereddish-toyellowish-brownBwhorizonsthatdonotexhibitsignificantincreasesinclaycontent.Huesofthesesubsoilhorizonsaretypically7.5YRto10YRwithchromasof4or6.ThesecharacteristiccolorsreflecttheweatheringofashtoformpoorlycrystallineFeoxidessuchasferrihy-drite(McDanielandothers2005).

Insomehigher-elevationareas,ash-capsoilsmayhaveundergonesufficientpodzolizationtodevelopE-Bh,Bhshorizonsequences.Thesesoilsaretypicallyinhigh-precipitationenvironmentsandhaveextremelyacidEhorizons.IntheSelkirkMountainsofnorthernIdaho,pHvaluesaslowas3.7havingbeenre-ported(McDanielandothers1997),thelowestpHvaluesfoundanywhereintheregionfornativesoils.

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Physical Properties ______________________________________________

Particle-Size Distribution

MuchofthetephradistributedthroughoutthePacificNorthwestwascarriedinsuspensionbyprevailingwesterlywinds,aprocessthathasresultedinfairlydistinctrangesofparticlesizesassociatedwithashcaps.Examinationofashcaptexturesacrosstheregionshowsadecreaseinoverallparticlesizewithincreas-ingdistancefromthesource(fig.1b).ThepumiceregionsofcentralOregonareincloserproximitytoCraterLake,andsand-sizedpumicedominatesinsoilslikeLapine.However,ashcapsfromtheBlueMountains(e.g.Angelpeaksoil),northernIdaho(Threebearsoil),andwesternMontana(Jimlakesoil)arepredominantlysilt-sized,reflectingthegreaterdistanceoftransport.Fine-earthfractionsoftheseashcapstypicallyhavesiltloamorloamtextures,whichcansubstantiallyenhancethewater-holdingcapacityofsoilprofiles.

Bulk Density

BulkdensityofAndisolstendstoberelativelylow,andthisisoneofthedefiningcharacteristicsofthesesoilsinSoilTaxonomyandtheWRB.Ofthe271horizonsexaminedacrosstheregion,theaveragebulkdensityis0.90gcm–3(fig.2).How-ever,only~60perecentofthehorizonshavebulkdensitiesof<0.90gcm–3,sug-gestingthatmixingand/orcompaction,bothofwhichwouldtendtoincreasebulkdensity,haveoccurredsincedeposition.Becausebulkdensityisinverselyrelatedtoporosity,anashcapwithabulkdensityof0.90gcm–3mayhave~40percentmoreporositythanatypicalmineralsoilwithabulkdensityof1.3gcm–3.

Figure 2—Bulk density data for andic soil horizons of the Pacific Northwest region. Taken from McDaniel and others (200�).

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Water-Holding Capacity

Andisols typically are able to retain large amounts ofwater (Dahlgren andothers2004).IntheInlandNorthwestregionwhereextendeddryperiodsoftenoccur during the growing season, this water-holding capacity is arguably themostimportantpropertyfromanecologicalstandpoint.Additionalwater-holdingcapacityassociatedwithash-capsoilsmaybecriticaltotheestablishmentandmaintenanceofsomeplantcommunities.Thebasisfortheadditionalwater-holdingcapacityimpartedtosoilsbyvolcanicashstemsfromthreefactors.First,asdis-cussedpreviously,ashcapshaverelativelyhighporosity.Secondly,muchofthedistalashissilt-sized,whichisdesirablefromthestandpointofwaterretentioncharacteristics.Thisisespeciallytrueinmanymountainousareaswherevolcanicashoverliescoarser-textured,oftenrockysoilswithrelativelylittleplant-available-water-holdingcapacity.Finally,weatheringofvolcanicashgivesrisetoacolloidalfractionthathashighsurfaceareaandisabletoretainconsiderablequantitiesofwater(Wada1989;Kimbleandothers2000;Dahlgrenandothers2004).

Theeffectofavolcanicashcaponthewater-holdingcapacityofanorthIdahosoilisillustratedinfigure3.Theashcapconsistsofthetoptwohorizons,whichhavemorethantwicethewater-holdingcapacity(onavolumebasis)oftheun-derlyingcoarser-texturedhorizonthathasformedinoutwash.Inthisexample,theashmantlecontributes8.6cm(~3.4inches)ofplant-availablewatertothesoilprofile.Ash-capsoftheregionhaveanaveragewaterretentionof~11percent(byweight)at1,500kPaofsoilmoisturetension(wiltingpoint)(McDanielandothers2005),orroughlytwicethatofsandysoils(BradyandWeil2004).

Figure 3—Water retention difference in the top �0 cm of the Bonner series (��ID017002) from north Idaho. Bars represent the difference in water content between –1,�00 and –�� kPa, expressed on a volume basis and corrected for rock fragment content. Shaded bars are for andic materials.

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Whencomparingorinterpretingwaterretentiondata,itisimportanttodistin-guishbetweendatareportedonagravimetric(weight)andthosereportedonavolumebasis.Becausesoilhorizonsformedinvolcanicashtypicallyhavebulkdensities<1.0g/cm3,watercontentswillbegreaterwhenexpressedonaweightbasisratherthanavolumebasis.Theoppositeistrueforothermineralsoilswithbulkdensities>1.0g/cm3.

A mantle or cap of volcanic ash also contributes to a more-favorable soilmoistureregimethroughamulchingeffect.Someoftheincreasesincropyieldsobservedafterthe1980MountSt.Helenseruptionwereattributedtothedeposi-tionoffreshashanditsroleinreducingevaporativewaterlosses(Dahlgrenandothers2004).

Other Physical Properties

SomeAndisolsexhibitthixotropy,whichisdescribedasareversiblegel-sol-geltransformation (Nanzyo and others 1993b). Upon application of pressure orvibration,awetsoilmasswillexperienceasuddenreversible lossofstrengthandbegintoflow.Itcaneasilybeobservedinawell-moistenedash-capsample.Byapplyingpressure to thematerialbetweenthethumbandforefinger,watercanbeforcedoutofthesoilmass;releaseofpressurewillcausethenwatertobere-absorbed.Onalargerscale,thixotropycanresult insoilcollapseunderroadwaysandbuildingfoundations(BradyandWeil2004).Thixotropyismostpronouncedinhighlyweatheredsoilsformedinvolcanicash(Buolandothers2003),andthereforeisnotoftenwellexpressedintherelativelyless-weatheredash-capsoilsoftheInlandNorthwestregion.Soilsformedinvolcanicashalsohavealowbearingcapacitybecauseoftheirlowbulkdensity(Kimbleandothers2000),andthismakesthemsusceptibletocompactionandcancreateproblemsfortraffickingandbuildingfoundations.

Mineralogical Properties _________________________________________

Volcanic Glass

TheprincipalcomponentofMazamavolcanicashisglass,whichwasformedbytheveryrapidcoolingofrhyodaciticmagmaasitwasejectedfromthevolcanoduringtheclimaticeruption(fig.4)(Bacon1983).Theamountofglassinvolcanicashmantlesacrosstheregionisvariable,andmostlikelyreflectspost-depositionalredistribution.IntheBlueMountainsofnortheasternOregonandsoutheasternWashington,relativelyundisturbedashmantlescontain60-90percentvolcanicglassandarefoundinmoisterlandscapepositionswithlowerfirefrequency(Wil-sonandothers2002).Inotherareaswithmoreactivesurficialprocessessuchaserosionafterfire,glasscontentsrangesfrom25-60percent,andthesearereferredtoasmixedmantles(Wilsonandothers2002).Theglasscontentof528ash-capsoilhorizonsoftheregionthatmeetandicsoilrequirementsinSoilTaxonomyaverages~42percentandexhibitsabimodaldistribution(McDanielandothers2005).Thissuggeststhatmixedmantlesarecommonacrosstheregion,andthatlocally,underlyingsoilshaveatleastsomeinfluenceonashcapproperties.



Analysis of Mazama ash deposits shows an elemental composition of 70-72percentSiO2(Bacon1983).Inaddition,McDanielandothers(1997)reportedthatthecombinedAl2O3andSiO2contentofMazamaglasswas~87.5percent,withtheCaO,MgO,andK2Ocontentscomprisingonly4.9percentbyweight.Forcomparativepurposes,Bohnandothers(2001)reportanaverageCaO,MgO,andK2Ocontentforavarietyofigneousrocksof12percent.Itisclearfromthesedata thatvolcanicglass isnota rich sourceofplantnutrientsandshouldnotbethoughtofasahavingremarkablefertility.MinoramountsofplantnutrientssuchasSO4

2–,Ca2+,andPO42–presentintheoriginalMazamaashfallmayhave

providedshort-termfertilityinputs,aswasthecasewiththe1980eruptionofMountSt.Helens(Fosbergandothers1982;Mahlerandothers1984).However,long-termfertilityoftheglassisrelativelylow.

Clay Mineralogy

Becauseofthelackofacrystallinestructure,glassweathersrelativelyrapidlytoformpoorlycrystallineornon-crystallinemineralsintheclayfraction.InslightlytomoderatelyweatheredAndisolssuchasthosefoundinthePacificNorthwestregion,therearethreemineralassemblagesthattendtodominate:(1)Al-humuscomplexes,oftenwithAl-hydroxy-interlayered2:1minerals; (2)allophaneandimogolite,and;(3)poorlycrystallineFeoxides(ferrihydrite)(Dahlgrenandothers2004).Asashweathers,AlandFereleasedintosoilsolutioninsurfacehorizonscanformstableorganiccomplexes,especiallyatsoilpH<5(ParfittandKimble1989);thesepHvaluesarecommonunderconiferousforestvegetationathigherelevationintheInlandPacificNorthwest.Al-humuscomplexesrepresentactive

Figure 4—Volcanic glass shard from north Idaho soil. Glass is from the Mazama eruption and measures ~0.1 mm across. Photo from University of Idaho.

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formsofAlandmaycontributetoAltoxicity(Dahlgrenandothers2004).Intherelativeabsenceofhumusandlessacidconditions,suchasinBandChorizons,AlandSiprecipitatetoformallophaneandimogolite.Thesetwoaluminosilicatemineralsmayhavesimilarchemicalcomposition,butallophanetypicallyconsistsofsphericalunitswhileimogoliteappearsasthreadsortubes(Kimbleandothers2000).Allophaneand imogolitebothhave relativelyhigh surfacearea.UsingdatapresentedbyHarshandothers(2002),WhiteandDixon(2002),andMalla(2002),thesurfaceareaofimogoliteisasmuchas100timesgreaterthanthatofkaoliniteand40percentgreaterthanthatofvermiculite.Thisimportantpropertyisdirectlyrelatedtoboththehighwater-holdingcapacityandchemicalreactivityoftheseminerals.

Chemical Properties _____________________________________________

Soil Reaction

Themajorityofandicsoilhorizonsoftheregionaremoderatelytoslightlyacid.Approximately70percentof thehorizonsexaminedhavepHvaluesbetween5.6and6.5,withanaveragepHof6.02(fig.5).ThispHrangeisgenerallysuitableforplantgrowth,asmanynutrientsarepresentinplant-availableformswithinthisrangeandthepotentialforaluminumtoxicityislow(BradyandWeil2004).

Cation/Anion Exchange

Oneoftheimportantfeaturesofsoilsformedinvolcanicashisthevariablechargeassociatedwith thecolloidal fraction.Variablecharge,also sometimesreferredtoaspH-dependentcharge,meansthatthenetsurfacechargedepends

Figure 5—Soil pH data for ash-cap soil horizons from the Inland Pacific Northwest region.



onthepHofthesoilsolution(SoilScienceSocietyofAmerica2001).Itisthisnetsurfacechargethatdeterminesthecationexchangecapacity(CEC)ofasoilanditsabilitytoretainandexchangenutrientcationssuchasCa2+,Mg2+,andK+.TheCECofandicsoilsdecreaseswithdecreasingpH,andmayinfact,bequitelowattheacidicpHvaluesthatcharacterizemanyforestsoils.Moreover,someandicsoilsmayhavesubstantialanionexchangecapacity(AEC)atacidicfieldpHvalues(Kimbleandothers2000),influencingtheretentionofionssuchaschlorideandnitrate.AlthoughPO4

2–andSO42–ionsaresorbedthroughanion

exchange, fixation of these ions (described below) is much greater than thisexchangereaction.

TheeffectofvariablesurfacechargeisillustratedbyCECandbasecationdatafromash-capsoilsacrosstheregion.Table1showsCECvaluesdeterminedfor515horizonsusingthreeprocedures.Thelowestvaluesareforeffectivecationexchangecapacity(ECEC),wheretheCECisdeterminedatthepHofthesoil.ThehighestvaluesareforCECdeterminedatpH8.2(CECpH8.2),whichisconsiderablyhigherthanthesoilpHoftheregion.AlthoughthislattermethodoverestimatestheCECthatexistsinthefield,thedifferenceinECECandCECpH8.2valuesillus-tratesthepotentialvariablechargethatexistsinthesesoils.Onaverage,CECpH8.2valuesareapproximatelyfourtimesgreaterthanECECvalues.

Table 1—Exchange properties of andic soil horizons of the Inland Northwest Region. Adapted from McDaniel and others (200�).

Ca2+ Mg2+ K+ Base saturation1 ECEC2 CEC pH 7 CEC pH 8.2

- - - - (cmolc /kg) - - - - (%) - - - - - - - - (cmolc /kg) - - - - - - - - Average 7.2 1.� 0.� 4�.0 �.� 19.1 2�.4 n �1� �1� �14 �14 11� �1� �1� Std. deviation �.7 2.0 0.7 2�.� �.4 11.� 1�.� Minimum 0 0 0 1 0.� �.4 7.0 Maximum ��.2 24.0 9.1 100 4�.7 101.0 12�.2

Anotherimportantcharacteristicofash-capsoilsistheirabilitytostronglyadsorbandimmobilizeplant-availableformsofPandS.Thisprocess,knownasfixation,resultsinlowconcentrationsofPO4

2–andSO42–insoilsolutionand

availableforplantuse(fig.6).TheabilitytofixPO42–resultsfromthestrongat-

tractionbetweenPO42–andtheedgesofallophane,imogolite,andferrihydrite,

andthereareprobablyseveralsorptionmechanismsinvolved(Wada1989).HighPO4

2–sorptionisoneofthecharacteristicsusedtodefineandicsoilpropertiesinSoilTaxonomyandandichorizonsinWRB.

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Figure 6—Sorption isotherms for (a) phosphorus additions (adapted from Jones and others 1979), and (b) sulfate additions (adapted from Kimsey and others 200�). Data points represent the proportions of sorbed (plant unavailable) and soluble (plant available) forms with additions of increasing quantities of phosphorus and sulfate.

Allophanic and Non-allophanic Andisols __________________________Becauseoftherangeincharacteristicsofvolcanicash-capsoils,arelatively

simpleclassificationschemehasbeendevisedthatrecognizesthesedifferencesfromamanagementperspective.Thissystemclassifiessoilsaseitherallophanicornon-allophanic(Nanzyoandothers1993a;Dahlgrenandothers2004).Allo-phanicAndisolsarethosethataredominatedbyinorganicweatheringproductssuchasallophaneandimogolite.Incontrast,organicallyboundcomplexesaremuchmoreimportantinnon-allophanicAndisols.Propertiesofthesetwoclassesofash-derivedsoilsaresummarizedintable2.Ingeneral,non-allophanicsoilspresentmoremanagementchallenges.Thesesoilshaveseveralpropertiesthatmayinhibitplantgrowth—greateracidityandgreaterpotentialforAl3+toxicity

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aretwoofthemostimportant.MostofthesoilsintheInlandNorthwestregionareclassifiedasallophanic(McDanielandothers2005).However,researchhasshown that shifts fromallophanic tonon-allophanicpropertiesandviceversacanoccurrelativelyquicklywithchangesinvegetation.AnexampleofthisfromnorthernIdahoisdescribedinthefollowingsection.

Response to Management _______________________________________

Erosion

Thescientific literaturecontainssometimes-contradictory informationaboutsusceptibilityofash-capsoilstoerosion.Soilsformedinvolcanicasharetypi-callydescribedashavingstrongresistance toerosion, largelyasa functionofstableaggregationandhighinfiltrationrates(Nanzyoandothers1993b;Dahlgrenandothers2004).However,becauseoftheirlowbulkdensitytheymaybeverysusceptibletowindandwatererosionwhenvegetativecoverisremoved(Kimbleand others 2000; Arnalds and others 2001). In the Inland Northwest region,maintenanceofforestcover,includingbothcanopyandlitterlayers,hasbeenanimportantfactorintheretentionofMazamaashcapsoverthemillenniasincedeposition(McDanielandothers2005).Thesefactorssuggestthaterodibilityofvolcanicashcapsisrelatedtodegreeofdisturbance;disturbancethatdestroysorremovescanopyandlitterlayerswilllikelyleadtosevereerosion.

Compaction

Perhapsoneof thebest-documentedresponsesofash-capsoils toland-useactivitiesiscompaction.Asdescribedearlier,andicsoilsaredefinedinpartbyabulkdensityof<0.90g/cm3.Numerousstudieshaveshownsignificantincreasesinash-capbulkdensityasaresultoftimberharvesttrafficking.IntheBlueMoun-tainsofeasternOregonandWashington,averagebulkdensity increasedfrom0.669gcm–3inunharvestedareasto0.725gcm–3inharvestedareas,represent-inganaverageincreaseof~9percent(Geistandothers1989).However,larger

Table 2—Comparison of properties of allophanic and non-allophanic Andisols (adapted from Dahlgren and others 2004; Nanzyo and others 199�a).

Property Allophanic Non-allophanicpH slightly to moderately acid strongly acid

Dominant mineralogy allophane/imogolite Al-interlayered 2:1 layer silicates and Al-organic complexes

Organic matter content moderate moderate to high

Al toxicity rare common

Compaction potential slight moderate



bulkdensity increaseswereassociatedwithskid trailsand landings.Similarly,Cullenandothers(1991)reportedsignificantincreasesinash-capbulkdensityinmoderatelyandseverelytraffickedareasascomparedtonon-harvestedcontrols.In addition, they were able to quantify changes in soil hydrologic propertiesaccompanyingbulkdensityincreases.Watercontentatrelativelylowsoilmois-turetensionsdecreasedsignificantlyintraffickedareascomparedtothecontrols(Cullenandothers1991)(fig.7a).One-hourinfiltrationdecreasedaswell,withvaluesdecreasingfrom38.5cminthecontrolto7.3cmintheseverelytraffickedareas(Cullenandothers1991).

Figure 7—Changes in (a) water content of ash-cap soils due to timber harvest trafficking, and (b) 1-hour infiltration in ash-cap soils due to timber harvest traf-ficking. *, ** indicate values that differ from the untrafficked controls at the 0.01 and 0.0� significance levels, respectively. Data are adapted from Cullen and others (1991).

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Chemical/Mineralogical Changes

WorkbyJohnson-Maynardandothers(1997)innorthernIdahosuggeststhatdevelopmentofnon-allophanicpropertiesaccompaniesestablishmentofbrackenferncommunitiesafterclearcuttinginaslittleas30yearsandmaycontributetotheobservedlackoftimberregeneration.ThissubjectisdiscussedinmoredetailbyFergusonandothers (thisproceedings).Recent literature suggests that thismineralogicalconversionisnotirreversible.Takahashiandothers(2006)reportedthatadditionsoflimetonon-allophanicsoilswillraisepHandreducetheactiveformsofAl3+,therebyrepresentingapotentialreclamationstrategy.

Knowledge Gaps ________________________________________________Twomajorquestions related to thepropertiesandmanagementof ash-cap

soilsintheInlandPacificNorthwestregionremain.Thefirstquestionis:How much ash is there and where is it?Thisseemslikeabasicquestion,yetitisoneforwhichagoodanswerdoesnotcurrentlyexist.TheNationalCooperativeSoilSurveyprogramhasproduced in~1:24,000-scale soil surveys forpartsof theregion,andtheseprovidesufficientdetailtoassesstheextentandspatialdistri-butionofash-capsoils.However,muchoftheforestedlandareawhereash-capsoilsare foundhasnotbeensubject to this typeofsoilmapping.Asa result,detailedinformationabout thequantitiesandspatialdistributionofashacrosstheregionisoftenlacking,poorlydocumented,and/ornotreadilyaccessible.Developmentofmodelsthatpredictashcapdepthanddistributionwill likelybegreatlyenhancedbynewtechnologies in remotesensing.However, in theabsenceofmoredetailedmappingorpredictivemodels,atestsuchastheNaFpHmeasurementcanprovideaquickandeasy indicatorofash-capsoils.AnNaFpHvalue>9.4generallyindicatesthepresenceofweatheredvolcanicash(FieldesandPerrott1966;Wilsonandothers2002)andservesasaguidelineforidentifyingash-capsoils.

Asecondquestionis:What is the relationship between ash-cap soil properties and soil performance?Themajorityofresearchtodatehasfocusedonmeasure-mentofash-cappropertiesratherthanperformance.Forexample,compactionofash-capsoilshasbeenwelldocumented,butcompactedbulkdensitiesaretypicallywellwithintherangeconsideredtobeacceptableforothermineralsoils.Sowithregardtoperformance,doesanashcapwithbulkdensity1.1gcm3be-havedifferentlythanasoilwithoutanashcaphavingthesamebulkdensity?Ananswertothisquestionwillrequireabetterunderstandingofthechangesinporesizeandconnectivitythatoccurwithcompactioninash-capsoils.Effortsshouldbefocusedondeterminingcriticalthresholdsinandicsoilpropertiesatwhichtreegrowthandthehydrologicperformancewillbeadverselyaffected.Long-termstudieswillbeneededtoaddresstheseissues,andsuccessfulmanagementwillultimatelyrequireanunderstandingofthemechanismsinvolved.

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ments.47:359-371.Bacon,C.R.1983.EruptivehistoryofMountMazamaandCraterLakecaldera,CascadeRange,

USA.J.Volcanol.Geotherm.Res.18:57-115.



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physical and chemical characteristics of ash-influenced ... · volcanic ash-cap textures are...

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