CompostApplicationRatesforCaliforniaCroplandsandRangelandsforaCDFAHealthySoilsIncentivesProgram

KellyGravuer,PhDCandidate,GraduateGroupinEcologyandGraduateStudentAssistant

UniversityofCalifornia,DavisCaliforniaDepartmentofFoodandAgriculture

PreparedincoordinationwithAmrith(Ami)Gunasekara,PhD

LiaisontotheEnvironmentalFarmingActScienceAdvisoryPanelCaliforniaDepartmentofFoodandAgriculture,Sacramento

AreportfortheEnvironmentalFarmingActScienceAdvisoryPanel

Draft(Version2.0–7/22/2016)

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TableofContents

ExecutiveSummary 2

Introduction 3

Methodology 4

Results 4Definitionofcomposteligiblefortheprogram 4Figure1.Distributionofapplicationratestodefine 6Compostapplicationratesforcroplands 6Table1.Averagepoundsofnitrogenpertondrycompostandaveragemoisturecontentforhigherandlowernitrogencomposttypes 7Figure2.Thenitrogencycle 8Box1.Exampleofcalculationstodeterminethepercentageoftotalplantrequirednitrogenrepresentedbycompost 9Table2.Proposedcompostapplicationratesforcroplands 10Compostapplicationratesforrangelands 10Table3.Literaturereviewoforganicamendmentadditionstosemi-aridrangelands 12Table4.Proposedcompostapplicationrateforrangelands 13Table5.Typesofsitesforrangelandcompostapplication 14Summaryofcompostapplicationratesforcroplandsandrangelands 15Table6.Recommendationsofthesubcommitteeforcompostapplicationtoagriculturallandsdistributedbytypeofagriculturalsystem,C:Nratioandtypeoffarming 15

OtherConsiderations 15Nitrousoxide(N2O)emissions 15Organically-managedcroplands 15Pathogens 16Monitoring 16Lifecycleconcerns 16Rangelandsiteassessments 17Technicalassistanceforprogramapplicants 17Primarypotentialenvironmentalimpactsofcompostapplicationtorangelands 171.Potentialimpactsonnitrate 172.Potentialimpactsonplantdiversity 173.NutrientRun-off 18Additionalconsiderationsforrangelands 18

LiteratureCited 20

ListofparticipantsforEnvironmentalFarmingActScienceAdvisoryPanelcompostsubcommittee23

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ExecutiveSummaryAspartofGovernorBrown’sHealthySoilsInitiative,theCaliforniaDepartmentofFood

andAgriculture(CDFA)isplanningtoestablishafinancialincentiveprogramforCalifornia’sfarmersandrancherstoimplementpracticesthatimprovesoilhealthandreducegreenhousegas(GHG)emissions.Theseincentives,asproposed,wouldbebasedontheUSDANaturalResourcesConservationService(NRCS)ConservationPracticesthatareincludedinCOMET-Planner.COMET-PlannerisatooldevelopedbyNRCStoestimatesGHGreductionsfrommanagementpracticechanges.CDFArecognizesthattheAirResourcesBoard(ARB)isresponsiblefordevelopingthequantificationmethodology(QM)associatedwithanyprogramfundedthroughtheCaliforniaClimateInvestmentsprogram,alsoknownastheGreenhouseGasReductionFund.Assuch,thisreportdescribesanalysesthatmaysupportthedevelopmentofaQMfortheCDFAincentiveprogram,ratherthanfurnishingtheQMtoolthatwillbeused.

OneagriculturalpracticewithconsiderablesoilhealthimprovementandGHGreductionpotentialisnotyetrepresentedasastand-aloneConservationPracticeintheCOMET-Plannertool:theapplicationofcomposttocroplandsandrangelands,animportantconservationpracticethatcanimprovesoilhealth.InordertomakethismanagementpracticeincludedinanyfutureincentiveprogrambyCDFA,compostapplicationratesthatwouldbecost-sharedbytheprogramneedtobeestablished. AttherecommendationoftheEnvironmentalFarmingActScienceAdvisoryPanel,CDFAconvenedasubcommitteeofscientiststoproposebest-availablescience-basedapplicationratesforcompost.Thissubcommitteeproposeddistributingcompostsintotwomajorcategories:thosewithhighernitrogen(C:N≤11)andthosewithlowernitrogen(C:N>11)content.ThegroupalsoproposeddividingCaliforniacroppingsystemsintotwomajortypes(annualcropsandtreecrops)andconsideringcroplandsandrangelandsseparately.

Basedonscientificliteraturereviews,therecommendationsofthesubcommittee,andpubliccomments,amaximumapplicationrateof8moist(i.e.,aspurchased)tonsofcompost/acre/yearwasdetermined.Applicationratesofmoistcompostapplicationforcroplandswere:forannualcrops,3-5tons/acre/yearforhighernitrogen(C:N≤11)compostand6-8tons/acre/yearforlowernitrogen(C:N>11)compost;andfortreecrops,2-5tons/acre/yearforhighernitrogencompostand6-8tons/acre/yearforlowernitrogencompost. BecausespecificfielddataonrangelandcompostapplicationinCaliforniaisstillverylimited,veryconservativeestimateswereusedinsettingrangelandapplicationratesto6-10tons/acreoflowernitrogencompostonly.Prioritysitetypesfortheseapplicationshavebeenidentified,consistentwithpubliccomment,andincluderangelandsthathavebeendepletedoftheirbaselinesoilorganicmatterthroughavarietyofagronomicpracticesorthathavebeenotherwisemanagedsuchthatnaturalplantcommunitiesareeithernolongerpresentorareofatypethatwouldnotbethreatenedbysoilamendments.Itisvitaltocontinuedocumentingeffectsofthispracticeandadjustingapplicationratesaccordingtositespecificconditions.Higherapplicationratesmaybepossibleoncemoredataisacquiredthroughongoingstudiesofthispractice.

Additionalinformationonthescienceofhowtheseratesweredeterminedisdescribedinthisreport.Also,wenotethatproducersparticipatingintheprogramwouldbeabletoapplycompostathigherratesthanthoseputforwardhere;however,theCDFAfinancialincentivewouldbelimitedtotheratesinthisreport.

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IntroductionInthe2015-16proposedbudget,GovernorBrownrecognizedtheimportanceofsoil

healthanddirectedtheCaliforniaDepartmentofFoodandAgriculture(CDFA)tocoordinateanewinitiativetosupportandenhancethiscriticalresource.Thebudgetlanguagestated“Astheleadingagriculturalstateinthenation,itisimportantforCalifornia’ssoilstobesustainableandresilienttoclimatechange.Increasedcarboninsoilsisresponsiblefornumerousbenefitsincludingincreasedwaterholdingcapacity,increasedcropyieldsanddecreasedsedimenterosion.Intheupcomingyear,theAdministrationwillworkonseveralnewinitiativestoincreasecarboninsoilandestablishlongtermgoalsforcarbonlevelsinallCalifornia’sagriculturalsoils.CDFAwillcoordinatethisinitiativeunderitsexistingauthorityprovidedbytheEnvironmentalFarmingAct”.

ConsistentwiththeGovernor’sinitiative,nowtitledtheHealthySoilsInitiative,CDFAworkedwithseveralstateagenciestoidentifyshortandlong-termactionsthatcouldimprovesoilhealthinCaliforniatoensureagriculturalsustainabilityandfoodsecurity(https://www.cdfa.ca.gov/EnvironmentalStewardship/pdfs/ShortTermActions.pdf).Oneoftheactionsidentifiedwastoincentivizemanagementpracticesthatbuildthecarboncontentinsoils.Increasingthecarboncontentofsoilshasbeenscientificallyshowntoleadtogreateragriculturalsustainabilityandensurefoodsecurity,especiallyinlightofclimatechange.CDFAplanstoimplementacost-shareincentivesprogramusingConservationPracticeStandardsestablishedbytheUnitedStatesDepartmentofAgriculture(USDA)NaturalResourcesConservationServices(NRCS).TheCDFAprogramwouldincludesoilhealth-promotingmanagementpracticesthatalsoreducegreenhousegas(GHG)emissions.MostcandidatepracticesthatcouldmeetthesegoalsareidentifiedintherecentlydevelopedCOMET-Plannertool1.COMET-PlannerprovidesestimatesofGHGreductionsfromeachincludedpractice,whichserveasaninputtotheCaliforniaAirResourcesBoard(ARB)’sprocessofdevelopingaquantificationmethodology(QM)thatwillmeettheneedsoftheCaliforniaClimateInvestmentsprogram.

OneagriculturalpracticewithconsiderablesoilhealthimprovementandGHGreductionpotentialistheapplicationofcomposttocroplandsandrangelands.IncentivizingtheuseofthispracticecanindirectlyachievelargeGHGemissionreductions2byincreasingdemand,spurringexpansionofcompostingfacilitiesandorganicwastediversionfromlandfillsthatproducemethane.MethaneisaGHGwitha100-yearglobalwarmingpotential28timesthatofcarbondioxide.Aerobiccompostingallowsthecarboninplantandanimalsourcematerialstobestabilizedintocarboncompoundsthatgenerallydecomposeslowlyafterthecompostisappliedtoland.Landapplicationofcompostalsodirectlystimulatesbiologicalprocesses,includingincreasesinsoilmicrobialandplantbiomass3,4,thatsequestercarbonintostablelong-termorganicmatterfractions5,6.Increasesintheseorganicmatterfractionsoffernumerousbenefitssuchasincreasingthewaterandnutrientretentioncapacityofsoils,providingareservoirofnutrientsforplants,improvingaeration,improvingwaterinfiltration,reducingsoilerosion,andsupportingtheabundanceanddiversityofsoilorganisms,whichcanimproveplanthealth.

CDFAmustdetermineapplicationratesofcompostthatwouldbesupportedbyanincentiveprogram.CDFAwillnotbeabletosupportunlimitedratesofcompostapplicationrequestedbyfarmersandranchersgiventhelimitedamountoffundingavailableasincentives,aswellastheneedtoensurethatenvironmentalconcernsareaddressed.Theamountof

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anticipatedgreenhousegasreductioncorrespondingtodevelopedapplicationratescanthenbeestimatedbasedonamodelrecognizedbyARB7.

CDFArecognizesthatARBisultimatelyresponsiblefordevelopingthequantificationmethodology(QM)associatedwithanyprogramfundedthroughtheCaliforniaClimateInvestmentsprogram,alsoknownasfundingfromtheGreenhouseGasReductionFund.Assuch,thisreportdescribessomequantitativeanalysesthatmaysupportthedevelopmentofaQMfortheCDFAincentiveprogram,ratherthanfurnishingtheQMtoolthatwillbeused.Methodology

OnJuly17,2015,CDFAconvenedameetingoftheEnvironmentalFarmingActScienceAdvisoryPanel(EFASAP)todiscusstheapplicationofcomposttoCaliforniacroplandsandrangelands.TheEFASAPisagroupoffarmersandscientistswhoprovidescientificguidancetotheSecretaryofCDFAandisaplatformforpubliccomment.TheEFASAPfunctionsundertheauthorityoftheEnvironmentalFarmingActof1995(https://www.cdfa.ca.gov/oefi/efasap/docs/Environmental_Farming_Act.pdf).TheJuly17thmeetingwasopentothepublicandattendedbyavarietyofstakeholders.AttendeesatthemeetingrecommendedthatCDFAconveneasubcommitteeofcompostexperts(fromacademiaandstateagencies)toevaluateandproposecompostapplicationrates,whichcouldthenbeconsideredforreviewbytheEFASAP,subjecttopubliccommentandproposedtotheSecretaryofCDFAtoimplementaspartofanyfutureHealthySoilsIncentiveProgram.

OnAugust28,2015,CDFAconvenedameetingofacompostsubcommittee.Thegroupconsistedofuniversityresearchersinsoilscience,compostmanagementandagronomyandincludedscientistsfromseveralpertinentstateagenciessuchasCalRecycle,CDFAandtheCentralValleyRegionalWaterQualityControlBoard(acompletelistofparticipantscanbefoundattheendofthisreport).ThegoalofthismeetingwastodeterminecompostapplicationratesthatcouldbesupportedbyaCDFAIncentivesProgramgiventhediversityofcroppingsystemsinCalifornia.AsecondmeetingofthesubcommitteewasheldonSeptember30,2015.SeveralliteraturereviewswereconductedtoevaluatethebestavailablesciencethatwouldsupportcompostapplicationratesforaCDFAIncentivesProgramfordiscussionatthetwosubcommitteemeetings.

AsecondEnvironmentalFarmingActScienceAdvisoryPanel(EFASAP)meetingonthistopicwasconvenedonJanuary15,2016.Adraftreportoftheresultsfromthesubcommitteemeetingswaspresentedtothemembers.Publiccommentonthereportwassolicitedatthismeeting,aswellasthroughafour-weekpubliccommentperiodextendingthroughFebruary12,2016.

TheResultssectionbelowsummarizestheproposedcompostapplicationratesrecommendedbythesubcommitteeandtakesintoconsiderationpubliccommentsreceived.ResultsDefinitionofcomposteligiblefortheprogram

ForthepurposesoftheCDFAHealthySoilsIncentivesProgram,compostisdefinedasallofthefollowing:

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• Theproductresultingfromthecontrolledbiologicaldecompositionoforganicwastesthataresourceseparatedfromthemunicipalsolidwastestream,orwhichareseparatedatacentralizedfacility.Feedstocksmayincludegreenmaterials,foodmaterials,woodwaste,yardtrimmings,agriculturalmaterialsorbiosolidsasdefinedin14CCRSection17852(www.calrecycle.ca.gov/laws/regulations/title14/ch31.htm)

• Mustbeproducedbyafacilitypermittedorotherwiseauthorizedbystateandlocalauthoritiesthatcandemonstratecompliancewithallstateregulationsregardinginspectionofincomingfeedstocks,finished-producttestingrequirementsincludingtheProcesstoFurtherReducePathogens(PFRP)asdescribedin14CCRSection17868.3,maximummetalconcentrationsforheavymetalsper14CCRSection17868.2,andphysicalcontaminationlimitsper14CCRSection17868.3.1.(14CCRSection17868:www.calrecycle.ca.gov/laws/regulations/title14/ch31a5.htm#article7)

• Note:STAcertifiedorCDFA-OIMcompostisrecommended.STAcertifiedcompostmeansthecomposthasbeentestedfornumerousproductparametersbyaSTA-certifiedlabwhichusesstandardizedtestingmethodologies(TMECC,developedbytheUnitedStatesCompostingCouncil),andtheresults(inatechnicaldatasheet)arereportedtothecompostproducer.Compoststowhichbiocharwasaddedduringthecompostingprocessasaprocess

amendmentarealsoeligiblefortheprogramaslongastheymeetalloftherequirementsabove.Biocharmaterialsaloneorbiocharmaterialsthathavebeenaddedtocompostincontextsotherthanasanamendmenttofacilitatethecompostingprocessarenoteligibleforthisincentivesprogram.ThereasonforexcludingbiocharintheCDFAIncentiveProgramatthistimeisbecauseregulatorystandardsareintheprocessofbeingdevelopedandtherearefewexperimentalfieldtrialsthatexaminetheapplicationrateofbiocharalongwithevaluatingitsbenefitsandlimitations.

Eachincentiveprogramparticipantmustensurethatthecompostproductstheyuseareincompliancewithanyadditionalregulationsthatmayapplytotheirparticularproductionsystem.Theseinclude,butarenotlimitedto,NationalOrganicProgramguidanceforUSDAcertifiedorganicgrowersandFoodSafetyModernizationAct(FSMA)ProduceSafetyRulerequirementsforgrowersoffreshproducethatiscoveredunderthisRule.Determiningapplicationrates

ThesubcommitteemeetingonAugust28,2015,reachedconsensusthatthereistoomuchvariationinthescientificdatawithinboth“croplands”and“compost”todefineasingleapplicationratetoallagriculturallands.Thesubcommitteefeltthat“croplands”couldbeusefullydividedintoannualcropsandtreecropsandthatbothconventionalandorganicmanagementsystemsshouldbeconsideredforeachoftheseproductionsystems.Rangelandshavedifferentconsiderationsandwarrantedtheirownseparatecategory.Compost,asdefinedabovecanbedividedintotwofurthercategories(carbon[C]:nitrogen[N]ratio[C:Nratio]lessthan11andC:Nratiogreaterthan11).Thisdifferentiationseparatescompoststhatprovidemorenitrogenatafasterrate(lowC:N)andthosethatprovidelessnitrogenataslowerrate(highC:N).

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C:Ntypicallyreflectsboththefeedstocksusedtoproducecompost(e.g.,manure-basedcompoststendtohavelowerC:Nthanplantwaste-basedcomposts)andthematurityofthecompostproduct(immaturecompostscanhavehigherC:Nthanmaturecomposts).Becauseonly“fullyfinished”compostswillbeeligibleforthisprogram,C:Ndifferencesamongeligiblecompostswillprimarilyrelatetodifferencesinthefeedstocksusedtoproducethem.Inadatasetof1364southwesternU.S.compostsamples(sharedwithCalRecyclebySoilControlLabs,Watsonville,CA),C:Ncorrelatedwithcompostpercentnitrogen(r=-0.44).Basedonthisobservationandtherecommendationofthesubcommittee,C:Nappearstobeareasonablemetriconwhichtobasecompostapplicationrates.Intotal,thesubcommitteeidentifiedtenapplicationratesforaCDFAIncentivesProgramonbuildingsoilcarbon(Figure1).

Figure1.Distributionofapplicationratestodefine,asestablishedatAugust28,2015subcommitteemeeting.Thesubcommitteeagreedtosettingtheupperlimitofeachapplicationraterangebasedonbest-availablescientificdataonthepotentialenvironmentalimpact(s)ofgreatestconcern.Thisstrategyisnotmeanttoimplythattheprimaryresultofcompostapplicationtocroplandsandrangelandswouldbeoneofenvironmentalimpact;rather,theintentwastofocusonthesignificantsoilhealthbenefitsthatcomposthasbeenshowntoprovidewhileatthesametimeminimizingpotentialforenvironmentalimpacts.Compostapplicationratesforcroplands

Forcroplands,thesubcommitteedeterminedleachingofnitrogen(intheformofnitrate)fromcomposttogroundwatertobetheenvironmentalimpactofgreatestconcern.ManyparticipatinggrowersintheCDFAIncentiveProgrammaychoosetoreducetheirsyntheticnitrogenfertilizerapplicationsastheygainexperiencewiththenitrogencontentin

APPLICATIONRATES

Rangeland2

Tree2crops

Annual2 crops

Conventional

Organic

Conventional

Organic

C:N2≤211 (Higher(Nitrogen)

C:N2>211 (Lower(Nitrogen)

C:N2≤2112(Higher(Nitrogen)

C:N2>2112(Lower(Nitrogen)

C:N2≤2112(Higher(Nitrogen)

C:N2>2112(Lower(Nitrogen)

C:N2≤2112(Higher(Nitrogen)

C:N2>2112(Lower(Nitrogen)

C:N2≤2112(Higher(Nitrogen)

C:N2>2112(Lower(Nitrogen)

1

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compoststominimizenitratesinsurfaceandgroundwater;however,noassumptionsaboutsuchbehaviorsweremadewhendevelopingcompostapplicationrates.

Compostscontainasmall(often<0.1%)percentagebyweightofnitrateaswellaslarger(often1-3%)percentagesbyweightofothernitrogencompoundsthatcouldeventuallybeconvertedtonitratebyresidentsoilmicrobes.Ascientificliteraturereviewwascompletedtoestimatethetotalnitrogenavailableovertimeinthetwotypesofcomposts.Estimatingnitrogenmineralizationfromcompost:Nitrogenincompostcanbedividedintothreemaintypes.Themajority(usually>95%)ofcompostNisorganicallybound(attachedtocarbon).Mostcompostalsocontainssmallamountsofinorganic(non-carbonbased)Nintheformofnitrate(NO3

-)andammonium(NH4+).Residentmicrobescanquicklyconvert

ammoniumtonitrate,andcanslowlyconvertorganicnitrogentoammoniumandthennitrateovertime(Figure2).Scientificliteraturewasreviewedtodevelopestimatesoftherateammonium+nitratereleasebycompost. Inadditiontoslowlyreleasingammonium+nitrate,compostwilllikelyaltersoilpropertiessuchthatlessnitrateleachesintogroundwaterperpoundofammonium+nitrateinthesoilascomparedtounamendedfields8,9.Forexample,compostgenerallyimprovessoilwaterholdingcapacity,suchthatlesswater–potentiallycarryingnitrate–mayleachbelowthecroprootzoneincompost-amendedfields.However,becausetheamountofthisreductionishighlydependentonsoiltype9alongwitharangeofothermanagementfactors,wecouldnotreliablyquantifyitatthistime.

Estimatingtherateofammonium+nitratereleasebycompostrequiresthreepiecesofinformation.Theyare:1.theamount(byweight)ofammonium+nitrateinthecompost,2.theamount(byweight)oforganically-boundnitrogeninthecompost,and3.amodelfortherateatwhichthisorganically-boundnitrogenwillbeconverted(mineralized)toammonium+nitrate.Estimatesforthefirsttwoinformationneedsabovewereobtainedusinglabanalysesfor1364SouthwesternU.S.compostsamplesfromavarietyoffeedstocksthatwasprovidedbySoilControlLabs(Watsonville,CA).Compostswerefirstdividedintotwocategories(C:N≤11andC:N>11)andaveragevaluesofthesequantitieswerecalculatedforeachcategoryseparately(Table1).

Table1.Average(median)pounds(lbs)ofnitrogenpertonofdrycompostandaveragemoisturecontentforhighernitrogen(C:N≤11)andlowernitrogen(C:N>11)composttypes,ascalculatedfromdataon1364compostsamplesprovidedbySoilControlLabs(Watsonville,CA).

HigherNcompost(C:N≤11) LowerNcompost(C:N>11)LbsNasammonium(NH4

+) 1.43 0.51LbsNasnitrate(NO3

-) 0.12 0.07LbsNasorganically-boundN 38.12 26.43Moisturecontent 27.11% 34.14%

Amodelfortherateatwhichorganically-boundnitrogenincompostismineralizedtoammonium+nitratewasdeveloped.Themodelwasdevelopedusinginformationfrom

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publicationsthatsynthesizedmanyindividualstudiesand/orstudiesthatwerespecifictoCalifornia10–15.Publicationsthatmetthesecriteriawerefairlyconsistentintheirmineralizationrateestimates,suggestingthattheseestimatesareareasonablebasisonwhichtodetermineapplicationrates.However,asadditionalCalifornia-specificstudiesbecomeavailable,theseratesshouldberevisited.Inaddition,itwouldbeusefultoconductaformalmeta-analysisofallavailablestudies,throughwhichtheinfluenceoffactorssuchasclimateandcomposttypecouldbequantified,andanalternativemethodofcalculatingCalifornia-appropriateestimatescouldbeemployedusingthoserelationships.Suchameta-analysiswasnotfeasiblewiththeresourcesavailableforthisstudy.

Figure2.Thenitrogencycleshowingthatorganically-boundnitrogen(suchasthatinplantresiduesandanimalmanures)ismineralizedtoammoniumandnitrate.

Forcompostwithhighernitrogen(C:N≤11),studiessuggestedthat5-15%(average≈10%)oftheorganically-boundnitrogenwouldbemineralizedinthefirstyearofapplication.Eachsubsequentyear,additionalremainingorganically-boundnitrogenwouldbemineralized,ataratethatwoulddeclinebyhalfeachyeartoaminimumofapproximately2%untilalloftheorganicNinthecomposthadbeenconsumed10.Asanexample,approximately10%oftheorganically-boundnitrogenwouldbemineralizedinthefirstyear,5%oftheremainingorganically-boundnitrogeninthesecondyear,2.5%oftheremainingorganically-boundnitrogeninthethirdyear,and2%oftheremainingorganically-boundnitrogeninthefourthyearandsubsequentyears.Forcompostwithlowernitrogen(C:N>11),studiessuggestedthat2-7%(average≈5%)oftheorganically-boundnitrogenwouldbemineralizedinthefirstyear,withasimilarpatternofmineralizationinsubsequentyears,includinga2%minimum.AtasecondscientificsubcommitteemeetingheldonSeptember30,2015,thesubcommitteeverifiedthatthemodelwasinagreementwithexistingscientificfindings.

Source:IPNIpublicationnumber4;www.ipni.net/publications

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Comparingnitrogenfromcomposttorecommendedplantnitrogenrequirements:Theamountofammonium+nitratereleasedfromcompostinagivenyearfollowingitsapplicationcanbeestimatedandcomparedwithplantrequirednitrogenrecommendations.PlantrequirednitrogenrecommendationsareavailableonCDFA’sFertilizerResearchandEducationProgram(FREP)andareaccessibleathttp://apps.cdfa.ca.gov/frep/docs/Guidelines.html.Forthisanalysis,plantrequirednitrogenrecommendationswereaveragedacrosstwomajorcroptypes:annualcropsandtreecrops.Forannualfruitandvegetablecrops(includingprocessingtomatoes,broccoli,lettuce,strawberries,cauliflower,andcorn),anaverageof161lbsofnitrogenperacreperyearwascalculated(withahighof270lbs/acreforcorn).Forestablishedtreecrops(includingestablishedalmonds,walnuts,citrus,pistachios,andplums),anaverageof115lbsofnitrogenperacreperyearwascalculated(withahighof380lbs/acreforalmonds).Usingthesevalues,theamountofammonium+nitratereleasedfromcompostcanbeexpressedinunitsofpercentageoftotalplantrequirednitrogenrepresentedbycompostforannualandtreecrops(Box1).Box1.Exampleofcalculationstodeterminethepercentageoftotalplantrequirednitrogenrepresentedbycompostfortreeorannualcrops.Inthisreport,applicationraterecommendationsforcompostareshownintermsof“tonsmoistcompost”toalloweasycomparisonwithcurrentapplicationratesusedbygrowers.However,percentmoisturevarieswidelyamongcomposts.Actualincentivizedrateswillbeintermsof“tonsdrycompost”,withthegrowerandcompostfacilityresponsiblefordeterminingtheequivalentmoistcompostapplicationratebasedonthepercentmoisturecontentofthespecificcompostpurchased.

Example1:ApplylowerNcompost(C:N>11)totreecrop• Nreleasedbycompostinyear1:1.91lbspertondrycompost

[ammonium-N+nitrate-N+5%oforganically-boundN]• AveragetotalNrequiredfortreecrops:115lbs/acre• Average%moistureoflowerNcompost=34.14%

• Ifapplying5moisttonsoflowerNcompost/acre(1ton(U.S.ShortTon)=2000lbs):

– 5*(1-0.3414)=3.29tonsdrycompostequivalent– 3.29*1.91=6.27lbsNappliedperacre– 6.27/115=5.5%oftotalrequiredNaddedbycompost

Example2:ApplyhigherNcompost(C:N≤11)toannualcrop

• Nreleasedbycompostinyear1:5.36lbspertondrycompost[ammonium-N+nitrate-N+10%oforganically-boundN]

• AveragetotalNrequiredforannualcrops:161lbs/acre• Average%moistureofhigherNcompost=27.11%

• Ifapplying4moisttonsofhigherNcompost/acre(1ton(U.S.ShortTon)=2000lbs):

– 4*(1-0.2711)=2.92tonsdrycompostequivalent– 2.92*5.36=15.6lbsNappliedperacre– 15.6/161=9.7%oftotalrequiredNaddedbycompost

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Table2.Proposedcompostapplicationratesforcroplands.TheratestouseforproposedCDFAIncentivesProgramarethe“equivalentdrycompostapplicationrates”(†),whichshouldbeconvertedtocorrespondingmoistcompostapplicationratesonabatch-specificbasisusingmoisturedatafromthecompostfacility.*AsC:Nratiorisesabove24,thelikelihoodofNimmobilizationincreases,whichmayleadtodecreasedNavailabilityforcrops.AscompostC:Nincreases,monitoringbecomesincreasinglyimportanttoensureadequatecropNsupply.

CropType CompostTypeMoistCompostApplicationRate

(tons/acre)

EquivalentDryCompostApplicationRate(tons/acre)†

Annual HigherN(C:N≤11) 3–5 2.2–3.6Annual LowerN(C:N>11)* 6–8 4.0–5.3Tree HigherN(C:N≤11) 2–4 1.5–2.9Tree LowerN(C:N>11)* 6–8 4.0–5.3

AtthesecondsubcommitteemeetingonSeptember30,2015,thescientistssupportedtheproposedratesinTable2,withminormodifications,tobeusedinaCDFAIncentiveProgram.Theserateswouldrepresent6.1-13.6%oftotalplantrequiredN,brokendownasfollows:7.3–12.1%forhigherNcompostonannualcrops,6.1–8.1%forlowerNcompostonannualcrops,6.8–13.6%forhigherNcompostontreecrops,and8.6–11.4%forlowerNcompostontreecrops.

Atthepresenttime,CDFAdoesnotplanonincentivizingthesameapplicanttoapplycompostinmultiplesuccessiveyearsgivenuncertaintiesinfunding.Potentialnutrientinputsfromcompostapplicationinmultiplesuccessiveyears,duetotheslow-releasenatureofcompostnutrients,couldbecomeanenvironmentalconcern.Itshouldbenotedhowever,thatthepercentageofplantnitrogenrequirementswouldbedoublethevaluesstatedaboveintheeighthandfifthsuccessiveyearofapplicationforhighernitrogenandlowernitrogencomposts,respectively.Thevalueswouldbetriplethosestatedaboveinthe17thandninthsuccessiveyearofapplicationforhighernitrogenandlowernitrogencomposts,respectively.

TheapplicationrateslistedinTable2donotlimitfarmersfromaddingadditionalcompost.ThelistedapplicationrateshavebeenestablishedsolelytosupportaCDFAIncentiveProgram.Participatinggrowersshouldberequiredtotestsoilnitrogenandphosphoruslevelsinfieldstowhichtheyareapplyingcompostatleastannually,tounderstanditseffectsonnutrientsupplyandbeabletoadjustsubsequentmanagementaccordinglywithcarbonsequestrationmanagementpractices.Compostapplicationratesforrangelands

InCalifornia,thebenefitsandpotentialdrawbacksofcompostapplicationhavereceivedlessattentiononrangelandscomparedtocroplands.ResultsfromonlytwonorthernCaliforniaexperiments(YubaCountyandMarinCounty,averageannualprecipitation730mmand950mmrespectively)havebeenpublished4,16,17.Atthesesites,adding31tons/acreofcompost(C:N=11)resultedinCsequestrationof51±77to333±52gC/m2overthreeyears,in

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additiontothecarbondirectlyaddedbytheamendment4.ThescientificsubcommitteecautionedagainstextrapolatingtheseresultstoallCaliforniarangelands,giventheconsiderablediversityofclimatesandsoilsthroughoutthestate18,19.Thus,whiletheseinitialnorthernCaliforniaresultsareencouraging,studiesatadditionalsitesacrossCalifornia’sclimateandsoilgradientsarenecessarytounderstandtherangeofpotentialcarbonsequestrationratesthatmightbeachieved.

Scientificsubcommitteediscussionsofpotentialenvironmentalimpactsofcompostapplicationtorangelandledtothreeconcerns:1.thepotentialforincreasednitrateleachingtogroundwater,2.thepotentialfordeclinesinplantdiversitysincenutrientadditioncoulddisproportionatelyfavorcertainplantspeciesand3.thestream-dissectedslopingrangelandlandscape,combinedwiththenitrogenandphosphoruscontentinmanycomposts,raisedtheconcernofnutrientmovementintosurfacewaterstreams.Forreasonsdetailedinthe“Primarypotentialenvironmentalimpactsofcompostapplicationtorangelands”sectionbelow,weusedthesecondconcern(potentialplantdiversitydecline)asameansofsettingtheupperapplicationrateforrangelandandnotedpotentialmethodsofaddressingtheothertwoconcerns.Definingratesbasedonpotentialplantdiversityimpacts:Aliteraturereviewoforganicamendmentapplicationstorangelandswasinitiated.Studiesmeetingthefollowingcriteriawereincludedinthereview:1.organicamendmenthadbeenaddedtoasemi-aridorMediterranean-climaterangelandcommunity(mostlygrasslands,sometimeswithscatteredtreesorshrubs),2.authorsreportedthepercentnitrogenoftheamendmentwithadequateinformationtoassignittothe“highN”(C:N≤11)or“lowN”(C:N>11)category,and3.plantcommunitydiversityhadbeenmeasuredatsomepointafteraddingtheamendmentandcomparedtothatofcomparablecontrolplot(s).Intotal,ninenon-redundantstudiesfitthereviewcriteria;fiveofwhichhadusednon-compostedamendments.Mostofthestudies(includingthoseofcompostedandnon-compostedamendments)hadappliedtheamendmentatmultiplerates,providing35datapoints(fromthenumberofstudiestimestheapplicationrate),nineofwhichrepresentedcompostedamendments.Acrossthesestudies,theplantcommunitywasobservedanaverageoffouryearsafteramendmentapplication.UsingtheC:Nandpercentnitrogendataprovidedinthestudies,thesamemineralizationmodelusedforcroplands(describedabove)wasthenusedtoestimatethecumulativeamountofavailablenitrogenthatwouldhavebeenreleasedfromtheamendmentbythetimetheplantdiversitydatawascollected.Thedatapointswerethensortedbythisestimateofnitrogenreleased(Table3).

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Table3.Literaturereviewoforganicamendmentadditionstosemi-aridrangelands,sortedbyNreleasedattimeofplantdiversitymeasurement.

AmendmentType Study Mg/haapplied

AmendmentNCategory

Yearsbetweenapplication&measurement

InorganicN(lbspertoncompost)

OrganicN(lbspertoncompost)

AvailablelbsNreleased/acreattime

ofmeasurement

Plantdiversitydecrease

non-composted Pierceetal.199820 5 LowerN 2 0.34 0.24 0.52 Nnon-composted Pierceetal.199820 10 LowerN 2 0.34 0.24 1.04 Nnon-composted Pierceetal.199820 15 LowerN 2 0.34 0.24 1.56 Nnon-composted Pierceetal.199820 20 LowerN 2 0.34 0.24 2.08 Nnon-composted Pierceetal.199820 25 LowerN 2 0.34 0.24 2.60 Nnon-composted Pierceetal.199820 30 LowerN 2 0.34 0.24 3.12 Nnon-composted Pierceetal.199820 35 LowerN 2 0.34 0.24 3.64 Nnon-composted Pierceetal.199820 40 LowerN 2 0.34 0.24 4.16 Nnon-composted Sullivanetal.200621 2.5 HigherN 13 7.48 82.32 17.21 N

compost Kowaljowetal.201022 40 LowerN 2 0.48 14.92 18.57 Ncompost Kowaljowetal.201022 40 LowerN 2 0.96 13.04 22.58 Ncompost Pedroletal.201023 20 LowerN 0.5 2.36 43.44 26.65 N

non-composted Sullivanetal.200621 5 HigherN 13 7.48 82.32 34.42 Nnon-composted Stavastetal.200524 12 HigherN 2 2.60 31.60 37.59 Y

compost Kowaljowetal.201022 40 LowerN 2 1.06 29.94 38.41 Ncompost Martínezetal.200325 40 LowerN 3 1.82 33.38 54.82 Ycompost Kowaljowetal.201022 40 LowerN 2 2.68 34.32 61.24 N

non-composted Sullivanetal.200621 10 HigherN 13 7.48 82.32 68.85 Ycompost Martínezetal.200325 80 LowerN 3 1.82 33.38 109.64 Y

non-composted Sullivanetal.200621 21 HigherN 13 7.48 82.32 144.58 Ycompost Ryalsetal.201626 70 HigherN 3 2.38 35.02 146.72 N

non-composted Martínezetal.200325 40 HigherN 3 4.02 48.98 158.30 Ynon-composted Fresquezetal.199027 22.5 HigherN 3 7.38 89.82 163.30 N

compost Martínezetal.200325 120 LowerN 3 1.82 33.38 164.46 Ynon-composted Jurado-Guerraetal.201328 30 HigherN 2 6.22 75.78 167.89 Ynon-composted Sullivanetal.200621 30 HigherN 13 7.48 82.32 206.54 Ynon-composted Jurado-Guerraetal.201328 45 HigherN 2 6.22 75.78 251.83 Ynon-composted Martínezetal.200325 80 HigherN 3 4.02 48.98 316.60 Ynon-composted Fresquezetal.199027 45 HigherN 3 7.38 89.82 326.61 Ynon-composted Jurado-Guerraetal.201328 60 HigherN 2 6.22 75.78 335.77 Ynon-composted Stavastetal.200524 107 HigherN 2 2.60 31.60 343.17 Ynon-composted Jurado-Guerraetal.201328 75 HigherN 2 6.22 75.78 419.72 Ynon-composted Martínezetal.200325 120 HigherN 3 4.02 48.98 474.90 Ynon-composted Jurado-Guerraetal.201328 90 HigherN 2 6.22 75.78 503.66 Ynon-composted Fresquezetal.199027 90 HigherN 3 7.38 89.82 653.21 Y

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ThreenitrogenapplicationratecategorieswereobservedinTable3;1.atapproximately35lbsavailableN/acre,impactstonativeplantspeciesareunlikely(0%ofdatapointsshowedimpact),2.between35and164lbsavailableN/acre,impactsareprobable(60%ofdatapointsshowedimpact)and3.aboveapproximately164lbsavailableN/acre,impactsarehighlylikely(100%ofdatapointsshowedimpact).Basedonthesefindings,onestrategyforminimizingimpactscouldbetoonlyallowlowernitrogenamendmentstobeusedonrangelands.Anotherstrategycouldbetosetapplicationratessothattheavailablenitrogenprovidedbycompostwouldbelessthan35lbs/acre.Combiningthesetwostrategiesandusingafive-yearpost-applicationtimeframeforcalculatingcumulativeavailableN,theimpactthresholdwouldoccuratapproximately13moisttons/acreoflowernitrogencompost.GiventhatthisfigureisclosetotheupperlimitoflowerNcompostapplicationratesforcroplands(8moisttons/acre)proposedtodate(Table2),ratesforallagriculturallandswerekeptconsistent(Table4).

However,thisimpactminimizationstrategyshouldstillbeviewedwithcautionbecauseliteraturereviewstudiesdifferedinvegetation(e.g.,perennialinsteadofannualdominated)andclimatefromCalifornia’srangelands.Giventhisuncertainty,acomplementarystrategywouldbetoconsidertherisktonativeplantdiversityfromtheperspectiveofthesitetowhichcompostwouldbeadded.Forinstance,sometypesofrangelandareespeciallysensitivetonutrientadditionand/orcontainhighconcentrationsofrarespecies.Forexample,impactsonspeciesofconservationconcerninserpentinegrasslandshaveoccurredatNadditionratesmuchlowerthanthethresholdratessuggestedbytheliteraturereview.29–31Discussionswiththescientificsubcommittee,aswellaspubliccommentsfromCaliforniarangelandexperts,suggestedthatcompostapplicationtosometypesofrangelandclearlyposedahigherrisk.

Basedonthisinformation,wegroupedpotentialrangelandsitetypesintothreecategories:1.“priority”sitetypes,wherecompostapplicationshouldhavetheleastimpactonnativeplantdiversity;2.“evaluate”sitetypes,wherecompostapplicationattheproposedratesmayhaveanimpact;and3.“ineligible”sitetypes,wherecompostapplication,evenatlowrates,wouldbelikelytoimpactnativeplantdiversity(Table5).Proposalstoaddcompostto“priority”sitetypeswouldrankhigherthansimilarproposalsfor“evaluate”sitetypes,whileproposalstoaddcompostto“ineligible”sitetypeswouldnotbeconsidered.Rangelandmanagementspecialistscouldbeconsultedtosortproposalsintothesesitetypes,andtoevaluatethepotentialimpactsvs.benefitsofcompostapplicationoneach“evaluate”site.

NRCSiscurrentlyconductingfieldtrialsofrangelandcompostapplicationandisevaluatingimpactsonnativeplantdiversityandCsequestration.AsresultsfromthesetrialsbecomeavailableandNRCS’sDraftConservationPracticeStandardisrevisedaccordinglyandfinalized,itmaybepossibletonarrowthe“evaluate”categoryandadjusttheapplicationrates.Table4.Proposedcompostapplicationrateforrangelands.TheapplicationrateisconsistentwithcroplandapplicationratesinTable2,andthecumulativenitrogenavailabilityislessthanthethresholdforimpactonplantdiversitysuggestedbyourscientificliteraturereview.

CompostTypeMoistCompostApplicationRate

(tons/acre)

EquivalentDryCompostApplicationRate

(tons/acre)

CumulativelbsavailableN/acreat5yearspost-

application

LowerN(C:N>11) 6–8 4.0–5.3 15.7–20.9

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Table5.Typesofsitesforrangelandcompostapplication.Compostapplicationisexpectedtohavetheleastimpactonnativeplantdiversityat“priority”sites,mayhaveanimpactat“evaluate”sites,andislikelytoimpactnativeplantdiversityat“ineligible”sites.Both“priority”and“evaluate”sitetypeswouldbeeligiblefortheincentivesprogram.SiteType Examples

Priority

• sitesthathavebeenplowed,irrigated,heavilyseeded,orotherwisedisturbedsuchthatthenaturalcommunitiesandsoilconditionsarenolongerpresent

• areasthathavebeendepletedoftheirbaselinesoilorganicmatterthroughavarietyofagronomicpractices–resultinginareassuchasdegradedrangelandsand/orabandonedagriculturallandsthataregenerallyconsideredtohavepoorsoilhealth

• retiredagriculturallandsthatarebeingrestoredorconvertedtorangelands• othersiteswheresoilshavebeenpreviouslytilledorsubjectedtomajorsoil

disturbance,e.g.,failedhomesteads• smallholding/feedingpastures• fallowedfields

Evaluate

• anyrangelandareanotdescribedunder“Priority”or“Ineligible”

Ineligible

• slopesgreaterthan15%• seasonalwetlandssuchasvernalpoolcomplexes(includingsurroundinguplands)• wetmeadowsorotherseasonallyinundatedrangelands,regardlessofslope

(e.g.,floodplains)• morepermanentwetlands,includinganyareawithhydricsoils• sagebrushsteppe• alkalisinks• desertgrasslands*• nativecoastalprairies• serpentineandserpentine-influencedsoiltypes• chaparral,coastalsagescrub,andothersystemsdominatedbynativeshrubs• grasslandscurrentlydesignatedasmitigationand/orconservationlands• sitescontainingfederal,state,and/orCNPSlistednativeplants;and/oranimals

thatrequirelow-staturerangelandsfortheirlifehistory,includingbutnotlimitedtoSanJoaquinKitFox,GiantKangarooRat,TigerSalamander,and/orBurrowingOwl

• sitesthathaverecentlyburned**• sitesinwatershedsalreadyimpactedbyNorP(i.e.,listedundersection303dof

theCleanWaterActfornutrientpollution),unlessappropriatemitigatingpracticesincluded

*CompostapplicationondesertgrasslandsiteswherevegetationisdominatedbyinvasiveEurasiangrasses,suchascheatgrass(Bromustectorum),maybeconsideredaspartofanoverallrestorationstrategy,wheregrazingispresent.**Compostapplicationasastrategyforrehabilitatingselectburnedsitesmaybeconsidered,wheregrazingisinvolved.

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SummaryofcompostapplicationratesforcroplandsandrangelandsAsummaryoftherecommendratesforcompostapplicationtosupportaCDFA

incentiveprogramonsoilhealthisprovidedinTable6below.Table6.Recommendationsofthesubcommitteeforcompostapplicationtoagriculturallandsdistributedbytypeofagriculturalsystem,C:Nratioandtypeoffarming.Theratestousefortheproposedincentivesprogramarethe“equivalentdrycompostapplicationrates”(†),whichshouldbeconvertedtocorrespondingmoistcompostapplicationratesonabatch-specificbasisusingmoisturedatafromthecompostfacility.

System Management CropType CompostType

MoistCompostApplication

Rate(tons/acre)

EquivalentDryCompost

ApplicationRate

(tons/acre)†Cropland Conventional Annual HigherN(C:N≤11) 3–5 2.2–3.6Cropland Organic Annual HigherN(C:N≤11) 3–5 2.2–3.6Cropland Conventional Annual LowerN(C:N>11) 6–8 4.0–5.3Cropland Organic Annual LowerN(C:N>11) 6–8 4.0–5.3Cropland Conventional Tree HigherN(C:N≤11) 2–4 1.5–2.9Cropland Organic Tree HigherN(C:N≤11) 2–4 1.5–2.9Cropland Conventional Tree LowerN(C:N>11) 6–8 4.0–5.3Cropland Organic Tree LowerN(C:N>11) 6–8 4.0–5.3Rangeland -- -- LowerN(C:N>11) 6–8 4.0–5.3OtherConsiderationsNitrousoxide(N2O)emissions.Anadditionalissuethatwasraisedwaswhethercompostapplicationtocroplandscouldcauseincreasesinnitrousoxide(N2O)emissions,becauseadditionalorganiccarboncouldpotentiallyincreaseN2Oemissionrateswhensoilsarerelativelysaturated(>80%water-filledporespace).However,undermostotherconditions,reactionsrelatedtonitrificationincludingammoniaoxidationandnitrifierdenitrificationarebelievedtobethedominantcontributortoN2OemissionsfromCalifornia’sagriculturalsoils32,33.Thesereactionsarecarriedoutbyautotrophsthatarenotstimulatedbyorganiccarbonaddition.N2Oproductionpathwaystendtobestimulatedbyadditionofammonium,suchthatanincreaseinN2Oemissionsmaybenotedwhencomparingcompost-amendedsoiltoanunamendedcontrolbecauseoftheammoniumprovidedbythecompost.However,theimpactofammoniumadditionviacompostwouldnotbeexpectedtobegreaterthanthatofadditionofanequivalentamountofammoniumfromanyothernitrogensource.Organically-managedcroplands.Thereisconsiderablevariationamongorganicgrowersintheuseofcompostforplantnutrientprovision;somegrowersapplysubstantialcomposttosupply

16

asignificantpercentageofcropnutrientneeds,whereasothersmayapplylittletonocompostandrelyonotherorganicnutrientsources,suchasmanure,certaincovercrops,andfeathermeal12.Atthesecondscientificsubcommitteemeeting,theapplicationrateseligibleforfinancialincentiveswasrecommendedtobethesamefororganicandconventionaloperations(Table6),withtheunderstandingthatorganicgrowers,ingeneral,mayapplygreateramountsofcompostintotal.Pathogens.Concernaboutthepotentialpresenceofpathogensincompostmaymakesomegrowershesitanttoadoptcompostapplication.However,theheatgeneratedduringthecompostingprocesskillsthevastmajorityofpathogenicmicrobes,typicallyreducingthembelowdetectablelevels34–36.Furthermore,anypathogensthatmightremainareoftenoutcompetedinthelaterstagesofcompostmaturation,becausethesimplecarboncompoundsthataretheirpreferredfoodsourcegetconsumed,leavingthemorecomplexcompounds-onwhichothermicrobialgroups(suchasfungi)arestronglyfavored-asthedominantfoodsource36,37.Finally,inCalifornia,mostcompostthatissold-andallcompostthatcouldbeappliedinthisincentivesprogram-issubjecttorigoroustestingforanyresidualpathogensandmustpassallsuchtestsbeforeitisclearedforsale(14CCRSection17868.3).Monitoring.Thedesirabilityofcollectingmonitoringdatainassociationwiththesecompostapplicationsonbothcroplandsandrangelandswasnotedinpubliccomments.Suchdatacollectionwouldbehelpfultoquantifyboththebenefitsandpotentialenvironmentalimpactsofcompostapplicationandmayallowfutureadjustmentofapplicationrates.Forsoils,standardphysicochemicalanalysisincludingallplantnutrientsandtoxins,soilorganicmatter(SOM),andcompactionandinfiltrationrateswouldbedesirableandshouldbecollectedwithasamplingdesignthatappropriatelycapturessitevariability.Onrangelands,additionalplantdatatocollectwouldincludepercentbareground,residualdrymatter(RDM),speciescomposition,vegetationproduction,andphotomonitoringofrepresentativesiteswherecomposthasbeenapplied(alongwithpairedcontrolsiteswhereithasnot).Lifecycleconcerns.Afrequently-raisedquestioniswhethertheCO2emittedintransportingcomposttotherangelandsitewouldbegreaterthantheCsequesteredasaresultofitsapplication.ThismightbethecaseifconsideringonlytheCsequesteredviabiologicalactivityonsite,whichforrangelandsisestimatedtobeapproximately50%oftheCO2emittedduringtransportbasedonalifecycleanalysisusingdatafromthesenorthernCaliforniarangelandsites6.However,thisbalancedependsonthesystemtowhichcompostisappliedandthemethodsusedtomakeemissionsestimates.Forexample,aCaliforniaAirResourcesBoardstudyofcompostapplicationtocroplandsestimatedthattransporttotheapplicationsitewouldemit0.008MTCO2epertonofcompostedfeedstockwhileon-sitesoilCincreases(estimatedusingbiogeochemicalprocessmodelingratherthanfielddata)wouldsequester0.26MTCO2epertonofcompostedfeedstock,onaverage7.Furthermore,ifincreaseddemandforcompostcreatedbyrangelandapplicationisassumedtobedirectlyresponsibleforincreaseddiversionoforganicwastefromlandfillsand/ormanurefromslurrypondsintoaerobiccompostingprocesses,thenthispracticereducesGHGsduetoavoidedmethaneemissions2whichis28timesmorepotentthancarbondioxide.Assessingthisclaimisbeyondthescopeof

17

thisreport,astherearenumerousotherdriversofdiversionoforganicwastesandmanurestocompostinginCalifornia,suchthatitisdifficulttoestimatethepresentandpotentialfuturecontributionsofrangelanddemand.Rangelandsiteassessments.Forrangelands,anin-personsiteassessmentbyaqualifiedprofessional,asstipulatedintheAmericanCarbonRegistryProtocol38,ishighlyrecommended,aswellascarefulconsiderationofapplicationratesinthecontextofsiteconditions.Thisprofessionalshouldsurveythesiteforspeciesofconservationconcern,identifyanypotentialplaceswherenutrienttransportposesaeutrophicationrisk,recommendBMPstomitigaterunoff,andassessotherresourceconcernsasappropriate.Comprehensivelyevaluatingapractice’spotentialeffectsonallnaturalresourcesisastandardNRCSprocedure,and,assuch,shouldbepartofanyCompostAdditiontoRangelandsConservationPracticeStandard.Technicalassistanceforprogramapplicants.TheHealthySoilsincentivesprogramisproposedtobebasedonconservationpracticesdevelopedbytheUSDANaturalResourcesConservationService(NRCS).TheapplicationofConservationPracticeStandardsisadaptedasneededforeachindividualsitebytechnicalpersonnel.Thesevisitsenablethepotentialbenefitsandenvironmentalimpactsofeachpracticetobeassessedinamoresite-specificmanner.SiteassessmentsareanimportantcomponentofanincentiveprogramandCDFAshouldevaluateifsuchaprocesscanbeestablished.ProducerswhohavecompostapplicationlistedasapracticeinanexistingCarbonFarmPlanorequivalentconservationplanwouldbewelcometoapplytotheincentivesprogramforcost-share(oftherateslistedhere).However,suchadocumentwouldnotberequiredforparticipationintheincentivesprogram,unlessaprocesscanbeestablishedtomaketheseplanningserviceseasilyavailabletoallCaliforniaproducers.Primarypotentialenvironmentalimpactsofcompostapplicationtorangelands1.Potentialimpactsonnitrate:Fornitrateleaching,rangelandsmightinterceptmoreoftheavailablenitrogenreleasedfromcompostthancroplandsduetoagreaterspatialandtemporalextentofplantcover.However,nodirectfieldmeasurementsofnitrateleachingfromcompost-amendedrangelandsareavailableinthescientificliterature.ForthenorthernCaliforniasites,Ryalsetal.17usedtheDAYCENTmodeltoestimatenitrateleachingintheirlaboratorystudy.TheDAYCENTestimatewasapproximately8.9lbsNO3-N/acre/yearforthefirst10yearspost-application,whichequatestoapproximately40%oftheNreleasedfromthecompostleachingoutasnitrateoverthatperiod(89lbsNO3-N/acreoftheestimated222lbsN/acrereleased).LeachingrateswereconsiderablylowerforsimulationsofC:N=20andC:N=30compoststhantheywerefortheC:N=11compostthatwasusedinthefieldstudy.Theseestimatesnowurgentlyrequirefield-validationandtestingatothersites.2.Potentialimpactsonplantdiversity:Californiarangelandssupportover400plantsofconservationconcern39,40andanumberofrangelandwildlifespecies,someofwhicharealsoimperiled,requirespecificplantsand/orvegetationstructurefortheirfoodandhabitat41–43.Concernsabouttheimpactofcompostadditiononplantdiversityaregroundedinafairlylargebodyofstudiesthathavedocumentedsignificantchangesinplantcommunitycomposition–andusuallydecreasesindiversity–inresponsetosyntheticNfertilizeraddition44.Ingeneral,

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addingNincreasesgrassbiomassmoresothanforbbiomass,suchthatafewhighly-responsivegrassspecies(mostlynon-native)canoutcompetemanyoftheforbs(mostlynative)45.

However,mostofthesestudieshaveappliedfairlyhighratesofN(80-100lbsN/acre/year)intheirexperiments.ThereisalackofscientificpeerreviewedstudiesinCaliforniagrasslandsthathaveaddedarangeofNratestodetermineathresholdrateofNadditionabovewhichdiversityislikelytodecline.AfewstudieshaveattemptedtodetermineN“criticalloads”atwhicheffectsontheecosystemarediscernable29–31,46.ThesestudiessuggestedthatacriticalloadforCaliforniagrasslandscouldbe6-9lbsN/acre/year,buttheyarebasedonlimitedobservationaldataalonganNdepositiongradientinserpentinegrasslands.Becausenutrient-poorserpentinegrasslandsmaybemoresensitivetonutrientadditionthanotherCaliforniagrasslandtypes,moreresearchisneededtoevaluatewhetherthisconstitutesabasisforanapplicationratelimitthatwouldberelevanttomostCaliforniarangelands.3.NutrientRun-off.Foreutrophicationconcernsonrangelands,mandatorybuffersaroundallsurfacewaterscombinedwithasite-specificriskfactoranalysisisanalternativestrategytoacross-the-boardlimitsonapplicationrates.Requiredbufferwidthfortheincentivesprogramwillbe30feet(ataminimum)aroundallsurfacewatersonoradjacenttotheparcel.Fortheriskfactoranalysis,similartothe“phosphorusindex”approachappliedinmanystatestoevaluateriskfromphosphorusapplicationtocroplands47,rangelandareaswithlowsoilNand/orPthatareataconsiderabledistancefromwaterwaysprobablywouldnotcreatesignificantriskandthereforemightbasetheirapplicationratesonotherconcerns.Forareasthatdohaveoneorbothoftheseriskfactors,amoredetailedriskassessmentcanbeconducted47,andsomeoralloftheriskcouldbemitigatedbyadjustingthecompostapplicationrateand/orusingbestmanagementpractices(BMPs)inadditiontotherequiredbuffers.Alternatively,potentiallyproblematicareasofthepropertycouldsimplybeavoidedifthereareothermoresuitableareas.TheAmericanCarbonRegistry’sMethodologyforCompostAdditionstoGrazedGrasslands38recommendsasitesurveybyaQualifiedExpert(i.e.,aCertifiedRangelandManager,NRCSSoilConservationistorQualifiedExtensionAgent)beforecompostisappliedtoassessthisandotherrisks.Inrelationtothesenutrientrun-offconcerns,somecompostexpertsalsonotedthatcompostuseinsoilerosionpreventionanddegradedsitemitigationisdocumentedatratesmuchhigherthanthoseproposedhere.48,49Additionalconsiderationsforrangelands.Becauseadditionofcomposttorangelandsmayincreasebiomassproduction4,whichcouldincreasefireriskinadditiontopotentialplantdiversityimpacts,applicationofcomposttograzedrangelandsonlyisrecommended.Applicationtoungrazedsiteswouldonlybeadvisableifpartofadegradedrangelandrestorationproject,guidedbyanappropriateconservationorrestorationplan.

Fiveoftheninestudiesintheliteraturereviewpresentedhereinvolvednon-compostedorganicamendments.Nitrogenmineralizationislikelytobefasterinnon-compostedthanincompostedamendments,suchthatlevelsofavailableNmaybeunderestimatedfornon-compostedamendmentsinTable3.

Nitrogenisnottheonlysoilnutrientthatcouldincreasewithcompostaddition,ascompostusuallycontainssignificantphosphorus,potassium,andothersecondaryplantnutrientsaswell.Here,ratesweredeterminedbasedonNreleasebecausetherearemore

19

studiesdemonstratingNimpactsonCaliforniagrasslandplantcommunitiesthanthereareforothernutrients44.However,othernutrientsandindirecteffectsmayhaveimportantconsequencesoverthelongerterm50. ItisimportanttoconsidertheecologyofCaliforniarangelandplantcommunitieswhenevaluatingfindingsof“noimpact”ontheirdiversity.Manyrangelandforbspeciesformseedbanksandonlyappearinyearsthatarefavorableforthem.Anychangeinsoilconditionsmayalterthedegreeoffavorabilityofsuchyearsforthesespecies,butthisalterationmaynotbedetectedwithinthetimeframeofmostpublishedstudies.ThesedynamicssuggestaprecautionaryapproachtopracticesthatcouldimpactCaliforniarangelandplantdiversity.

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ListofparticipantsfortheEnvironmentalFarmingActScienceAdvisoryPanelcompostsubcommitteeAcademicResearchersKateScow,PhD(UCDavis)WhendeeSilver,PhD(UCBerkeley)DavidCrohn,PhD(UCRiverside)PeterGreen,PhD(UCDavis)MariadelaFuente,PhD(UCCooperativeExtension)WillHorwath,PhD(UCDavis)CarolShennan,PhD(UCSantaCruz)DougParker,PhD(UCOfficeofthePresident)JojiMuramoto,PhD(UCSantaCruz)KellyGravuer,MSc(UCDavisandCaliforniaDepartmentofFoodandAgriculture)AgencyScientistsDennisChessman,PhD(USDANaturalResourcesConservationService)AmrithGunasekara,PhD(CaliforniaDepartmentofFoodandAgriculture)RobertHorowitz(CalRecycle)EvanJohnson(CalRecycle)TungLe(CaliforniaAirResourcesBoard)SueMcConnell(CentralValleyRegionalWaterQualityControlBoard)BrendaSmyth(CalRecycle)BrianLarimore(CalRecycle)DavidMallory(CaliforniaAirResourcesBoard)BruceGwynne(CaliforniaDepartmentofConservation)ScottCouch(StateWaterBoard)KylePogue(CalRecycle)CarolynCook,MSc(CaliforniaDepartmentofFoodandAgriculture)BarzinMoradi,PhD(CaliforniaDepartmentofFoodandAgriculture)JenniferKiger(CaliforniaAirResourcesBoard)