HS948 Reduction of the Impact of Fertilization and

HS948ReductionoftheImpactofFertilizationandIrrigationOnProcessesintheNitrogenCycleinVegetableFieldswithBMPs1Eve-MarieCockxandEricH.Simonne21.ThisdocumentisHS948,oneofaseriesoftheHorticulturalSciencesDepartment,FloridaCooperativeExtensionService,InstituteofFoodandAgriculturalSciences,UniversityofFlorida.Publicationdate:September2003.PleasevisittheEDISWebsiteat(N)cycleisasetoftransformationsthataffectNinthebiosphere.Throughaseriesofmicrobialtransformationsinthesoil,Nismadeavailabletovegetablecrops.Thus,knowledgeofthiscyclebywhichNpassesfromairtosoiltoorganismsandbacktoair,andhowthecomponentsofthecycleareaffectedbyhumanactivities,isrequiredtodesigneffectivestrategiesfordecreasingundesirablelossesofNfromvegetableproductiontotheenvironment.AdequatemanagementoffertilizationandirrigationhasalwaysbeenrecognizedasoneofthekeystosuccessfulvegetableproductioninFlorida.Thus,fertilizationandirrigationpracticeshaveaimedatsupplyingenoughnutrientsandwatertoensureeconomicalyields.Sinceupto200lbs/AofexogenousNarerecommendedforvegetableproductioninFlorida,andfertilizeruseefficiencyseldomexceeds75%,itislikelythatfertilizationaffectstheNcycle.BestManagementPractices(BMPs)aimatreconcilingtheneedsofeconomicalvegetablecropproductionwiththoseofenvironmentalprotection.EffectiveBMPimplementation,therefore,requiresanunderstandingofhowcurrentculturalpracticesaffectcertainprocessesintheNcycleincommercialvegetablefields.ItislikelythatacompleteunderstandingoftheseissuesbyfarmersandvegetableprofessionalswillbeaprerequisiteforthesuccessoftheBMPprogram.Thegoalsofthisarticleareto(1)presenttheNcycleasitrelatestocropproduction,(2)describehowfertilizationandirrigationaffecttheprocesseswithinNcycle,and(3)explainhowtheproposedBMPsmayhelpreducethenegativeenvironmentalimpactoftheseculturalpractices.TheNitrogenCycleinaTypicalEcosystemBecausetheNcycleisa“cycle”,ithasnoclearbeginningandnoend(Pidwirny,2002).Hence,forthesakeofpresentation,thisdescriptionofthecyclestartswithNinthesoilorganicmatterwhereNisintheformofaminoacids,proteins,andnucleicacids(Fig.1).Inthesoil,NfoundindecomposingorganicmattermaybeconvertedintoinorganicNformsbysoilmicroorganisms(bacteriaandfungi)inReductionoftheImpactofFertilizationandIrrigationOnProcessesintheNitrogen....2aprocesscalledmineralization(step1).Thesebacteriaandfungi,alsocalleddecomposers,maybefoundintheuppersoillayer.TheychemicallytransformtheNfoundinorganicmatterfromamino-N(NH2)toammonium(NH4+)(Pidwirny,2002).Step1:Organicmatter---AmmoniumR-NH2---NH4+Figure1.Impactoffertilization,irrigationandotherculturalpracticesinvegetablefields(inred)onthestepsofnitrogencycle(ingreen)withbestmanagementpractices(inblue).NitrogenintheformofNH4+canthenbeadsorbed(step2)ontothesurfacesofclayparticlesinthesoil.TheNH4ionthathasapositivechargemaybeheldbysoilcolloidsbecausetheyhaveanegativecharge.Thisprocessiscalledmicellefixation(Pidwirny,2002).Step2:Ammoniuminsolution---absorbedammonium---ammoniumbackintosolutionNH4+aqueous---NH4+...-soilcolloid---NH4+aqueousAsthisfixationisreversible,NH4+maybereleasedfromthecolloidsbywayofcationexchange.Whenreleased,NH4+maybechemicallyalteredintonitrite(NO2-)byaspecifictypeofautotrophicbacteriabelongingtothegenusNitrosomonasorganisms.NitrosomonascansynthesizetheirownorganicNcompoundsfrominorganicNsources(step3a).Then,NO2-maybequicklyconvertedintonitrate(NO3-)byanothertypeofbacteriabelongingtothegenusNitrobacter(step3b).Bothoftheseprocessesinvolvechemicaloxidationandtogetherareknownasnitrification(Pidwirny,2002;Mahendrappaetal.,1966).BothbacteriautilizetheenergyreleasedbytheoxidationofNcompoundsintheirmetabolismofwhichNO2-andNO3-areby-productsoftheirmetabolicpathways.This2-stepprocessinvolvesacomplexseriesofreactionsthatcanbesummarizedas:ReductionoftheImpactofFertilizationandIrrigationOnProcessesintheNitrogen....3Step3a:Ammoniuminsolution---nitriteinsolution55NH4++76O2+109HCO3----C5H7O2N+54NO2-+57H2O+104H2CO3Step3b:Nitriteinsolution---Nitrateinsolution400NO2-+NH4++4H2CO3+HCO3-+195O2---C5H7O2N+3H2O+400NO3-Theseequationshighlighttwoimportantpoints:nitrificationrequiresoxygenanditaffectsbulksoilpH.First,approximately4.3mgO2areconsumedforeverymgofNH4+oxidizedintoNO3-.Second,aquitesubstantialamountofalkalinityintheformofHCO3-isconsumedwhenNH4+isoxidized,thereby,indirectlydecreasingsoilpH(Ano