Plant Micronutrients and Heavy MetalsOutlineRenewed interest in micronutrientsOrders of magnitude difference in micronutrient needDeficiency vs. toxicityMicronutrients can be toxic athigh concentrations (heavy metals)Zinc deficiency:Symptoms on new tissueBoron deficiencyNutrient deficiencies in grapeMicronutrient cycling, not volatileConditions conducive to deficienciesAvailability influenced by:pH and micronutrientsIron deficiency: interveinal chlorosisFe deficiency in sorghumIron deficiency of beanPeriodic table of the elements: redox rxns in soilOxidation/ReductionMn+3 available under reducing conditionsAcidification/chelation of lupine rhizosphere for iron uptakeChromium, CrMicronutrient interactions are highly complex and emphasize need for nutrient balanceIron deficiency induced enhancement of manganese reductionFe-deficiency induced acid secretion in rhizosphere of cucumber (high pH)Common plants/micronutrient deficiencyHow can you know positively?How to provide micronutrientsWhat is a heavy metal?Periodic table of the elements Problem heavy metals with arrowsWhy are heavy metals impt in org ag?How’d they get into the soil?Contamination vs. pollutionSome heavy metals and their environmental and physiological effectsThe Fertilizer Loophole in 1976 RCRA- Reduce, Recycle, ReuseSome industrial wastes with arsenic, cadmium, lead, mercury, dioxins, etc., are “recycled” through ordinary fertilizer withoutWastes in FertilizersGovernment and industry’s positionFour heavy metalsMercury (Hg) ppmArsenic: SourceAs- sources and background levelsArsenic (As) ppmArsenic and human healthAs- regulationArsenic in drinking water in USAs behavior in soil and plantsLead (Pb)Lead (Pb) ppmCadmium (Cd) ppmCadmium (Cd) ppmCd- Sources of exposureCd has increased in soils due to P fertilizer usePhosphate fertilizers as source of Cd, Pb, As: Western states to 340 mg/kg CdLimits in sewage sludge and metal additions for WASoil applied sewage sludge: Pb stays in soil, all others 40-60% lost (20-80%?)Reduction of HM in sewage sludge 1976 to 1990Heavy metals in the food chain:soil to plantPlant uptakeConcentration of Pb and As in plantsDuff Wilson’s exposeFeds not moving to regulate (USDA or EPA), States regulationMessageInternational Pb and Cd limits in foods-- no established US limitsHeavy metals in the food chain: Plants to animalsHM in earthworms after application of sewage sludge- concentrate Cd, ZnCd uptake in snailsHeavy metals in the food chain: Soil to animalsAnimal uptake of soil-- not via plant!How can we manage Pb and As contaminated soil soils?Organic matter binds heavy metals (make sure OM not contaminated) --the case of CrAdd lime (make sure source not contain heavy metals)PhytoremediationTable 15.6 Metal Accumulators have been found widelyPlant “hyperaccumulation”HyperaccumulatorBioremediation/accumulation by fungiThe Natural Step addresses problem of mined products, and system is used by some industry, communities www.naturalstep.orgSummary and conclusionsStudy QuestionsReferencesPlant Micronutrients and Heavy MetalsMartha E. RosemeyerEcological AgricultureFebruary 28, 2006Outline What are micronutrients? Availability How to manage them What are heavy metals?  How are they bound to the soil Food chainz Plant uptake of heavy metalsz Animal uptake Reducing heavy metal contaminationRenewed interest in micronutrients Exportation from soils with high yields High-analysis fertilizers have decreased trace nutrient “contaminants” Ability to diagnose plant deficiencies Increasing evidence that food grown on deficient soils doesn’t meet human nutritional goals although plants don’t suffer Some are being applied to soils with heavy metals at levels that cause animal toxicityOrders of magnitude difference in micronutrient needFigure 15.3Deficiency vs. toxicityMicronutrients can be toxic athigh concentrations (heavy metals)Zinc deficiency:Symptoms on new tissueBoron deficiencyNutrient deficiencies in grapeOH-form dominant in alkaline soilsMicronutrient cycling, not volatileFigure 15.6Conditions conducive to deficiencies Parent Material  Leached, sandy acid soils Organic soils, esp. Cu is a problem Intensive cropping pH extremes– low Mo def, Mn toxichigh pH- Fe, Mn, Zn, B, Cu unavailable ErodedAvailability influenced by: pH, e.g. lime induced Chlorosis Oxidation state and pH- Fe, Mn, Ni, Cu occur in more than one valence state, depends on aeration Interaction of pH and aeration (degree of oxidation):z Fe, Mn toxic at low pH, reduced form (flooded or wet conditions)z At high pH the hydroxide form of Fe, Mn precipitates out but Mo can be toxicz Certain plants less sensitive to low Fe, Mn because lower pH and secrete compounds that reduce the Fe Other inorganic reactions, e.g. Fe precipitated by PO4-2 OM, organic compounds as chelates Role of mycorrhizae- in uptake/EM protect excesspH and micronutrientsIron deficiency: interveinalchlorosisFe deficiency in sorghumIron deficiency of beanYellow chlorosis (yellowing) between green veinsPeriodic table of the elements: redox rxns in soilOxidation/Reduction OIL RIG (oxidation is loss OIL/ reduction is gain RIG) or LEO/GER O2 + 4Fe +2+ 4H+ 4Fe+3+ 2H2O  4 Fe +24Fe+3+ 4e-(iron gives up electrons) O2+ 4H++4e-2H2O (oxygen accepts electrons) Fe+2 (ferrous, more soluble) oxidized to Fe +3(ferric, less soluble) and O2is being reduced (gaining elections) At low pH (high acidity) oxidation of Fe takes place because oxygen needs H to accept electronsMn+3available under reducing conditionsFigure 15.9Acidification/chelation of lupine rhizosphere for iron uptakeFe+3reductionto Fe+2Marschner et al. 1986Function of chelate is to help ion remain available, diffuses to soilAnd then reduced at root surfacedicotsmoncotsChromium, Cr Trace amounts are essential but excess are carcinogenic Cr +3(reduced) and Cr+6 (oxidized) Cr +3not available at pH 5.5 wet (anaerobic) Cr +6contamination of ground water with mutagen, movie Erin BrockavichMicronutrient interactions are highly complex and emphasize need for nutrient balanceIron deficiency induced enhancement of manganese reductionMnO2reducedPeanut in calcareous soilwith low available FeMarschner et al. 1986Fe-deficiency induced acid secretion in rhizosphere of cucumber (high pH)Fe+3(ferric) chelation/reduction and uptake using organic acidsAcidification increases Fe uptake an order of magnitudeCommon