Biogeochemistry of Wetlands

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1 Institute of Food and Agricultural Sciences (IFAS) Biogeochemistry of Wetlands Si Science and da Applications Electrochemical Properties Wetland Biogeochemistry Laboratory Soil and Water Science Department University of Florida Instructor K. Ramesh Reddy 6/22/ Chemical Reactions in Natural Systems Reactions in which neither protons nor electrons are exchanged Fe 2 O 3 + H 2 O = 2 FeOOH Reactions involving protons H 2 CO 3 = H + + HCO - 3 Reactions involving electrons Fe 2+ = Fe 3+ +e - Reactions in which both protons and electrons are transferred 2Fe(OH) 3 + 3H + + e - = 2Fe H 2 O 2 1

2 Institute of Food and Agricultural Sciences (IFAS) Electrons and Protons e - e - e - eē- e - e - e - e - e - H + e - H e e e - H H + H + H + H + e e - - e - e - e - e e - e - H + H H + H + - e + - e - e - e - H + e - H e - e - e - H + + e - e - e - H + H + e - e - H e Electrochemical Properties Topic Outline Introduction Oxidation-reduction reactions Nernst Equation Eh - ph relationships Buffering of redox potential Measurement of redox potentials Soil and water column ph Redox couples in wetland soils Redox gradients in wetland soils s Specific conductance Soil oxygen demand Walther Nernst The Nobel Prize in Chemistry

3 Electrochemical Properties Learning Objectives Basic concepts related to oxidationreduction reactions Use of Nernst Equation to calculate redox potential (Eh) Relationship between redox potential (Eh) and ph Laboratory and field measurements of redox potentials Diel changes in water column ph Redox couples and microbial metabolic activities in wetlands Redox gradients and aerobic/anaerobic interfaces in wetlands Soil oxygen demand and nutrient fluxes Source: D. R. Lovley, Nature Reviews 4: Oxidation-Reduction Reductant Oxidant + e - Reductant = Electron donor [Organic matter, NH 4+, Fe 2+, Mn 2+, S 2-, CH 4, H 2, H 2 O] Oxidant + e - Reductant Oxidant = Electron acceptor [O 2, NO 3-, MnO 2, Fe(OH) 3, SO 4 2-, CO 2, and some organic compounds] 6 3

4 Oxidation-Reduction [Aerobic Respiration] Oxidation C 6 H 12 O 6 + 6H 2 O = 6CO 2 Reductant Reduction 6O H e - = 12H 2 O Oxidant Oxidant + 24H e - Reductant C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O Oxidation - Reduction Oxidation-Reduction [Nitrate Respiration Dentrification] Oxidation 5C 6 H 12 O H 2 O = 30CO 2 Reductant + 120H e - Reduction 24NO H e - = 12N H 2 O Oxidant Oxidant Reductant 5C 6 H 12 O NO H + = 12N CO H 2 O Oxidation - Reduction 4

5 Oxidation-Reduction [Sulfate Respiration] Oxidation C 6 H 12 O 6 + 6H 2 O = 6CO 2 Reductant + 24H e - Reduction 3SO H e - = 3S H 2 O Oxidant Oxidant Reductant C 6 H 12 O 6 + 3SO 2-4 = 3S CO 2 + 6H 2 O Oxidation - Reduction Oxidation-Reduction UPLAND SOILS O 2 NO - 3 Mn 4+ Fe 3+ Reduction FLOODED SOILS H 2 O N 2 NH 4 + Mn 2+ Fe 2+ SO 4 2- CO 2 PO 4 3- H 2 O S 2- Oxidation CH 4 PH 3 H

6 Nernst Equation m (OXIDANT) + m H + + n e - = m (REDUCTANT) Eh = E o - [0.059/n] log [Reductant/Oxidant] [m/n] ph E = Electrode potential (volts) E o = Standard electrode potential (volts) F = Faraday s constant ( kcal/volt mole or kj/volt mole R = Gas constant ( kcal/mole degree or kj/mole degree T = Temperature ( K ( o C)) n = number of electrons involved in the reaction 11 Wetland Soil Drained Soil Anaerobic Aerobic Highly Reduced Reduced Moderately Reduced Oxidized Oxidation-Reduction Potential (mv) 12 6

7 Oxidation-Reduction ure Electron Press Strongly reduced Strongly oxidized Oxidation-Reduction Potential (mv) 13 Electron donors [Organic matter, NH 4+, Fe 2+, Mn 2+, S 2-, CH 4, H 2, H 2 O] e - e - e - eē- e - e - e - e - e - e - e - H + e - H e e e - H e - H e - + e - H + H + H + H + e - e - e - H + - e - H + H + H + e e - e - e - e - O 2 NO 3 - Electron acceptors Energy Mn 4+ Fe 3+ SO 4 2- CO 2 H2 O Oxidation-Reduction Potential (mv) 14 7

8 How much energy is released during oxidation - reduction reactions? [+] [-] Elec ctrode Potentials SO CO 2 SO 4 2- Fe (III) N-Oxides Mn (IV) O 2 En nergy Yield Ease of Reduction 15 Oxidation-Reduction Mn 4+ Mn 2+ CO 2 CH 4 SeO 2-3 Se(0); Se 2- SeO 2-4 SeO 2-3 SO 2-4 S 2- Fe 3+ Fe 2+ NO - 3 N 2 O 2 H 2 O Redox Potential, mv (at ph 7) 16 8

9 Iron Redox Couple and Eh-pH l (mv) Re edox Potentia Fe 3+ O 2 H 2 Fe 2 O 3 Fe 2+ H 2 O H 2 O FeCO 3 FeS 2 Fe 2+ FeCO 3 Fe 3 O ph Oxidation-Reduction Uplands Electron acceptor non- limiting Electron donor limiting Wetlands and Aquatic Systems Electron acceptor limiting Electron donor non-limiting 18 9

10 Re elative Concentr ration Sequential Reduction of Electron Acceptors SO 4 2- NO 3 - O 2 Organic Substrate [e - donor] Mn 2+ Fe 2+ S 2- CH 4 Oxygen Nitrate Iron Methanogenesis Manganese Time or Soil Depth Sulfate 19 Redox Zones with Depth WATER SOIL Depth I II III IV V Oxygen Reduction Zone Oxygen Reduction Zone Eh = > 300 mv Nitrate Reduction Zone Mn 4+ Reduction Zone Eh = 100 to 300 mv Fe 3+ Reduction Zone Eh = -100 to 100 mv Sulfate Reduction Zone Eh = -200 to -100 mv Methanogenesis Eh = < -200 mv Aerobic Facultative Anaerobic 20 10

11 Regulators of Eh Water-table table fluctuations. Activities of electron acceptors. Activities of electron donors. Temperature ph 21 Field Redox Electrodes Copper wire Heat shrinking tube Volt meter Calomel Reference Epoxy Platinum wire Water Soil Platinum electrodes 22 11

12 Laboratory Redox Electrodes Copper wire Platinum Glass Electrode Saturated KCl Heat shrinking tube Glass tube Glass tube Calomel + Mercury Mercury Platinum wire Salt bridge Epoxy Mercury Platinum Calomel Reference Electrode wire 23 Okeechobee Basin Wetland Soils and Stream Sediments 24 12

13 Flooded Organic Soils: Everglades Agricultural Area 25 Flooded Paddy Soils: Louisiana Redox Potential, mv Aerobic Anaerobic Time, days 26 13

14 Electron Acceptors - Redox Potential Eh (mv) Oxygen Nitrate Sulfate Bicarbonate Time (wk) 27 Electron Donor [Organic Matter] Redox Potential Redox potential (m mv) Low Organic Matter Soil High Organic Matter Soil Time after flooding 28 14

15 Depth (mm) Redox Gradients in Sediments 0-20 Aerobic Layer Anaerobic Layer Redox Potential (mv) 29 Sediment Microbial Fuel Cell Source: D. R. Lovley, Nature Reviews 4:

16 Eh mv] Redox Potential and ph ph Baas Becking et al. 31 Limitations of Redox Potentials Most of the redox couples are not in equilibrium except in highly reduced soils. In biological systems, electrons are added and removed continuously. Platinum electrodes respond favorably to reversible redox couples. Redox potential is closely related to ph. Platinum electrode surface can be contaminated by coatings of oxides, sulfides and other impurities

17 Soil and Water Column - ph Reactions involving protons CO 2 + H 2 O = H 2 CO 3 H 2 CO 3 = H + + HCO - 3 HCO 3- = H + + CO 2-3 Reactions in which both protons and electrons are transferred 2Fe(OH) 3 + 3H + + e - = 2Fe H 2 O 33 Water Column ph: Experimental Ponds Lake Apopka Basin 10 ph Water hyacinth Algae Cattails and Egeria Time, hundred hours 34 17

18 Effect of Flooding on Soil ph 8 Clay loam [ ph = 8.7; OM = 2.2%; Fe = 0.63%] 7 ph Clay [ ph = 3.4; OM = 6.6%; Fe = 2.8%] Clay [ ph = 3.8; OM = 7.2%; Fe = 0.1%] Time after flooding 35 Effect of Flooding on Soil Porewater Ionic Strength Fe 2+ Fe 2+ A Soil Fe 2+ Fe 2+ Ca 2+ NH 4 + Mn 2+ K + Solid Phase Soil Solution Ionic Streng gth B Time after flooding 36 18

19 Redox Couples in Wetlands C 6 H 12 O 6 /CO 2 and O 2 /H 2 O C 6 H 12 O 6 /CO 2 and NO 3- /N 2 C 6 H 12 O 6 /CO 2 and MnO 2 /Mn 2+ C 6 H 12 O 6 /CO 2 and FeOOH/Fe 2+ C 6 H 12 O 6 /CO 2 and SO 2-4 /H 2 S H 2 /H + and CO 2 /CH 4 37 Aerobic Respiration and Energy Yield C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O G r = kcals/mole ADP + P i = ATP G r =-7.7 kcals/mole 38 19

20 Institute of Food and Agricultural Sciences (IFAS) Biogeochemistry of Wetlands Si Science and da Applications Soil Oxygen Demand Wetland Biogeochemistry Laboratory Soil and Water Science Department University of Florida Instructor K. Ramesh Reddy 6/22/ Oxygen Oxygen is an electron acceptor Reduction [Electron acceptor] O 2 + 4H + + 4e - = 2H 2 O : Oxidant Oxidation [Electron donor] C 6 H 12 O 6 + 6H 2 O = 6CO H e - Reductant 40 20

21 Oxygen Consumption Heterotrophic microbial respiration C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O Chemolithotrophic oxidation of reduced inorganic compounds NH O 2 = NO 3- + H 2 O + 2H + Chemical oxidation of reduced inorganic compounds 4Fe H 2 O + O 2 = 4Fe(OH) 3 + 8H + 41 Oxidation-Reduction Carbon Nitrogen O 2 O 2 Floodwater CO 2 O 2 + CH 4 Aerobic soil Anaerobic soil OM CH 4 Floodwater NO 3 O 2 + NH 4 Aerobic soil Anaerobic soil OM NH

22 Oxidation-Reduction Iron Manganese O 2 O 2 Floodwater Floodwater Fe 3+ O 2 + Fe 2+ Mn 4+ O 2 + Mn 2+ Aerobic soil Aerobic soil Anaerobic soil Anaerobic soil FeOOH Fe 2+ MnO 2 Mn Oxidation-Reduction Carbon Sulfur O 2 O 2 Floodwater Floodwater CO 2 O 2 + OM SO 4 O 2 + H 2 S Aerobic soil Aerobic soil Anaerobic soil Anaerobic soil SO 4 H 2 S 44 22

23 Oxygen Consumption Low organic matter soil [C/C o ] Consumption during chemical oxidation Consumption during biological oxidation High organic matter soil Time (hours) 45 Aerobic Respiration Ox xygen consumpt tion, mg/kg day Talladega, AL Salt marsh, LA Belhaven, NC Unimpacted Everglades, FL Lake Apopka marsh, FL Prairie pothole, ND Crowley, LA Houghton Lake marsh, MI Impacted Everglades, FL y= ln(x) R 2 = ,000 1,500 2,000 2,500 3,000 Dissolved organic C, mg/kg 3,

24 DEPTH (cm m) WATER FILAMENTOUS ALGAE Impacted 12N 6P 12M 6A MACRO-LITTER AND ALGAE Unconsolidated Peat Unimpacted WATER PERIPHYTON Peat DISSOLVED OXYGEN (% SATURATION) 47 Oxygen - Periphyton Depth (mm) % O 2 Saturation Irradiance (μmol m -2 s -1 ) S. Hagerty, SFWMD unpublished results 48 24

25 Lake Apopka Marsh De epth, cm Soluble P, mg L -1 Dissolved Fe, mg L Phosphorus Iron Water 0-10 Soil Mobile and Immobile Iron rface Dep pth below soil su Fe 2+ Aerobic Insoluble Fe Anaerobic % Fe 50 25

26 OXYGEN: Sources and Sinks Plants and Algae Water Air Soil Oxygen Release by Plant Roots Respiration Oxidation of Reductants Chemolithotrophic oxidation Chemical oxidation 51 Electrochemical Properties & Soil Oxygen Demand Summary Oxidation-reduction reactions regulate several elemental cycles Wetland soils are limited by electron acceptors and have abundant supply of electron donors. Upland soils are usually limited by electron donors, and have abundant supply of electron acceptors (primarily oxygen). Nernst Equation is used to calculate redox potential (Eh) Redox potentials (Eh) are inversely related to ph (59 mv/ph unit) Redox potential of soils are affected by (i) activities of electron donors (ii) activities of electron acceptors and (iii) temperature Laboratory and field electrodes can be used to measure redox potentials of soils 52 26

27 Electrochemical Properties & Soil Oxygen Demand Summary Distinct Eh gradients are present at (i) the soil-floodwater interface, (ii) root-zone of wetland plants, and (iii) around soil aggregates in drained portions of wetlands during low water-table depths. Water column ph is affected by photosynthesis Soil ph is affected by reduction of electron acceptors The rate of oxygen consumption is related to the concentration of reductants Oxygen consumption at the soil-floodwater interface regulates nutrient fluxes