Institute of Food and Agricultural Sciences (IFAS) Biogeochemistry of Wetlands Si Science and da Applications Biogeochemical Properties of Wetlands Wetland Biogeochemistry Laboratory Soil and Water Science Department University of Florida Instructor K. Ramesh Reddy 1 1 Biogeochemical Properties of Wetlands Topic Outline Definition Wetland soils Biogeochemical features of wetlands Presence of molecular oxygen Sequential reduction of inorganic electron acceptors Oxidized soil-floodwater interface Exchange of dissolved substances between soil and water column Presence of hydrophytic vegetation Types of wetland soils 2 1
Biogeochemical Properties of Wetlands Learning Objectives Define biogeochemical features of wetlands, specifically Why anaerobic conditions exist in wetlands How microbes use alternate inorganic electron acceptors to support their metabolic activities Juxtaposition of aerobic and anaerobic zones Exchange of dissolved substances between soil and water column Adaptation of wetlands to anaerobic soil conditions Understand the differences among different wetland soils 3 WETLAND DEFINITION "HYDRIC SOILS (WETLAND) ARE THOSE IN THEIR NATURAL CONDITIONS ARE SATURATED, FLOODED, OR PONDED LONG ENOUGH DURING THE CROPPING SEASON TO DEVELOP ANAEROBIC CONDITIONS THAT FAVOR THE GROWTH AND REGENERATION OF HYDROPHYTIC VEGETATION. Source: USDA. Natural Resource Conservation Service 4 2
WETLAND DEFINITION Wetland contains biologically active soil or sediment in which the content of water in or the overlying water column is great enough to inhibit oxygen diffusion into the soil/sediment and stimulate anaerobic chemical and biological i l processes, that help biotic communities to adapt to anaerobic conditions. 5 Main Characteristics of Wetlands Wetlands typically lie between terrestrial (upland) and aquatic (water) systems. Wetlands are seasonally and periodically flooded or have saturated soils for significant periods of time. Presence of unique soil characteristics. Presence of special vegetation adapted to saturated soil conditions. 6 3
Wetland Ecosytem TERRESTRIAL SYSTEM WETLAND DEEPWATER AQUATIC SYSTEM Import/Export of nutrients and species Hydrology Dry Biogeochemical Role Source, sink, or transformer Productivity Low to Medium Intermittently to permanently flooded d Source, sink, or transformer Generally high, sometimes WBL low Permanently flooded Sink or transformer Generally low 6/22/2008 7 Wetland Functions - Biogeochemical Function Nutrient/contaminant storage (long-term) Nutrient/contaminant sinks Nutrient/contaminant source Transformers Adsorption and precipitation Microbial breakdown of OM Denitrification Methanogenesis 8 4
Wetland Biogeochemistry Depending on wetland type, hydrologic regime, and nutrient/contaminant inputs, wetland can serve as: SINK SOURCE TRANSFORMERS 9 WETLAND SOIL Drained Soil Flooded Soil Pore spaces Soil aggregate Porewater Porewater 10 5
WETLAND SOIL Oxygen Oxygen Oxygen Water Soil [saturated] Soil [saturated] Soil [drained] Soil [saturated] Flooded Drained 11 Biogeochemical Characteristics of Wetlands Restricted oxygen supply Presence of aerobic soil-floodwater interface Reduction of alternate electron acceptors Exchange of dissolved species between soil and water column Accumulation of organic matter Presence of hydrophytic vegetation 12 6
Anaerobic-Aerobic Interface OXYGEN Oxygen NITROGEN PHOSPHORUS Carbon Water SULFUR XENOBIOTICS METALS Soil 13 Anaerobic-Aerobic Interface 20 Dissolved oxygen (mg L -1 ) 0 2 4 6 Air Dept th (mm) 10 0-10 Water Soil Aerobic Anaerobic -20 14 7
Anaerobic-Aerobic Interface [Flooded Paddy Soil] High SOM Floodwater Low SOM Floodwater Aerobic Zone Anaerobic Zone 15 Aerobic and Anaerobic Soils Oxidized Soil Profile Reduced Soil Profile Wade Hurt 16 8
Gas Exchange in Soil-Water-Plant System Drained Soil Flooded Soil O 2 O 2 CO 2 Dissloved metals sulfides, and organic acids CO 2, CH 4, and other gases 17 Biogeochemical Characteristics of Wetlands Restricted oxygen supply Presence of aerobic soil-floodwater interface Reduction of alternate electron acceptors Exchange of dissolved species between soil and water column Accumulation of organic matter Presence of hydrophytic vegetation 18 9
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] 19 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 2 20 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 Sulfate 21 Biogeochemical Characteristics of Wetlands Restricted oxygen supply Presence of aerobic soil-floodwater interface Reduction of alternate electron acceptors Exchange of dissolved species between soil and water column Accumulation of organic matter Presence of hydrophytic vegetation 22 11
Exchange of Dissolved Solutes NH 3 NO 3 - SO 4 2- Water column Aerobic NO 3 - NH + Mn 2+ 4 DRP CH 4 Soil column Fe 2+ Anaerobic 23 10 ENR WCA-1 Depth (cm) 0 (10) (20) April 1991 October 1990 October 1990 April 1991 (30) 0 2 4 6 8 10 1 2 3 4 Ammonium N (mg/l) 24 12
Wetland receiving Secondarily Treated Sewage Effluent 10 Depth (cm) 0 (10) (20) (30) Water Soil Station 1 Station 2 Station 4 Station 6 (40) 0 1 2 0 1 2 0 1 2 0 1 2 Nitrate N (mg/l) 25 Sulfate-reducing and Methanogenic Zones in Lake Apopka Marsh Profile 20 Water Depth, cm 0-20 CH 4 -C 4 Floc sediment Peat soil -40 SO 4 -S 0 3 6 9 1 Porewater concentration, 2 mg/l 26 13
Biogeochemical Characteristics of Wetlands Restricted oxygen supply Presence of aerobic soil-floodwater interface Reduction of alternate electron acceptors Exchange of dissolved species between soil and water column Accumulation of organic matter Presence of hydrophytic vegetation 27 Organic Matter Accumulation Detrital Plant Biomass Water table Depth Aerobic Anaerobic Detritus Peat Decomposition Burial Compaction 28 14
Carbon Accumulation in Wetlands [g C/m 2 year] Alaska - Sphagnum 11-61 Finland - Sphagnum Carex 20-28 Ontario - Sphagnum bog 30-32 Georgia - Taxodium 22.5 Florida - Cladium 70-105 Florida Typha 300-400 29 Biogeochemical Characteristics of Wetlands Restricted oxygen supply Presence of aerobic soil-floodwater interface Reduction of alternate electron acceptors Exchange of dissolved species between soil and water column Accumulation of organic matter Presence of hydrophytic vegetation 30 15
Cattail Roots [Typha latifolia] Aerenchyma (intercellular air space) 31 H. Brix [Denmark] Oxidized Root Zone [Oryza sativa] Reddish brown color on root surface is due to oxidation of ferrous iron to ferric iron 32 16
Oxidized Root Zone [Oryza sativa] Reddish brown color on root surface is due to oxidation of ferrous iron to ferric iron 33 WETLAND SOILS Wade Hurt 34 17
Waterlogged Mineral Soils Increase in organic matter accumulation in surface horizon Mottled zone (gley horizon) with Fe and Mn accumulation Permanently reduced zone (Bg or Cg) Formation of secondary minerals Pyrite (Fe S 2 ) Siderite (Fe CO 3 ) Vivianite (Fe 3 (PO 4 ) 2.8H 2 O) Jarosite (K Fe 3 (SO 4 ) 2 (OH) 6 ) 35 Oxidation-Reduction WELL DRAINED SOIL Aerobic Fe 3+ Mn 4+ Aerobic Mn 4+ Fe 3+ Fe 3+ Mn 4+ Mn 2+ Fe 2+ Anaerobic RECENTLY FLOODED Aerobic FLOODED FOR LONG PERIOD Mn 2+ Fe 2+ Anaerobic Mn 4+ Fe 3+ Fe 2+ Mn 2+ Mn 2+ Fe 2+ Anaerobic RECENTLY DRAINED Wade Hurt 36 18
Organic Soils Organic matter content (> 12% of total C) in the upper soil profile Soils formed due to plant litter accumulation Soils typically have low bulk density Soils have high water holding capacity Cation exchange capacity is saturated with H + ions Wade Hurt 37 Everglades Agricultural Area Soil Subsidence 38 19
Paddy Soils Mineral and organic soils managed for rice cultivation Plowing and land leveling Flooded with 5-15 cm water Alternate flooding and draining during growing season Soils drained after rice harvest and kept fallow for 4-6 months Development of hard plow pan 39 Paddy Field - India 40 20
Marsh Soils Accumulation of plant residues Permanently reduced Bg or Cg horizon Freshwater marsh Saltwater marsh Wade Hurt 41 Freshwater Marsh Upland marshes (ombrotrophic) rainfed, hydrodynamically isolated, some ground water exchange ph 3 to 4.5 Lowland marshes (rheotrophic) receive water and nutrient inputs from adjacent areas ph 5 to 6 42 21
Saltwater Marsh Neutral ph Presence of Fe S 2 Drained soils have ph < 3.5 Fe (OH) 2 + H 2 S = FeS + 2H 2 O (Mackinawite) FeS + S 2- = Fe S 2 (Pyrite) Fe S 2 + O 2 = Fe (OH) 3 + H 2 SO 4 Thiobacillus ferroxidans Thiobacillus thioxidans 43 Biogeochemical Properties of Wetlands Summary Wetland soils exhibit unique features with aerobic and anaerobic zones Wetland soils are long-term integrators of elemental storage and ecosystem processes Wetland soils support a range of microbial communities and associated metabolic pathways Wetland accumulate organic matter Wetlands accumulate reduced chemical substances Wetlands are source green house gases including methane and nitrous oxide 44 22
http://wetlands.ifas.ufl.edu http://soils.ifas.ufl.edu 45 23