Transport & Transformation of chemicals in an ecosystem, involving numerous interrelated physical, chemical, & biological processes

Transcription

1 OPEN Wetland Ecology Lectures Wetland Biogeochemistry What is biogeochemical cycling? Transport & Transformation of chemicals in an ecosystem, involving numerous interrelated physical, chemical, & biological processes Examples of movement include Water-sediment exchange Plant Uptake Organic exports Two major categories of wetland biogeochemistry include: Intrasystem cycling through various transformation processes Exchange of chemicals between a wetland & its surroundings Open vs Closed Ecosystems BHF + Tidal Marshes have significant exchange of materials with surroundings CLOSED River flooding + Tidal Exchange Bogs + Cypress Domes Little material exchange except for gaseous matter! Biogeochemically open vs closed BGC Open Abundant exchange of materials with surroundings Tidally driven or riparian BGC Closed Little movement of materials across the ecosystem boundary Cypress swamps or very stagnant areas So, are wetlands open or closed systems? They can be both! Wetland Soil Wetland soil is: Medium in which many of the wetland chemical transformations occur The primary storage of available chemicals for wetland plants

2 Two major types of wetland soils Organic & Organic Mineral Soils Mineral Organic Soil + Organic Mineral Soils Defined under two saturation conditions 1. Saturated with water for long periods Have > 18% organic carbon Have > 12% organic carbon if no clay present Have a proportional content of organic carbon (12 18%) if clay content (0 60%) 2. Soils are never saturated with water for more than a few days and have > 20% organic carbon %Corg = %OM/2 %Corg = percentage of organic carbon %OM = percentage of organic matter Bulk density & porosity have lower bulk densities (dry weight of soil/volume). Low due to high porosity (peat soils ~ 80% air space) Hydraulic conductivity Depends on degree of decomposition. Organic soils hold more water (do not necessarily allow more water to pass) Nutrient availability Organic soils have more minerals tied up in organic form. Cation Exchange Capacity Organic soils have greater CEC (sum of exchangeable ions) Organic soils (What order?) are classified into four groups; 3 are hydric Saprists (muck) > 2/3 of the material is decomposed, < 1/3 of plant fibers are identifiable Fibrists (peat) - < 1/3 of material is decomposed and > 2/3 of plant fibers are identifiable Hemists (mucky peat or peaty muck) Conditions fall between saprist & fibrist soil Folists organic soils caused by excessive moisture (precip > evapotranspiration) that accumulate in tropical & boreal mountains; not classified as hydric because saturated conditions are the exception rather than the rule Mineral Wetland Soil When flooded for extended periods mineral soils develop certain characteristics that allow for their identification Redoximorphic features (mediated by microbes) The rate these features are formed depend on three conditions (all must be present) Sustained anaerobic conditions

3 Sufficient soil temp (5 C biological zero ) Organic Matter (substrate for microbial activity) Hydric mineral soils are characterized by: Gleying Oxidized Rhizosphere Mottles (aka Redox Concentrations) Current nomenclature 1. Redox concentrations Accumulation of Fe & Mn in 3 different structures Nodules & Concretions (firm extremely firm irregularly shaped bodies with diffuse boundaries) Masses formerly called reddish mottles Pore linings Formerly included oxidized rhizospheres 2. Redox depletions Low-chroma (<2) bodies with high values (>4) including: Iron depletions: Gray mottles or Gley mottles Clay depletions: Contain less Fe, Mn, and clay than adjacent soils 3. Reduced matrices: Low-chroma soils REDOX in Wetlands When mineral or organic soils are flooded anaerobic conditions result. Water fills pore spaces and rate of oxygen diffusion through soil is drastically reduced Rate of oxygen depletion depends on: Ambient Temperature Availability of organic substrates for microbial respiration Chemical oxygen demand from reductants such as ferrous Fe Resultant O 2 deficiency prevents plants from normal aerobic root respiration and affect nutrient availability and adds toxic materials in the soil Usually thin layer formed and is related to: Eh ranges Rate of O 2 transport across the atmosphere-surface water interface Small population of O 2 consuming organisms Photosynthetic O 2 production by algae within water column Surface mixing by convection currents & wind action If DO is present, the redox potential range +400 to + 700mV

4 If O2 disappears, Eh range from +400 down to -400mV As organic substrates in a water logged soil are oxidized (donating) the redox potential drops = a sequence of reductions (gains) takes place 1 st reaction to occur after becoming anaerobic is the reduction of NO3- (nitrate) first to NO2- (nitrite) and ultimately to N2O or N2 ph Nitrate becomes an acceptor ~ 250mV At 225mV Mn is transformed from manganic to manganous At -75 to -150mV Fe is transformed from ferric to ferrous; while sulfates are reduced to sulfides Soil and overlying waters of wetlands occur over a wide range of ph s Organic soils often more acidic Mineral soils often neutral or alkaline Alkaline soils previously drained decrease in ph because of buildup of CO 2 then carbonic acid Acid soils previously drained increase in ph because of reduction of ferric iron hydroxide Nitrogen Transformations Nitrogen is often the most limiting nutrient in flooded soils Limitations reported in salt marshes, freshwater inland marshes, & freshwater tidal marshes Involve complex microbial processes NH4 is the primary form of mineralized N in wetlands Mineralization Often referred to as ammonification NH 2 CONH 2 + H 2 O 2NH 3 + CO 2 Nitrification NH 3 + H 2 O NH 4 + OH - Once ammonia has formed, it can take several possible pathways Aerobic environment ammonium is oxidized (nitrification) in two steps by Nitrosomonas sp. 2NH O 2 2NO H 2 O + 4H + + energy By Nitrobacter sp. 2NO O 2 2NO energy

5 Denitrification Denitrification is carried out by facultative bacteria under anaerobic conditions nitrate is the terminal electron acceptor C 6 H 12 O 6 + 4NO 3 6CO 2 + 6H 2 O + 2N 2 Most significant path of nitrogen loss from wetlands Usually lost as N 2 & N 2 O N fixation Conversion of N2 gas to organic nitrogen Favored by low oxygen concentrations Rhizobium species Cyanobacteria (blue-green algae) are also common in Louisiana, northern bogs, & in rice cultures Fe & Mn Transformations Found in reduced forms in wetlands Readily available Toxic levels More soluble Ferrous Fe reduced form of Fe Ferric Fe oxidized form of Fe Oxidized form creates barrier; may prevent plant from uptaking other nutrients Reduced form reacts with P making it unavailable Sulfur Rarely limiting to plant or animal growth in wetlands Hydrogen sulfide (H 2 S) is toxic (rotten-egg smell) Ferrous sulfide is responsible for black color of wetland soils (highly reduced sediment) Negative effects of sulfides on higher plants are attributable to a number of causes Direct toxicity of free sulfide as it comes into contact with plant roots Reduced availability of sulfur for plant growth because of its precipitation with trace metals Immobilization of zinc & copper by sulfide precipitation Carbon Cycle Photosynthesis & aerobic respiration dominate the aerobic horizons Fermentation

6 Methanogenesis Occurs when certain bacteria (methanogens) use CO 2 as an electron acceptor for the production of gaseous methane (CH 4 ) Requires extremely reduced conditions Phosphorus Redox potential range from -250 to -350mV One of the most important chemicals in wetland systems Most limiting in northern bogs, freshwater marshes, & southern deepwater swamps Retention is one of the most important features for natural & constructed wetlands Principle inorganic form = orthophosphate PO 4 3- (ph > 13) H 2 PO 4 - (ph 2-7) HPO 4 (ph 8-12) Predominant form dependent on ph P is not directly altered by Eh changes Affected by association with other elements especially Fe P is rendered relatively unavailable to plants & microconsumers by: 1. Precipitation of insoluble phosphates with ferric Fe, Ca, & Al under aerobic conditions 2. Adsorption (particle surface) of phosphate onto clay particles, organic peat and ferric & aluminum hydroxides & oxides 3. Binding of phosphorous in OM as a result of its incorporation into the living biomass of bacteria, algae, & vascular macrophytes Chemical Transport into Wetlands Geologic inputs from weathering of rock Type of soil direct reflection of parent material Biologic inputs Photosynthetic uptake of carbon, nitrogen fixation, & biotic transport of materials by animals (birds) Hydrologic inputs Major inputs into wetlands Precipitation Burning of fossil fuels Increased [ ] s of sulfates & nitrates in atmosphere Streams, Rivers, & Groundwater

7 As precip reaches the ground it will: Infiltrate into the ground Return to atmosphere via evapotranspiration Flow on surface as runoff Groundwater influence: Climate: Chemical characteristics of streams & rivers depend on the degree to which the water has previously come into contact with underground formations & types of minerals present in those formations Balance of precipitation & evapotranspiration Type of vegetation present Geographic effects: amount of dissolved & suspended materials that enter streams, rivers, & wetlands depend on: Size of watershed Steepness or slop of landscape Soil texture Streamflow/Ecosystem effects: The water quality of surface water runoff, streams, & rivers varies seasonally Human effects: water that has been modified by humans through sewage effluent, urbanization, & runoff from farms alters the chemical composition of streamflow & groundwater that enters wetlands Drainage from agriculture fields: Higher [ ] s of sediments, nutrients, herbicides, & pesticides might be expected Drainage from urban & suburban areas: Estuaries May have high [ ] s of trace organics, oxygen demanding substances, & some toxic materials Quality differs from that of rivers Seawater chemical composition is fairly constant worldwide 33 0/00 to 37 0/00 Mass balances A quantitative account of the inputs, outputs, & internal cycling of materials in an ecosystem Mass balances help determine:

8 Ecosystem functions Determine the importance of wetlands as sources, sinks, and transformers of chemicals Inputs primarily through: Exports: Hydrologic Biotic Precipitation Surface & groundwater inflow Tidal Exchange Atmospheric carbon fixation (photosynthesis) Atmospheric nitrogen (nitrogen fixation) Surface water & groundwater Long-term burial of chemical in the sediments Intrasystem cycling involves exchanges among various pools, or standing stocks of chemicals in within a wetland. Involves pathways such as: Litter production Remineralization Chemical transformations Translocation of nutrients through plants Generalizations 1. Wetlands serve as sources, sinks, or transformers of chemicals, depending on the wetland type, hydrologic condition, & length of time the wetland has been subjected to chemical loadings 2. Seasonal patterns of nutrient uptake & release Temperate regions retention is higher in growing season Increased microbial activity Higher macrophyte activity 3. Wetlands are frequently coupled to adjacent ecosystems through chemical exchanges that affect both systems Downstream ecosystems benefit from retention or from exportation 4. Wetlands are either highly productive (eutrophic) or low productivity (oligotrophic) 5. Nutrient cycling in wetlands differs from aquatic & terrestrial systems More nutrients in sediment & peat

9 Aquatic systems have autotrophic activity more dependent on nutrients in water column than in sediments. Wetland plants obtain nutrients from sediment 6. Anthropogenic changes have led to changes in nutrient cycling in many wetlands The capacity of wetlands to assimilate anthropogenic wastes from the atmosphere or hydrosphere is not unlimited!