Wetland Biogeochemistry Narin Boontanon Faculty of Environment and Resource Studies Mahidol University
Wetland Biogeochemistry What is biogeochemistry? The field of biogeochemistry involves scientific study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment (including the biosphere, the hydrosphere, the pedosphere, the atmosphere, and the lithosphere), and the cycles of matter and energy that transport the Earth's chemical components in time and space. 2
Wetland Biogeochemistry In ecology and Earth science, a biogeochemical cycle is a circuit or pathway by which a chemical element or molecule moves through both biotic ("bio-") and abiotic ("geo-") compartments of an ecosystem. In effect, the element is recycled, although in some such cycles there may be places (called "sinks") where the element is accumulated or held for a long period of time. 3
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Wetland Biogeochemistry Biogeochemistry is an interdisciplinary science. It has it roots in biochemistry to understand the metabolic reactions of organisms, which obtain raw materials from the environment and cast their chemical waste into nature ( ex . cycle of reduction and oxidation) 5
Wetland Biogeochemistry Wetland Soils Must show signs of being a wetland ( hydric ) soil » mottling » color » deep organic layer Soils can still be saturated with a water table 1 foot or more below the surface 6
Wetland Biogeochemistry Wetland Soils Types and Definitions Wetland soil are both the medium in which many of the wetland chemical transformations take place and the primary storage of available chemicals for most wetland plants. They are often described as hydric soils : formed under conditions of saturation long enough to develop anaerobic conditions. Gray or neutral color is caused by “reduction”, which is a bio-chemical process involving soil microbes in waterlogged soils. 7
Wetland Biogeochemistry Wetland Soils Types and Definitions Wetland soil are of two types: 1. mineral soils: less than 20-35% OM (on dry-weight basis) 2. organic soils have a specific definition dependent upon degree of saturation and soil texture. Organic soils differ from mineral soils in these categories: - Bulk density and porosity (lower bulk density) - Hydraulic conductivity (depends on degree of decomposition) - Nutrient availability (more nutrients are tied up in unavailable organic forms) - Cation exchange capacity (greater cation exchange capacity) 8
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Wetland Biogeochemistry Wetland Soils Identification Hydric soils Lacking oxygen Soil Color grayish and may have black and white mottling patterns. Soil Permeability organic (high) or mineral (poor) Soil Texture fine particles, silts, and clays Soil Smell sulfurous (rotten egg) 10
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Wetland Biogeochemistry Jarosite is a basic hydrous sulfate of potassium and iron with a chemical formula of KFe 3+ 3 (OH) 6 (SO 4 ) 2 . This mineral is formed in ore deposits by the oxidation of iron sulfides. 13
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Wetland Biogeochemistry Chemical Transformations in Wetlands Oxygen and Redox Potential Oxygen diffuses slowly in water, so slowly, in fact, that it is often used up by microbial activity faster than it can be replenished. This affects root respiration, and impacts nutrient availability. Some soil components are changed to toxic forms. Redox potential is the measure of electron availability in a solution. Reduction is the opposite of oxidation. It involves releasing oxygen, gaining hydrogen, or gaining an electron. It is driven down by microbial activity, as metabolizing organisms seek terminal electron acceptors to allow their harvest of energy from substrate compounds. 15
Wetland Biogeochemistry Oxygen and Redox Potential Organic decomposition can occur in the presence of any number of - , Mn 4+ , Fe 3+ , SO 4= . It terminal electron acceptors, including O 2 , NO 3 occurs most rapidly in the presence of oxygen, and slower for other electron acceptors. Redox potential drops through the sequence of electron acceptors, as O 2 is the acceptor at 400-600 mV. Nitrate becomes an acceptor at 250 mV, manganese at 225 mV, iron between +100 and -100 mV, and sulfides at -100 to -200 mV. Carbon, or CO 2 , will become the terminal electron acceptor below -200 mV. 16
Wetland Biogeochemistry Sequence in time of transformations in soil after flooding. 17
Wetland Biogeochemistry The thickness of oxidized layer is directly related to: 1. The rate of oxygen transport across the atmospheric-surface water interface 2. The small population of oxygen-consuming organisms present 3. Photosynthetic oxygen production by algae within the water column 4. Surface mixing by convection currents and wind action 18
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Wetland Biogeochemistry Carbon Transformations Methanogenesis occurs when certain bacteria use CO 2 as an electron acceptor and produce gaseous methane (CH 4 ). 20
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Wetland Biogeochemistry Nitrogen Transformations One of the more significant ways that nitrogen is lost to the atmosphere is in wetlands. Organic N is mineralized to ammonium + . In aerobic conditions, nitrification occurs through the NH 4 mediations of Nitrosomonas then Nitrobacter , resulting in nitrite then nitrate. Nitrate is often then subjected to uptake or leaching, as it is very mobile in solution. If not, it may be subjected to denitrification, which results in gaseous nitrogen forms that are lost to the atmosphere. Denitrification is inhibited in acid wetland soils. 22
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Wetland Biogeochemistry Phosphorus Transformations Phosphorus is a major limiting nutrient in freshwater marshes, northern peatlands and deepwater swamps. It is more available in agricultural wetlands and saltmarshes. Phosphorus retention is often considered to be an important ecosystem function in wetlands and is often designed into constructed wetlands. Phosphorus occurs in a sedimentary rather than gaseous cycle like nitrogen. It is often present in wetlands as a cation. It may be tied up in organic litter in peatlands or in inorganic sediment in other wetlands. It can be made unavailable for uptake as the result of precipitation as phosphates with ferric iron and aluminum (acid soils), or calcium and magnesium (basic soils) under aerobic conditions. In water columns, anaerobic conditions render it soluble. 24
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Wetland Biogeochemistry Sulfur Transformations At low redox potentials, sulfur is reduced and H 2 S, hydrogen sulfide, is released. Because the concentration of sulfates is higher in salt water wetlands, sulfide emission is also higher, and toxicity greater. Toxicity can occur as the result of contact with roots, or with reduced availability of sulfur to plants because it precipitates with trace metals. Zinc and copper can also be limiting because they precipitate with sulfur. If ferrous iron is present, it will precipitate with sulfides. Ferrous sulfide (FeS) give many wetland soils their black color, and is the source of sulfur commonly found in coal deposits. 26
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Wetland Biogeochemistry Chemical Transport Into Wetlands Precipitation Levels of chemicals entering wetlands in precipitation are variable, but such solutions are very dilute. Higher magnesium and sodium are associated with maritime influences, while calcium is associated with continental influences. Sulfate concentrations from industrial atmospheric pollution could have an impact in oligotrophic systems, though sulfate levels have decreased. Nitrates from auto exhausts have not decreased and may similarly impact poorly buffered systems. 28
Wetland Biogeochemistry Chemical Transport Into Wetlands Streams, Rivers, Groundwater Dissolved substances in groundwater often depend upon the ease of dissolution of the mineral, with limestone and dolomite yielding high levels of dissolved materials and granite and sandstone low levels. Arid regions tend to have higher salt concentrations in surface waters. Geography: there is often an inverse correlation between streamflow and dissolved materials; sediment load and dissolved materials. Human uses impact sediment, nutrients, herbicides, pesticides, and organic loading (BOD). 29
Wetland Biogeochemistry Chemical Transport Into Wetlands Estuaries Estuaries have quite variable chemistry, different from both that of the adjacent ocean or the tributary rivers. 30
Wetland Biogeochemistry Chemical Mass Balances of Wetlands Wetlands may serve as sources, sinks or transformers of chemicals. There are seasonal patterns of uptake and release, and they are different for colder, low productivity systems and warmer, high productivity systems. Wetlands are frequently couple to adjacent ecosystems through chemical exchanges that are significant to both systems. Wetlands can be highly productive or systems of low productivity. Nutrient cycling is different in aquatic and terrestrial systems. 31
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