Formation, Chemistry, and Biology of Wetland Soils

1. Formation, Chemistry, and Biology of Wetland Soils Maverick, Dana, Devon

2. General Information on Soils • Unconsolidated, natural material • Supports or capable of supporting vegetation • Can be described as an independent body (soil type) having specific properties and morphological characteristics that can be used to differentiate it from adjacent soil types

3. Soil Forming Factors • Climate • Parent material • Time • Topography • Living organisms

4. Climate • Weathering forces such as heat, rain, ice, snow, wind, sunshine, and other environmental forces, break down parent material and affect how fast or slow soil formation processes go

5. Parent Material • The primary material from which the soil is formed. • Soil parent material could be • bedrock • organic material • old soil surface • deposits from water, wind, glaciers, volcanoes, or material moving down a slope

6. Topography • The location of a soil on a landscape can affect how the climatic processes impact it. • Soils at the bottom of a hill will get more water than soils on the slopes • soils on the slopes that directly face the sun will be drier than soils on slopes that do not. • Also, mineral accumulations, plant nutrients, type of vegetation, vegetation growth, erosion, and water drainage are dependent on topographic relief.

7. Living Organisms • All plants and animals living in or on the soil • The amount of water and nutrients plants need affects the way soil forms. • The way humans use soils affects soil formation. • Animals living in the soil affect decomposition of waste materials and how soil materials will be moved around in the soil profile. • On the soil surface remains of dead plants and animals are worked by microorganisms and eventually become organic matter that is incorporated into the soil and enriches the soil.

8. Time • All of the aforementioned factors assert themselves over time, often hundreds or thousands of years. • Soil profiles continually change from weakly developed to well developed over time.

9. Properties important to the development and identification of wetland soils • Horizonization • Organic matter content • Texture • Permeability • Drainage • Color

10. Horizonization • Soil Horizon- layer of soil parallel to the land surface which can be differentiated from adjacent layers, or horizons, by identifiable physical, chemical, and biological characteristics MDEQ 2001

11. Organic Matter Content Mitsch and Gosselink, 2000.

12. Texture • Relative proportion of sand, silt, clay • Influenced by interaction of geologic and environmental factors • Important property affecting permeability Soil Survey Manual, USDA, 1993

13. Permeability • Measure of the ability of gases and liquids to move through a layer of soil • Sand has high permeability • Clay has low permeability • Arrangement or aggregation in soil structure also affects a soil’s permeability Sand Clay

14. Drainage • Used to describe amount of water present and it’s influence on potential use of that soil • Indicate frequency and duration of wet periods that may occur • Seven drainage classes • Very poorly drained • Poorly drained • Somewhat poorly drained • Moderately well drained • Well drained • Somewhat excessively drained • Excessively drained • Poorly drained and very poorly drained usually indicators of wetlands

15. Color • Color and location within profile can indicate conditions of soil development • Affected primarily by • Presence of iron and manganese • Organic matter content • Dominant color referred to as soil matrix • Contrasting colors or areas with spots are mottles

16. Definitions of Wetlands • U.S. Fish and Wildlife • Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification, wetlands must have one or more of the following three attributes: • at least periodically, the land supports hydrophytes; • the substrate is predominantly undrained hydric soil; • the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year • U.S.A.C.E. • Those areas that are saturated or inundated by surface or groundwater at a frequency and duration sufficient to support, and under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas (USACE, 1987).

17. Hydric Soils! • Formation influenced by interactions of soil-forming factors, but overriding factor is water • Hydric soils • soil that formed under conditions of saturation, flooding or ponding long enough during the growing season to develop anaerobic conditions in the upper part.

18. Hydric Soils • Critical factors • Saturation • Reduction • Redoximorphic features • Two types • Organic • Peat or muck • When waterlogged and decomposition is inhibited, histosols • Mineral • Inorganics

19. What is Peat? • Partially decomposed remains of dead plants which have accumulated on top of each other in waterlogged places for thousands of years. • Areas where peat accumulates are called peatlands. • Brownish-black in color. • Consists of Sphagnum moss along with the roots, leaves, flowers and seeds of heathers, grasses and sedges. • Occasionally trunks and roots of trees such as Scots pine, oak, birch and yew • Composed of 90% water and 10% solid material • Waterlogged soils cause anaerobic conditions, hinder growth of micro-organisms (bacteria and fungi). • thus, limited breakdown of plant material.

20. Hydric Soil Indicators for Non-Sandy Soil • Organic soils (histosols) • Histic Epipedons • Sulfidic material • Aquic moisture regime • Reducing soil conditions • Soils colors • Gleyed soils (gray colors) • Soils with bright mottles and/or low matrix chroma (dullness or neutral color) • Iron and Manganese concretions

21. Hydric Soil Indicators for Sandy Soils • High organic matter in surface horizon • Streaking of subsurface horizons by organic matter • Organic pans

22. Hydric Non-Hydric

23. Different Wetlands = Different soils? All hydric, but still vary • Tidal Marshes • Fens • Bogs • Pocosins • Non-tidal marshes • Wet meadows • Prairie potholes • Vernal pools • Playa lakes • Swamps • Forested swamps • Bottomland hardwoods • Shrubs • mangroves

24. Tidal Marsh • Salt marsh develops its own soil • Accumulated mud • Roots and organic material from the decay and breakup of salt-marsh plants. • Soils in coastal fresh marshes are generally alluvial • Fine material rich in organic materials and nutrients.

25. Bogs • Poor draining, waterlogged • Peat depth varies from 2 to 12m (slow decomposition rate). • Cool climates • May be up to 98% water • Water is held within the dead moss (e.g. sphagnum) fragments • Consists of two layers • The upper, very thin layer, known as the acrotelm • only some 30cm deep • consists of upright stems of the present mosses (water moves rapidly through this layer) • Below is a much thicker bulk of peat, known as the catotelm • where individual plant stems have collapsed under the weight of mosses above them to produce an amorphous, chocolate-colored mass of moss fragments • water moves more slowly through this layer • Bogs are ombrotrophic- water supply is from the mineral-poor rainwater

26. Fen • Glacial origins • Hydrology • waterlogged • mostly groundwater, some surface water. • Mineratrophic water- usually high in calcium, other ions from mineral-rich groundwater • Some drainage • slightly alkaline or neutral (pH of 7 to 8) • Soil is made of peat • large amount of decomposing plant material. • The technical term for this type of soil is muck • Average peat depth up to 2m • Wet meadows are similar • Don’t have organic soil • Don’t have year-round water

27. Pocosin • Like bogs, they have lots of sphagnum moss and nutrient-poor acidic soil and water • Like bogs, they get most of their moisture from precipitation • usually organic soil, and partly or completely enclosed by a sandy rim • Slow decay of dead vegetation contribute to the deep peat and acidic soils of these areas. • Naturally low nutrient levels in the soil

28. Vernal Pool • Ancient soils with an impermeable layer such as a hardpan, claypan, or volcanic basalt • Hardpans and claypans are mostly impervious to the downward percolation of rainwater • The restrictive soil layers are duripansor claypans, and the bedrock types are volcanic mud or lavaflows • Dependant on Rainfall • Makeup similar to surrounding soils, just hydric

29. Forested Swamp • Occur in a wide variety of situations ranging from broad, flat floodplains to isolated basins • Meandering river channels • Natural levees adjacent to rivers • Meander scrolls created as meanders become separated from the main channel • Texture ranges from mucks and clays to silts and sands • Organic levels may reach up to 36% Compared to content of upland soils (0.4-1.5%) (wharton et al. 1982). • Peat depostition is characteristic • Slow decomposition rates • Thickness decreases toward shallow end of swamp

30. Bottomland Hardwood • Alluvial soils as a result of flood pulses • High organic matter • Acidic • Typically high clay contents • Poorly drained • Low permeability • Some sandier blackwater environments an exception

31. Chemistry of Wetland Soils

32. Introduction • Classification of Wetland Soils • General chemical characteristics of organic and inorganic wetland soils • Primary chemical reactions in wetland soils and ways of measuring them • Case study: Lagoon of Venice, Italy

33. Classification of Wetland Soils • Techniques for classifying soil types: • Organic versus Inorganic: • Bulk density and porosity • Hydraulic conductivity • Nutrient availability • Cation exchange capacity • Organic soils are further classified by: • Percent organic carbon and clay • Hydroperiod

34. Organic vs. Inorganic • Bulk Density: dry weight of a soil sample • Organic soils weigh less than more inorganic soils • Hydraulic conductivity: capacity of soil to conduct water flow • Depends on the levels of decomposition in the soil • Organic soils hold more water than inorganic soils • Nutrient availability: availability of nutrients and minerals to plants • Organic soils can actually have low nutrient availability because it is all tied up in decomposition and peat formation

35. Organic vs. Inorganic • Cation exchange capacity: total amount of positive ions (cations) that a soil can hold • Organic soils have a higher capacity for H+ • Inorganic soils have a higher capacity for positive metal ions (Ca2+, Mg2+, K+, and Na+)

36. Organic Soils • Can be further classified by the percent of carbon in soil: • Organic soil material: 10% organic carbon • Mucky mineral soil material: 5-10% organic carbon • Mineral soil material: <5% organic carbon

37. Chemical Reactions • Oxidation-Reduction Reactions (Redox) • Carbon Transformations • Phosphorous Transformations • Sulfur Transformations • Nitrogen Transformations

38. Redox Reactions • Reduction: process of gaining an electron or hydrogen atom during a chemical reaction • Oxidation: process by which a compound loses an electron or hydrogen atom during a chemical reaction • In wetland soils, redox occurs during the transport of O2 • The anerobic conditions in wetland soils leads to high rates of reduction in the soil

39. Redox Reactions • Anerobic Conditions: • O2 diffusion rates through the soil is determined by how saturated the soil is • O2diffuse slower through more aqueous mediums • Causes reduced soil conditions • Takes longer for oxygen depletion to occur

40. Oxygen depletion depends on: Temperature Availability of organics When Oxygen is depleted, oxidized conditions occur Causes the soil to be red-brown Reduced soil is grey-blue Oxidized soil layer can sometimes form but depends on several factors: Transportation rate of O2 between the surface water and the atmosphere Production of oxygen by algae Number of oxygen consuming organisms in residence The amount of surface mixing that occurs Redox Reactions

41. Measuring Redox Reactions • Eh = E0 + 2.3[RT/nF]log[{ox}/{red}] • E0 = potential of reference (in millivolts) • R = gas constant (81.987 cal deg^-1 mol ^-1) • T = temperature (in Kelvin) • n = number of moles of electrons transferred • F = Faraday constant (23,061 cal/mole-volt) • A normal redox potential is between +400mV and +700mV

42. Carbon Transformations • Aerobic carbon transformations: • Photosynthesis: H2O is oxidized • Aerobic respiration: Oxygen is reduced • Decomposition of organic matter this way is efficient

43. Carbon Transformations • Anerobic carbon transformations: • Fermentation: organic matter is reduced by the anerobic respiration of microorganisms • Methanogenesis: CO2 is reduced by bacteria • Result can be methane gas • Can only occur in extremely reduced wetland soils, with a reduction potential of less than -200mV • Gas production affected by temperature and hydroperiod • Methane levels higher in freshwater wetlands than in marine wetlands

44. Carbon Transformations • Gas Transport: • Released from sediment into water column • Diffuses through sediment and mixes with the atmosphere at the surface • Carbon-Sulfur: • In some wetland soils, sulfur cycle necessary for the oxidation of organic carbon • Methane concentrations low in soil with high concentrations of sulfur • Competition for substrate between bacteria • Sulfate inhibits methane bacteria • Methane bacteria dependent on products of sulfur reducing bacteria • Redox potential not low enough to reduce CO2 due to sulfate

45. Sulfur Transformations • General information: • Never found in low enough concentrations to be called a limiting factor in wetlands • Most likely to occur at a redox potential of -100mV to -200mV • Sulfur is used as a electron receptor by bacteria in anerobic respiration • Sulfides are usually oxidized by microorganisms • Some wetland plants get energy from the oxidation of H2S into sulfur

46. Sulfur Transformations • Toxic Sulfides: • H2S can be toxic to rooted hydrophytes if the concentration of sulfates in the soil is high • Effect on plants is caused by: • Free sulfide is highly toxic to plant roots • Sulfur will precipitate with metals, limiting availability • Stops precipitation of some metals in the soil

47. Phosphorous Transformations • One of the most limiting elements in wetland soil • Northern bog, freshwater marshes, southern deepwater swamps • Inorganic form • Dependent on pH • Organic form • Bound in peat/organics • Does not have a gaseous cycle • Not affected by redox potential

48. Phosphorous Transformations • Can be made inaccessible to plants as a nutrient by the follow processes: • Precipitation of insoluble phosphorous with metals in aerobic conditions • Phosphate absorbed into peat, clay metal hydroxides and oxides • Phosphate bound in organic matter if consumed by bacteria, algae, or macrophytes

49. Nitrogen Transformations • One of the major limiting factors in saturated wetland soils • Considered one of the best electron acceptors for redox reactions in the soil (after oxygen) • Nitrogen levels in wetlands have increased due to runoff from fertilizers

50. Chemical Transport • Precipitation: sulfates and nitrates • Influenced by the burning of fossil fuels • Groundwater: • High in dissolved ions from the chemical weathering of soils or rocks, also dissolution, and redox reactions • Stream flow: • varies seasonally with the wet and dry seasons • Estuaries: • Where ocean water meets brackish river water many chemical reactions can occur • Dissolution, flocculation, biological assimilation and mineralization