Wetland Soils (Wetland Biogeochemistry)

 

The transport and transformation of chemicals in ecosystems is known as biogeochemical cycling.  The diverse hydrologic conditions in wetlands have a major influence on biogeochemical cycles.

 

Wetland Soils

 

Types and Definitions

 

Hydric soils: formed under conditions of saturation long enough to develop anaerobic conditions.

Mineral soils: less than 20-35% OM.

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)

 

Organic Wetland Soils

 

          Characteristics depend on botanical origin

                   1. mosses

                   2. herbaceous material

                   3. wood and leaf litter

 

Decomposition:  as OM decomposes, it becomes more dense and less permeable

 

Organic soil types: saprists (muck), fibrists (peat), hemists (mucky peat), folists (tropical and boreal montane soils)

 

Mineral Wetland Soils

 

Flooded mineral soils develop redoximorphic features.  These are caused by the reduction, translocation and/or oxidation of iron and manganese oxides.  Redoximorphic features are developed by biological processes.  Also required are sustained anoxia, soil temperatures above 5oC, and organic matter to serve as a microbial substrate.

 

Gleying.  Flooded soils will develop black, grey, bluish or greenish color as a result of the reduction of iron.  Oxidized iron is reddish; oxidized manganese is black.  When reduced they become colorless and can be leached out leaving the natural grey or black color of the parent material.

 

Oxidized Rhizosphere.  Oxidized iron is left along the traces of small roots.

 

Mottles and concretions.  Wetted and dried soils develop spots of highly oxidized materials.  They are reddish-brown and relatively insoluble, enabling them to remain in the soil long after it has been drained.

 

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.

 

Organic decomposition can occur in the presence of any number of terminal electron acceptors, including O2, NO3-,  Mn2+, Fe 3+, SO4= .  It occurs most rapidly in the presence of oxygen, and slower for other electron acceptors.  Redox potential drops through the sequence of electron acceptors, as O2 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 CO2, will become the terminal electron acceptor below -200 mV.

 

pH

 

Organic wetland soils tend to be acidic, particularly in oligotrophic peatlands.  Mineral wetland soils are more neutral or sometimes alkaline.  The consequence of flooding previously drained wetland soils is usually to push the pH toward neutrality, whether formerly acid or alkaline.

 

Nitrogen Transformations

 

One of the more significant ways that nitrogen is lost to the atmosphere is in wetlands.  Organic N is mineralized to ammonium NH4+.  In aerobic conditions, nitrification occurs through the 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.

 

Iron and Manganese Transformations

 

          Iron is reduced from insoluble ferric Fe 3+ to soluble and toxic ferrous Fe2+.  Manganese is reduced from insoluble manganic Mn 4+ to soluble and toxic manganous Mn 2+.

 

Sulfur Transformations

 

At low redox potentials, sulfur is reduced and H2S, 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.

 

Carbon Transformations

 

          Methanogenesis occurs when certain bacteria use CO2 as an electron acceptor and produce gaseous methane (CH4).

 

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.

 

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.

 

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).

 

Estuaries

 

Estuaries have quite variable chemistry, different from both that of the adjacent ocean or the tributary rivers.

 

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.  

 

Wetlands as Sources, Sinks and Transformers

 

Many wetlands act as sinks for inorganic substances and may be major providers of organic material for downstream systems.

 

Seasonal Patterns of Uptake and Release

 

Uptake is high during the growing season, and export is high during the season of senescence (often, maximum export occurs in early spring when warmer weather makes decomposer activity pick up after being slow through the winter.)

 

Coupling with Adjacent Systems

 

Hydrologically more open systems often have greater productivity and more export of materials.

 

High- and Low-nutrient Wetlands

 

There are oligotrophic and eutrophic wetlands.  Wetlands may also differ in productivity as a result of salinity stresses.

 

 

Terrestrial Compared with Aquatic Systems

 

 

Upland Plants

Wetland Plants

Water Stress

Seasonal dry periods, cyclic drought

Adequate water, except difficulty in obtaining water in saline environments because of osmotic potential

Herbivory

Can be as high as 90% of aboveground in savanna.  Defenses like alkaloids, silicon cells, thorns

5-10% in most salt marshes and freshwater wetlands.  Can be high in areas with muskrat, nutria.

Soil Environment

Dry, complex soil biota, cycling

Wet, simple benthic biota, import/export

Toxic Substances

Salts, anthropogenic

Fe, Mn, sulfides at low redox potential

Waste products

Soil CO2 levels elevated

Ethylene and CO2 cannot move through saturated soils and must be vented internally.

Carbon Limitation

Photosynthesis limited by competition-induced shading

Photosynthesis limited by inundation from floods, tides

Oxygen Limitation

Roots can get oxygen externally

External oxygen is limited by low diffusion rates in saturated soils.

Nutrient Availability

Variable, but cycling is very important

N,P limited, but import and export are important in open systems

Climate Extremes

Can range from mesic to extremes of hot and cold, wet and dry.

Fluctuations damped because of constant presence of water and its high heat capacity

Physical Disturbance

Accessible.  Agriculture, urbanization and grazing cause disturbance.

Human, animal access limited.

Decomposition Rates

Slow only in cold and dry environments.

Slowed in saturated soils.  Very slow in cold, acid soils.

Nutrient Cycling

Important

Important in low energy environments and low nutrient environments

Import of Substances

Depends on landscape position, but less important than in aquatic systems.

Important in high energy environments.

Export of Substances

Export occurs on slopes, heavily grazed systems

Common in high energy, productive systems

Stability

Subjected to climatic drivers, grazers.  Many systems fire regenerated

Stable except in pulsing systems with high energy.  Moderated climate, moisture stress, herbivory

 

 

 

Anthropogenic Effects

 

Land  clearing, erosion, channelization, dams as nutrient traps, pollutants.