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%
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 (
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.