Formation, Chemistry, and Biology of Wetland Soils

Maverick, Dana,

Devon

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

Soil Forming Factors

• Climate• Parent material• Time• Topography• Living organisms

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

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

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.

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.

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.

Properties important to the development and identification of wetland soils

• Horizonization

• Organic matter content

• Texture

• Permeability

• Drainage

• Color

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

Organic Matter Content

Mitsch and Gosselink, 2000.

Texture• Relative proportion

of sand, silt, clay • Influenced by

interaction of geologic and environmental factors

• Important property affecting permeability

Soil Survey Manual, USDA, 1993

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

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

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

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:

1. at least periodically, the land supports hydrophytes; 2. the substrate is predominantly undrained hydric soil; 3. 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).

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.

Hydric Soils• Critical factors

– Saturation

– Reduction

– Redoximorphic features

• Two types– Organic

• Peat or muck

• When waterlogged and decomposition is inhibited, histosols

– Mineral• Inorganics

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.

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

Hydric Soil Indicators for Sandy Soils

• High organic matter in surface horizon

• Streaking of subsurface horizons by organic matter

• Organic pans

Hydric Non-Hydric

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

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.

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

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

                                 

                   

                                                                                                                                 

               

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

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

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

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

Chemistry of Wetland Soils

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

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

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

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+)

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

Chemical Reactions

• Oxidation-Reduction Reactions (Redox)

• Carbon Transformations• Phosphorous Transformations• Sulfur Transformations• Nitrogen Transformations

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

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

Redox Reactions

• 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

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

Carbon Transformations

• Aerobic carbon transformations:– Photosynthesis: H2O is

oxidized

– Aerobic respiration: Oxygen is reduced

• Decomposition of organic matter this way is efficient

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

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

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

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

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

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

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

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

Temporal changes and spatial variation of soil oxygen consumption, nitrification, and dentrification rates in a tidal salt marsh of the Lagoon of

Venice, Italy.P.G. Eriksson, J.M. Svensson, and G.M.

CarrerEstuarine, Coastal, and Shelf Science

2003 pgs.1-11

• Purpose of study:– To determine seasonal and spatial patterns of O2 in

marsh soil, along with patterns of nitrification, dentrification, and flux of dissolved inorganic nitrogen (DIN)

• Location:– Lagoon of Venice, Italy

• 540 square kilometers• Lagoon surrounded by tidal salt marsh• Study conducted in salt marsh on west side of lagoon

• Study length:– Tests conducted April-October of 1999

Study Location

Marsh Vegitation

Methods

• Data was collected monthly at high tide in the study area• Took fully enclosed core samples

– 6 samples in areas vegetated by Limonium serotinum– 12 samples taken in April– 6 samples taken in May from areas vegetated by Juncus maritimus

and Halimione portulacoides– Also took water samples in sealed containers from same area– Some samples taken from near by creek bed

• Put core samples in a box, unsealed, and covered with water samples from same location– Kept water aerated and maintained temperature of original marsh

location

Methods

• Incubated for 2-6 hours• Water was then collected and filtered for nitrate and

ammonium, then frozen for later testing• Then the same cores were incubated for another 5-6 hours

in the dark (sealed)– Measured O2 flux, nitrates, and ammonium

• Used isotope-pairing techniques to measure rates of dentrification in the core samples

• Sieved remaining marsh sediment from core samples and collected microfauna– Dried and weighed sediments

Temporal Results• Ammonium:

– Released into the water in all core samples

– Highest release rate in April, June, July

• Nitrate:– Twice as high in April as in

September or October– Net removal in areas with a higher

vegetation densities• Oxygen soil consumption:

– Increased with temperature over time

• Dentrification:– Higher rates in spring and fall– Coincides with nitrate levels

Spatial Results

• Oxygen soil consumption:– Greater in creek soils then in vegetated areas

• DIN:– Highest fluxes and dentrification rates in non-

vegetated creek soil

• Lagoon retains nitrate and releases ammonium into the water column

Biology of Hydric Soils

Dana Rohrbacher

Hydric Soils

• Hydric soils contain complete complex communities, each with very distinct features.

• They have many important ecological functions, and help sustain the system as a whole.

Functions of Biological Soil Components

• Fertilize soil

• Break down dead organisms

• Release nutrients for use by living plants

• Maintain viable soils

• Contribute to long term sustainability

• Clean air and water

• Act as biological indicators

Soil Communities

• Biological crust

• Fungi

• Bacteria

• Protozoa

• Nematodes

• Annelids

• Arthropods

• Seed bank

• Root System

Biological Crusts

• Consist of algae, cyanobacteria, bacteria, lichens, mosses, liverworts, and fungi that grow on or just beneath the soil layer.

• Variable in appearance. • Formation of crusts is a result of soil

chemical and physical characteristics, and weathering patterns.

• They have many functions including serving as habitat for fauna, aiding in making soil more fertile, and helping to retain moisture.

Fungi

• Mycorrhizal fungi colonize roots of plants in a symbiotic relationship that aids the plant in the acquisition of nutrients and water necessary for growth. In return the plant provides energy to the fungus.

• Not all fungus is mycorrhizal however, some fungus play a role in decomposition, but to a lesser extent than bacteria.

Fungal Decomposition• Fungal decomposition starts while dead

plants are still standing, before they fall into the water.

• The decomposition process begins, and is greatest during early Spring.

• In estuarine systems there is generally greater colonization in non-impacted tidal wetlands than in tidally impacted wetlands.

Bacteria

• Ubiquitous, single celled organisms.• Some are primary producers and some are

decomposers.• The decomposers consume organic matter

releasing the nutrients for use by other living organisms.

• These decomposers are particularly important in several nutrient cycles. (ie-Nitrogen and Carbon cycles)

• They are important in water-holding capacity, soil stability, and aeration.

• They can also help filter and degrade anthropogenic pollutants in the soil and ground water.

Nutrient Cycles

• Both fungi and bacteria play important roles in the making nutrients such as nitrogen and carbon available for living plants.

Protozoa

• Single celled organisms that eat bacteria.• Classified into 3 categories, all of which

need water to move but can rely on a very thin film surrounding the particles.

• They play a very important role in the soil food web.

Diatoms• Benthic pennate

diatoms found in the Cape Fear River

• Scanning electron microscope image of Pseudo-nitzchia australis.

Nematodes

• Tiny ubiquitous roundworms classified according to their eating habits.

• They eat bacteria, fungi, roots, and even some tiny animals.

• They also need a thin film of moisture to survive, but they have an ability to become dormant until more favorable conditions arise.

• Beneficial in boosting the nutrient supply, assisting in decomposition, and can even be useful for pest control of insects.

• Serve as a food source for other animals.

                                                               

Annelids• Segmented worms• 2/3 live in the sea, while the rest are terrestrial.• Some are parasitic, while others are filter feeders.• Their major

role is

in reworking

the soil.

Annelids ~cont.~

• Annelids include:PolychaetesOligochaetesLeaches

• Most species prefer soft soils; often found under rocks.

• Serve as a food source for other animals.

Arthropods

• Jointed invertebrates generally referred to as…BUGS!• Range in size from microscopic to large enough to see

with the naked eye.• They eat everything from plants, animals, and even fungi.• They aerate the soil, shred organic matter, assist in the

decomposition process, distribute beneficial microbes, and serve as a food source for larger animals.

• They also help in the regulation of populations of other organisms (ie-protozoa) to maintain a more healthy soil food web.

Arthropods ~cont.~

• Can include many different types including:

insects

crustaceans

arachnids

myriapods

scorpians• Fiddler crabs play an important role in aeration.• Serve as a food source for other animals.

Seed Bank

• Consist of viable, ungerminated seeds in or on the soil.

• Significantly different from upland soil banks because of hydrology and soil properties.

• It is important for the emergence, maintenance, and diversity of plants in a system.

• Also, the seed bank is a mechanism for plant species to colonize newly disturbed areas. This is particularly important in those wetland systems that are frequently disturbed.

Seed Bank ~cont.~

• Seed banks provide a way in which several species can co-exist over time. This temporal variation is observed when a successful species experiences conditions less favorable for its dominance, and a new, less competitive species, existent in the seed bank, is allowed to take advantage.

• In river and tidal systems, seeds can be dispersed by water. The ease of transport of these seeds is in part a contributing factor to the biodiversity of these systems.

Seed Bank ~cont.~

• Leck and Robert (1987) Estimated seed bank (seeds per m2) in the top 10cm of soil in three wetland sites based on 1982 and 1983 soil samples collected in March and June. Values were obtained by extrapolation of depth data for 0-2, 4-6, and 8-10 cm.

• Indicates that the shrub forest generally has higher seed density within the soil. While all three locations have an overall higher seed density in March.

Root System• Aid in the stabilization of

wetland environments. This stabilization is especially important in some systems due to their unstable nature.

• Some types of roots are better at stabilizing than others. A tap root sends a single main root down, while more adventitious root systems have several branching roots. The later type tends to be a better stabilizing root system.

Roots ~cont.~

• They also add biomass to soil, and can be colonized by several different species.

• Some root systems,

such as those seen in

mangrove swamps,

play important above

ground roles.

Air and Water Quality

• Several soil species are important in managing soil organic matter which is a key factor in controlling air and water quality.

• Nutrient loads decline in both the soil and water when biological species thrive.

• Vegetation health increases which in turn provides habitat which contributes to the overall wetland quality.

Biological Indicators

• Because of the characteristic differences among different species, organisms can serve as indicators, offering a signal of the biological condition of a wetland.

• They can indicate soil types, wetland types, and the presence of pollution or other negative anthropogenic influences.

• Some organisms prefer specific conditions or tend to be sensitive to pollution. Thus, when conditions are altered or a pollutant is introduced, this can be measured by the absence of those organisms that cannot tolerate the new conditions.

Indicators ~cont.~

• Some organisms, such as macroinvertebrates, such as leaches, actually tend to thrive in moderately polluted areas.

• Several worm species are often indicators of “dirty water”.

• Other species such as the water penny beetle and the dobsonfly larvae are sensitive benthos. There absence in areas that they generally inhabit can be an indication of pollutants.

• Several species are also important in managing soil organic matter which is a key factor in controlling air and water quality.

                                                    

                                              

Case StudyMicrocrustacean communities in streams from two

physiographically contrasting regions of Britain.

• This is a study by Simon D. Rundle and Paul M. Ramsay that looked at benthic microcrustaceans from forty-three streams at two different locations in Britain; lowland southern England, and upland Whales.

• The test sights consisted of two areas of varying geology, vegetation, chemical, and compositional components.

• Organisms were sampled, preserved, identified, and counted.

• Results showed that lowland areas have significantly higher species richness than upland areas.

• There were also large differences observed in community structure between the two sights.

• It is important to understand the species ecology when assessing important issues such as pollution impacts.

References

• Brij Verma, Richard D. Robarts, John V. Headley. Seasonal Changes in Fungal Production and Biomass on Standing Dead Scirpus Lacustris Litter in a Northern Prarrie Wetland. Applied and Environmental Microbiology, Feb. 2003, p.1043-1050, vol. 69 no. 2.

• Biological soil communities. www.blm.gov/nstc/soil/. 12/4/98.• Matthew Ramsey, Yongjiang Zhang, Sarah Baker, and Scott Olmsted.

Collecting and germinating seeds from soil seed banks. June 10, 2003• Indicator Species. www.epa.gov/bioindicators/html/indicator.html.

10/29/03.• Rundle, Simon D. and Ramsay, Paul M. Microcrustacean

communities in streams from two physiographically contrasting regions of Britain. Journal of Biogeography. Vol. 24, No 1, p.101-111.

References

• Mitsch, William J., and James G. Gosselink; Wetlands: Third Edition. John Wiley & Sons, Inc., New York: 2000, p.155-187.

• P.G. Eriksson, J.M. Svensson, and G.M. Carrer; Temporal changes and spatial variation of soil oxygen consumption, nitrification, and dentrification rates in a tidal salt marsh of the Lagoon of Venice, Italy; Estuarine, Coastal, and Shelf Science. July 2003; p.1-11.

• http://www.uib.es/depart/dba/botanica/herbari/alfabetica/L.html; UIB, University of Illes Balears, Dept. of Biology; 2002.

References

• http://www.frtr.gov/matrix2/section4/4-50.html; Remediation Technologies Screening Matrix and Reference Guide