WETLAND BIOGEOCHEMISTRY

• cycling of materials between soil/sediment,

water column, and atmosphere

a) organic inorganic

b) particulate dissolved gas

BIO GEO CHEMISTRY

• mediated by biotic and abiotic factors

• availability of nutrients (i.e.fertility)

= sink

= source

= transformer

exchanges within the ecosystem

mangrove seagrass

exchange between ecosystems

What might determine a wetland’s ability to be a source, sink or transformer of materials?

Why is this important for the ecosystem?

Why might exchanges of materials within a system (i.e. recycling) be more important than

exchanges between systems?

Redox Potential is a measure of the electron pressure (availability) in a solution: measured in mV.

Oxidation occurs during the uptake of oxygen or when a chemical gives up an electron.

Reduction takes place when oxygen is released or an electron is gained.

Mineralization is the process of decomposition and transformation of organic matter to forms of inorganic matter.

Immobilization or Fixation is the process in which inorganic matter is converted to organic matter.

Heterotroph an organism that utilizes organic materials as a source of energy and nutrients (consumers).

Autotroph an organism that assimilates energy from the sun or inorganic compounds into biomass (producers).

SOIL OXYGEN POOL IN A WETLAND

SOIL O2

AIR ENTRY(WATER TABLE DRAWDOWN)

DIFFUSION

ADVECTION

BIOLOGICAL(AUTOTROPHY,

O2 PUMPING

CHEMICAL OXIDATION

DECOMPOSITION

RESPIRATION

Oxygen is not always totally depleted in a wetland soil. There is often a thin, oxic layer at the surface. The thickness of this layer is dependent upon:

1. The rate of oxygen transport across the “air”-”surface water” boundary.

2. The respiring organisms on the soil surface.

3. O2 production by benthic algae.

4. Mixing and bioturbation

Plants can also serve as conduits for gas

exchange between the soil and atmosphere and can effectively oxidize areas of the

soil.

Oxidized rhizosphere• plaque found along roots

• indicates presence of oxygen

• reddish hue = oxidized iron (Fe3+)

• plant is a conduit for gases

MACROPHYTE EFFECTS ON REDOX POTENTIAL

Wetland Soils

• Hydric Soils: soils that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part (12 inches or about 30 cm).

Redoxymorphic features in a wetland soil are areas of oxidized iron (Fe3+) that usually occur along the length of animal burrows or live roots.

Soil Color

• Organic soils are dark black to dark brown.

• Mineral soils range from black, gray, to greenish- or blue-gray in color.

• Gleization: transformation of non-hydric mineral soil to hydric mineral soil. Results from reduction of Fe3+, Mn3+, and Mn4+ to Fe2+, Mn2+, respectively.

• presence of oxidized rhizosphere

Redoximorphic features

Soil Color

• Used to identify wetland soils

• Identification and delineation

• wetlands have low chroma (≤ 2)

Chroma

Munsell Soil Color

Munsell Notation

Hue Value

Hue Value Chroma

Soil Color Chart Pages

What makes a soil organic?

• Composed of organic material:1. herbaceous plant material2. wood and leaf litter3. roots

• Processes that control accumulation1. Accumulation rates

• litterfall• belowground production• aboveground production

2. Decomposition rates (factors affecting)• Oxygen availability• pH• Temperature• Nutrient availability

Organic vs. Mineral Wetland Soils

% organic carbon ≈ % organic content / 2

RESPIRATION DRIVES REDOX POTENTIAL

REDUCTION HALF REACTIONS FOR ORGANIC MATTER OXIDATION

RXN EH (mV)

O2 + 4e- ----> 2H2O ~ 400

2NO3 + 10e- ----> N2 ~ 250

Mn4+ + 2e- ----> Mn2+ ~ 225

Fe3+ + e- ----> Fe2+ ~ 120

SO4

2- + 8e- ----> HS- ~ -100

CO2 + 8e- ----> CH4 ~ - 300

Go Kcal/m

-686

Microbial competition for electron acceptors

Chemical comparison of rivers and the oceans

Influenced by:

1. Groundwater2. Climate3. Geomorphology4. Stream flow5. Ecosystem6. Humans

SEASONAL VARIABILITY

WINTER

SUMMER

GENERALIZED CARBON CYCLE

What happens now?

Primer on decomposition

• Represents a major flux of fixed C and nutrients• Controlled by:

1. litter quality or nutritional value (nutrient content, lignin content, etc.)

2. abiotic conditions (temp., pH, moisture)3. microbial and faunal communities

• Characterized by 3 phases:1. leaching2. fragmentation3. mineralization

fastest in wetlands

slowest in wetlands

depends on fauna

DECOMPOSITION

exchanges within the ecosystem

mangrove seagrass

exchange between ecosystems

Simplified Estuarine Food Web

GENERALIZED CARBON CYCLE

QUIZ #7

1. Describe a process that contributes to the accumulation of organic matter (i.e., peat) in wetland soils?

2. Why are oxidized areas of a wetland soil generally reddish in color?

3. In which wetland type are internal recycling processes most important in controlling the availability of nutrients?

4. In a newly inundated wetland, what is the first terminal electron acceptor used in organic matter oxidation?

5. a) List 2 factors/processes that lead to an increase in soil oxygen pools in wetlands

b) List 2 factors/processes that lead to a decrease in soil [O2].

6. TRUE/FALSE The net flow of O2 into a wetland is greater than the net flow out of wetland soils.

RESPIRATION DRIVES REDOX POTENTIAL

REDUCTION HALF REACTIONS FOR ORGANIC MATTER OXIDATION

RXN EH (mV)

O2 + 4e- ----> 2H2O ~ 400

2NO3 + 10e- ----> N2 ~ 250

Mn4+ + 2e- ----> Mn2+ ~ 225

Fe3+ + e- ----> Fe2+ ~ 120

SO4

2- + 8e- ----> HS- ~ -100

CO2 + 8e- ----> CH4 ~ - 300

Go Kcal/m

-686

Microbial competition for electron acceptors

GENERALIZED CARBON CYCLE

NITROGEN CYCLE…IMPORTANCE OF REDOX BOUNDARY

Organic• amino acids• proteins• nucleic acids

Inorganic• di-nitrogen (N2; gas)• ammonia (NH3; volatile)• ammonium (NH4

+)• nitrite (NO2

-)• nitrate (NO3

-)• nitrous oxide (N2O)

THE NITROGENCYCLE

Origin of Reduced Nitrogen Nitrogen Fixation: N2 + 3H2 ----> 2NH3 (160 Kcal energy consumption)

This reaction requires a lot of energy expenditure and is only conducted by a certain group of microorganisms. The process is inhibited by oxygen and high ammonium. It is the only pathway for molecular nitrogen from the atmosphere to the biota.

Ammonification: NH2-CO-NH2 +H20 ----> 2NH3 + CO2; NH3 + H20 ----> NH4

+ + OH- (176 Kcal energy yield)

This is the process by which organically-bound nitrogen is released back to the environment in inorganic form.

RXNs OF THE N CYCLE

Fate of Reduced Nitrogen Nitrification: NH4

+ + 3O2 ----> 2NO2- + 2H2O + 4H+ , NO2

- +O2 ----> 2NO3- (66 Kcal energy yield)

This reaction yields energy and microbes (Nitrosomonas and Nitrobacter) will use it as an energy source with which to fix carbon. It requires aerobic conditions and an adequate supply of ammonium.

Fate of Oxidised Nitrogen Assimilatory nitrate reduction Reduction of nitrate after uptake by plants, algae, or bacteria. Nitrate is reduced to C-NH2 by nitrate reductase enzyme. Dissimilatory nitrate reduction Denitrification: C6H12O6 + 4NO3

- ----> 6CO2 + 6H20 + 2N2 (545 Kcal energy yield) This a microbially-mediated reaction where the microbes (such as Pseudomonas denitrificans) are using nitrate as the terminal electron acceptor to breakdown organic matter. Denitrification specifically occurs when the end product is a gas, nitrous oxide or molecular nitrogen. It requires anaerobic conditions and an adequate nitrate source. Compare nitrogen energy yield to respiration using oxygen (686 Kcal energy yield)

NH4+ NO2

-

R-N

H 2

NO3-NO2

-

NO

N2O

N2

assimilation

assimilation

ammonification

nitrification

deni

trific

ation

N fixation

exchanges within the ecosystem

mangrove seagrass

exchange between ecosystems

THE NITROGEN CYCLE IN DIFFERENT WETLANDS

THE SULFUR CYCLE

Organic - amino acids

1. cysteine2. methionine

- enzymes and co-enzymes

Inorganic sulfate (SO4

2-) sulfide (H2S) sulfur (S) sulfur dioxide

(SO2) pyrite (FeS2)

SO4-2 H2S

H2S

FeS, FeS2

sulfide oxidation

sulfate reductionuptake

diffusion

emissiondeposition

diffusion

Surface water input

SULFUR CYCLE simplified

PHOSPHORUS

Biogeochemical control on

Phosphorus availability

redox-driven example: In iron-containing soils Eh > 120 mV = Fe3+ Eh < 120 mV = Fe2+ reddish hue = oxidized iron (Fe3+) Fe3+ is solid and binds readily with

P (PO43-)

Fe2+ is soluble no rxn w/ P

PHOSPHORUS…INDIRECTLY INFLUENCED BY REDOX

Biogeochemical control on P-availability

pH-driven example:

Calcium carbonate soils

Diurnal DO fluctuations in water column

In high pH = precipitated Ca-carbonate

Under low pH = dissolved Ca-carbonate

Precipitated Ca-carbonate/Aragonite scavenges P

SMALL SCALERoot enclosures:exchanges with water columninfluence of epibiontseffects of water source/salinity

moreSMALL SCALEExamples

LARGESCALE

1. What is the renewal rate of a wetland that has a maximum volume of 25,000 m3 and an inflow rate of 750 m3 day-1?

2. A constructed wetland has an average depth of 20 cm and a surface area of 15000 m2. What is the volume of water in the wetland?

3. There is a single channel feeding this wetland and its average depth is 20 cm, channel width is 15 m, and mean current velocity is 0.10 m second-1. Calculate the discharge of water into the wetland.

4. What is the wetland’s residence time (in days)?

5. T/F Darcy’s Law states that groundwater flow is proportional to the slope of the piezometric surface (hydraulic gradient) and the water velocity (m sec-1).

Quiz 5

QUIZ #7

1. Describe a process that contributes to the accumulation of organic matter (i.e., peat) in wetland soils?

2. Why are oxidized areas of a wetland soil generally reddish in color?

3. In which wetland type are internal recycling processes most important in controlling the availability of nutrients?

4. In a newly inundated wetland, what is the first terminal electron acceptor used in organic matter oxidation?

5. a) List 2 factors/processes that lead to an increase in soil oxygen pools in wetlands

b) List 2 factors/processes that lead to a decrease in soil [O2].

6. TRUE/FALSE The net flow of O2 into a wetland is greater than the net flow out of wetland soils.

QUIZ #6• Why do wetland soils go anaerobic when flooded or saturated

for extended periods?

• What is the regulatory term used for wetland soil?

• Why are oxidized areas of a wetland soil generally reddish in color?

• In which wetland type are internal recycling processes most important in controlling the availability of nutrients?

• TRUE/FALSE Mineralization is a process that results in the transformation from inorganic to organic matter.

QUIZ #6

• Why is the rate of oxygen depletion accelerated in the warmer months of the year?

• After oxygen has been depleted from a wetland soil, what is the next terminal electron acceptor used for respiration? Why?

• Why is sulfate reduction more predominant in coastal wetland soils than in FW wetlands?

• Why is nitrogen fixation limited by high ammonium concentrations?

QUIZ #7

• Describe a process that contributes to the accumulation of organic matter (i.e., peat) in wetland soils?

• Describe a process that contributes to the loss of organic matter in wetland soils?

• List two abiotic factors that govern the net accumulation (balance between accumulation and consumption) of organic matter in wetland soils?

• Generally, what color are organic soils?

QUIZ #8

• Besides abiotic factors (e.g., temperature, oxygen availability, and pH), what factors determine the rate at which a dead plant will decompose?

• Which terminal electron acceptor is used by soil microbial communities after oxygen has been depleted? Why?

• Why is sulfate reduction more prevalent in coastal wetland soils than in freshwater wetland soils?

• TRUE/FALSE: Technically, a mineral soil with high iron can be entirely reddish in color (indicating presence of Fe3+) and still be considered a wetland (i.e., hydric) soil?