Biogeochemical Properties of Wetlands

Institute of Food and Agricultural Sciences (IFAS)
Biogeochemistry of Wetlands
S i
Science
and
dA
Applications
li ti
Biogeochemical
Properties of Wetlands
Wetland Biogeochemistry Laboratory
Soil and Water Science Department
University of Florida
Instructor
K. Ramesh Reddy
6/22/2008
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Biogeochemical
Properties of Wetlands
Topic Outline
Definition
Wetland soils
Biogeochemical features of wetlands
Presence of molecular oxygen
Sequential reduction of inorganic electron
acceptors
Oxidized soil-floodwater interface
Exchange of dissolved substances
between soil and water column
Presence of hydrophytic vegetation
Types of wetland soils

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Biogeochemical
Properties of Wetlands
Learning Objectives
Define biogeochemical features of wetlands, specifically
Why anaerobic conditions exist in wetlands
How microbes use alternate inorganic electron acceptors
to support their metabolic activities
Juxtaposition of aerobic and anaerobic zones
Exchange of dissolved substances between soil and
water column
Adaptation of wetlands to anaerobic soil conditions
Understand the differences among different wetland soils

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WETLAND DEFINITION
• "HYDRIC
HYDRIC SOILS (WETLAND) ARE THOSE
IN THEIR NATURAL CONDITIONS ARE
SATURATED, FLOODED, OR PONDED
LONG ENOUGH DURING THE CROPPING
SEASON TO DEVELOP ANAEROBIC
CONDITIONS THAT FAVOR THE GROWTH
AND REGENERATION OF HYDROPHYTIC
VEGETATION.”
– Source: USDA. Natural Resource Conservation Service
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WETLAND DEFINITION
• Wetland contains biologically active soil
or sediment in which the content of
water in or the overlying water column is
great enough to inhibit oxygen diffusion
into the soil/sediment and stimulate
anaerobic
bi chemical
h i l and
d bi
biological
l i l
processes, that help biotic communities
to adapt to anaerobic conditions.
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Main Characteristics of
Wetlands
• Wetlands typically lie between terrestrial
(upland) and aquatic (water) systems.
• Wetlands are seasonally and
periodically flooded or have saturated
soils for significant periods of time.
• Presence of unique soil characteristics.
characteristics
• Presence of special vegetation adapted
to saturated soil conditions.
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Wetland Ecosytem
TERRESTRIAL SYSTEM
WETLAND
DEEPWATER AQUATIC
SYSTEM
Import/Export of
nutrients and
species
Hydrology
Dry
Intermittently to
permanently
fl d d
flooded
Permanently
flooded
Biogeochemical Role
Source, sink,
or
transformer
Productivity
Low to
Medium
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Source, sink,
or
transformer
Generally
high,
sometimes
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low
Sink or
transformer
Generally
low
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Wetland Functions Biogeochemical Function

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Nutrient/contaminant storage (long-term)
Nutrient/contaminant sinks
Nutrient/contaminant source
Transformers
Adsorption and precipitation
Microbial
Mi bi l b
breakdown
kd
off OM
Denitrification
Methanogenesis
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Wetland Biogeochemistry
Depending on wetland type
type, hydrologic
regime, and nutrient/contaminant inputs,
wetland can serve as:
SINK
SOURCE
TRANSFORMERS
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WETLAND SOIL
Drained Soil
Flooded Soil
Pore spaces
Soil aggregate
Porewater
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Porewater
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WETLAND SOIL
Oxygen
Oxygen
Oxygen
Water
Soil
[saturated]
Soil
[saturated]
Soil
[saturated]
Drained
Flooded
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Soil
[drained]
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Biogeochemical
Characteristics of Wetlands

Restricted oxygen supply
Presence of aerobic soil-floodwater
interface
Reduction of alternate electron acceptors
Exchange of dissolved species between soil
and
d water
t column
l
Accumulation of organic matter
Presence of hydrophytic vegetation
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Anaerobic-Aerobic Interface
Oxygen
OXYGEN
PHOSPHORUS
NITROGEN
Carbon
Water
Soil
SULFUR
METALS
XENOBIOTICS
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Anaerobic-Aerobic Interface
Dissolved oxygen (mg L-1)
0
2
4
6
20
Air
Deptth (mm)
10
Water
0
Soil
Aerobic
-10
Anaerobic
-20
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Anaerobic-Aerobic Interface
[Flooded Paddy Soil]
High SOM
Low SOM
Floodwater
Floodwater
Aerobic Zone
Anaerobic Zone
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Aerobic and Anaerobic Soils
Oxidized Soil Profile
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Reduced Soil Profile
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Wade Hurt
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Gas Exchange in
Soil-Water-Plant System
Drained Soil
Flooded Soil
O2
O2
CO2
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Dissloved metals
sulfides, and
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organic
acids
CO2, CH4, and
other gases
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Biogeochemical
Characteristics of Wetlands

Restricted oxygen supply
Presence of aerobic soil-floodwater interface
Reduction of alternate electron acceptors
Exchange of dissolved species between soil
and water column
Accumulation of organic matter
Presence of hydrophytic vegetation
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Cattail Roots
[Typha latifolia]
Aerenchyma (intercellular air space)
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H. Brix [Denmark]
Oxidized Root Zone
[Oryza sativa]
Reddish brown color on root surface is due to
oxidation of ferrous iron to ferric iron
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Oxidized Root Zone
[Oryza sativa]
Reddish brown color on root surface is due to
oxidation of ferrous iron to ferric iron
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WETLAND SOILS
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Wade Hurt
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Waterlogged Mineral Soils
• Increase in organic matter accumulation in
surface horizon
• Mottled zone (gley horizon) with Fe and Mn
accumulation
• Permanently reduced zone (Bg or Cg)
• Formation of secondary minerals
– Pyrite (Fe S2)
– Siderite (Fe CO3)
– Vivianite (Fe3 (PO4)2 . 8H2O)
– Jarosite (K Fe3 (SO4)2 (OH)6)
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Oxidation-Reduction
WELL DRAINED SOIL
Aerobic
Mn4+
Fe3+
Aerobic
Fe3+
FLOODED FOR LONG PERIOD
Mn4+
Fe3+
Fe2+
Mn4+
Mn2+
Fe2+
Mn2+
Anaerobic
RECENTLY FLOODED
Anaerobic
Aerobic
Fe3+
Mn4+
Fe2+
Fe2+
Mn2+
Mn2+
Anaerobic
RECENTLY DRAINED
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Organic Soils
• Organic matter content (>
( 12% of
total C) in the upper soil profile
• Soils formed due to plant litter
accumulation
• Soils typically have low bulk
density
• Soils have high water holding
capacity
• Cation exchange capacity is
saturated with H+ ions
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Everglades Agricultural Area
Soil Subsidence
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Paddy Soils
• Mineral and organic soils managed for rice
cultivation
• Plowing and land leveling
• Flooded with 5 - 15 cm water
• Alternate flooding and draining during
growing season
• Soils drained after rice harvest and kept
fallow for 4-6 months
• Development of hard plow pan
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Paddy Field - India
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Marsh Soils
•
•
•
•
Accumulation of plant residues
Permanently reduced Bg or Cg horizon
Freshwater marsh
Saltwater marsh
Wade Hurt
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Freshwater Marsh
• Upland marshes (ombrotrophic)
– rainfed, hydrodynamically isolated, some
ground water exchange
– pH 3 to 4.5
• Lowland marshes (rheotrophic)
– receive water and nutrient inputs from
adjacent areas
– pH 5 to 6
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Saltwater Marsh
• Neutral pH
• Presence of Fe S2
• Drained soils have pH < 3.5
– Fe (OH)2 + H2S = FeS + 2H2O
(Mackinawite)
2 = Fe
– FeS
F S + S2F S2 (Pyrite)
(P it )
– Fe S2 + O2 = Fe (OH)3 + H2 SO4
– Thiobacillus ferroxidans
– Thiobacillus thioxidans
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Biogeochemical
Properties of Wetlands
S
Summary
Wetland soils exhibit unique features with aerobic and
anaerobic zones
Wetland soils are long-term integrators of elemental storage
and ecosystem processes
Wetland soils support a range of microbial communities and
associated metabolic pathways
Wetland accumulate organic matter
Wetlands accumulate reduced chemical substances
Wetlands are source green house gases including methane
and nitrous oxide
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6/22/2008
http://wetlands.ifas.ufl.edu
http://soils.ifas.ufl.edu
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Biogeochemical Properties of Wetlands

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