Biogeochemistry

Soil and Redox Chemistry

  • Redox chemistry is crucial for soil development, especially in relation to nutrients like nitrogen, carbon, and sediment dynamics in wetlands.

    • The biogeochemistry of wetlands plays a pivotal role in reducing pollution and enhancing water quality.

Key Processes in Redox Chemistry

  • Redox Potential:

    • Measures electron movement or availability in a solution.

    • Oxygen has a high propensity for electron exchange compared to other elements, facilitating energy production at cellular levels (e.g., ATP formation).

  • Cellular Respiration and Metabolism:

    • Redox processes in soils can be measured in millivolts, reflecting electron exchange rates.

  • Impact of Flooding on Redox Chemistry:

    • When soils are flooded, the diffusion rate of oxygen into the soil decreases significantly (about one ten-thousandth of the rate in unsaturated conditions).

    • Flooding leads to oxygen depletion, affecting microbial communities and making species reliant on other electron acceptors more dominant (e.g., sulfate-reducing bacteria).

Oxygen Consumption and Microbial Activity

  • Oxygen consumption post-flooding can be rapid, impacting redox potential.

    • Levels of manganese, iron, and sulfide can change based on flooding duration and soil conditions.

    • The microbial activity surrounding these elements reflects the redox conditions, which influences carbon decomposition rates.

Redox Reactions Explained

  • Oxidation:

    • The process where a substance (e.g., ferrous iron) donates electrons, often releasing a hydrogen ion (H⁺).

    • Example: Ferrous iron (Fe²⁺) oxidized to ferric iron (Fe³⁺) when reintroduced to oxygen.

  • Reduction:

    • The opposite process where a substance accepts electrons.

    • Example: Ferric iron (Fe³⁺) gaining electrons to become soluble ferrous iron (Fe²⁺).

Transformation of Nitrogen in Wetlands

  • Nitrogen Compounds:

    • Nitrate (NO₃⁻) is a crucial nitrogen source for plant growth.

    • Nitrate is prone to leaching into aquatic systems, contributing to eutrophication.

  • Denitrification Process:

    • Wetlands chemically reduce nitrate to nitrogen gas (N₂) and nitrous oxide (N₂O), which reduces nitrate levels and serves as an ecosystem service by removing nitrogen from the system.

    • The denitrification pathway (NO₃⁻ to N₂ gas) requires both aerobic and anaerobic conditions, highlighting the importance of wetland functions in nutrient cycling.

Variability of Elemental Forms in Wetlands

  • Wetlands provide various elemental forms, oscillating between oxidized and reduced states depending on environmental conditions (flooding, oxygen availability).

    • Oxidized Forms:

    • Nitrate (NO₃⁻), Ferric Iron (Fe³⁺), Manganese (MnO₄), Sulfate (SO₄²⁻).

    • Reduced Forms:

    • Ammonium (NH₄⁺), Ferrous Iron (Fe²⁺), Hydrogen Sulfide (H₂S), Methane (CH₄).

Sequence of Redox Processes in Flooded Conditions

  • Starting with flooding of soil:

    1. Oxygen concentration rapidly declines.

    2. Microbes utilize available nitrate as an alternative electron acceptor leading to declining nitrate levels over time.

    3. As nitrate depletes, ammonium levels rise.

    4. Eventually, sulfate becomes a respiration electron acceptor, contributing to hydrogen sulfide production.

    5. Methane generation indicates prolonged inundation of soils.

Microbial Activity Measurement

  • Microbial activity can be assessed by measuring CO₂ production rates based on available electron acceptors (oxygen > nitrate > sulfate), revealing metabolic rates.

    • Aerobic conditions yield higher CO₂ production due to efficient decomposition processes.

pH and Soil Chemistry in Wetlands

  • Wetland soils often trend towards neutral pH levels, influenced by organic matter, carbon dioxide buildup, and mineral content.

    • Peatlands may show markedly lower (more acidic) pH due to organic matter accumulation.

Nutrient Cycling: Carbon, Nitrogen, Phosphorus, and Sulfur

  • Carbon Cycling:

    • Carbon pathways involve aerobic assimilation through photosynthesis, decomposition of organic matter, and fermentation in anaerobic conditions.

    • Dissolved organic carbon (DOC) undergoes further microbial transformation, leading to CO₂ and CH₄ emissions.

  • Nitrogen Dynamics:

    • Moves from organic form in dead plants to ammonium (NH₄⁺) through mineralization, followed by nitrification and potential denitrification under anaerobic conditions.

  • Sulfur Pathways:

    • Sulfur undergoes processes similar to nitrogen, with sulfate often serving as a microbial electron acceptor converted back to hydrogen sulfide (H₂S) in anaerobic environments.

  • Phosphorus Cycling:

    • Unlike other elements, phosphorus lacks a gaseous phase and is primarily taken up by organisms in soluble forms, often becoming sequestered or unavailable in sediments.

Wetland Function and Element Management

  • Wetlands serve as sinks or sources for various nutrients (carbon, nitrogen, phosphorus, and sulfur), affecting landscape-scale nutrient management.

    • The balance of nutrient inputs and outputs determines the role of wetlands in carbon storage and the cycling of other essential elements, contributing to ecosystem services.