9.3 Wetland Biogeochemistry

Wetlands are unique ecosystems where water meets land, creating a hotbed of biogeochemical activity. These areas are characterized by anaerobic conditions, leading to specialized processes like methane production and slow decomposition, which contribute to their role as carbon sinks.

Wetland biogeochemistry plays a crucial role in environmental regulation. These ecosystems filter pollutants, sequester carbon, and influence greenhouse gas emissions. Understanding these processes is key to managing wetlands effectively and harnessing their potential for climate change mitigation.

Wetland Biogeochemistry Processes

Biogeochemical processes in wetlands

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• Anaerobic respiration
  • Occurs in oxygen-depleted soils where water saturates pore spaces
  • Utilizes alternative electron acceptors when oxygen is unavailable
    • Nitrate reduction converts nitrate to nitrogen gas
    • Iron reduction transforms ferric iron to ferrous iron
    • Sulfate reduction produces hydrogen sulfide
• Methane production (methanogenesis)
  • Carried out by methanogenic archaea in anoxic sediments
  • Occurs in strictly anaerobic conditions after other electron acceptors are depleted
  • Produces methane as a byproduct of organic matter decomposition (swamp gas)
• Carbon cycling
  • High rates of primary production from abundant aquatic plants and algae
  • Slow decomposition in anaerobic conditions leads to peat formation
  • Accumulation of organic matter creates carbon sinks (peatlands)
• Nitrogen cycling
  • Denitrification in anaerobic zones removes excess nitrogen
  • Nitrogen fixation by cyanobacteria and other microorganisms adds new nitrogen
• Phosphorus cycling
  • Adsorption to soil particles (clay, iron oxides) retains phosphorus
  • Release under reducing conditions can lead to internal loading

Wetlands for environmental regulation

• Water quality regulation
  • Filtration of pollutants and sediments through physical and biological processes
  • Nutrient removal through plant uptake and microbial transformations (phytoremediation)
  • Reduction of nitrogen through denitrification improves downstream water quality
• Carbon sequestration
  • Long-term storage of carbon in soil and biomass (centuries to millennia)
  • Peatlands serve as significant carbon sinks (30% of global soil carbon)
  • Factors affecting sequestration rates include hydrology, vegetation, and climate
• Greenhouse gas emissions
  • Methane production and release from anaerobic decomposition
  • Carbon dioxide uptake through photosynthesis by wetland plants
  • Nitrous oxide emissions from denitrification in fluctuating water levels
• Net climate impact
  • Balance between carbon sequestration and greenhouse gas emissions varies
  • Variability among wetland types and environmental conditions (temperature, hydrology)

Wetland Management and Ecosystem Dynamics

Impacts of wetland modification

• Wetland drainage effects
  • Increased oxidation of stored organic matter releases CO₂
  • Release of stored nutrients can cause eutrophication downstream
  • Altered hydrological patterns affect regional water balance
• Restoration impacts
  • Re-establishment of anaerobic conditions slows decomposition
  • Changes in vegetation composition alter habitat structure
  • Recovery of nutrient retention capacity improves water quality
• Nutrient cycling changes
  • Shifts in nitrogen and phosphorus dynamics affect productivity
  • Alterations in carbon storage and release influence climate regulation
• Ecosystem function recovery
  • Timescales of functional restoration vary (decades to centuries)
  • Challenges in restoring original ecosystem services due to altered landscapes

Biogeochemistry across wetland types

• Freshwater marshes
  • Dominated by emergent vegetation (cattails, reeds)
  • High primary productivity supports diverse food webs
  • Rapid nutrient cycling facilitates water purification
• Peatlands
  • Accumulation of partially decomposed organic matter forms peat
  • Low pH and nutrient availability limit decomposition
  • Slow decomposition rates lead to long-term carbon storage
• Mangroves
  • Adaptation to saline conditions through salt exclusion and excretion
  • High carbon sequestration potential in biomass and sediments
  • Unique sulfur cycling processes due to marine influence
• Comparative aspects
  • Differences in organic matter accumulation rates (peatlands > mangroves > marshes)
  • Variations in methane production and emissions (freshwater > saltwater)
  • Distinct nutrient limitation patterns (N-limited vs P-limited)
  • Salinity effects on biogeochemical processes alter microbial communities