Productive pothole wetland in an agricultural area.
Wetlands may sequester twice as much carbon as the surrounding agricultural soils
 

Carbon sequestration and greenhouse gas emissions in wetlands

Potential of agricultural wetlands:

Globally, wetlands cover approximately 6 per cent of the land surface but contain 14 per cent of the terrestrial biosphere carbon pool. The enhanced capability of wetlands to store carbon is largely a result of their productivity. Excluding peatlands, wetlands are among the most productive ecosystems in the world. Additionally, wetlands are often anaerobic (without oxygen) which greatly reduces the rate of decomposition relative to aerobic systems. Due to these facts production usually exceeds decomposition in wetlands and results in the net accumulation of organic matter and carbon.

Recently a study by Euliss et al. (2006) found that wetlands in the Prairie Pothole Region of Canada and the U.S. only comprise approximately 17 per cent of the landscape but may sequester twice as much carbon as the surrounding agricultural soils employing no tillage management. These authors concluded that restoring wetlands on the Prairies may sequester 378 Tg of carbon over a 10 year period and estimated that Prairie wetlands have the potential to offset 2.4 per cent of the CO2 emissions produced annually by fossil fueling burning in North America at 1990 levels.

Greenhouse Gas Emissions from Wetlands:

Although wetlands can sequester carbon they can also act as sources of greenhouse gases (GHGs). The most important GHGs released from wetlands are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).

Carbon Dioxide (CO2):

Carbon dioxide is the predominant GHG in the atmosphere responsible for climate change. Carbon dioxide emissions are primarily from anthropogenic (human) fossil fuel combustion and land management practices such as wetland drainage and deforestation. Wetlands can act as a source of CO2 when decomposition of organic matter outpaces production.

Methane (CH4):

CH4 is a more potent GHG than CO2 and is approximately 21 times more effective in retaining atmospheric energy.  Anthropogenic (human) sources of CH4 account for 70 per cent of CH4 emissions while the remainder comes from natural sources.

Wetlands are a natural source of CH4, which results from methanogenesis occurring in the sediments. Methanogenesis takes place in anaerobic environments when other inorganic substances that typically fuel microbial processes are depleted. Once produced in the sediments, CH4 can be released as bubbles or diffused through the water column and then released to the atmosphere. CH4 can also be transferred directly from the sediments where it is produced to the atmosphere by aquatic plants with hollow stems.

Only a fraction of the CH4 produced in the sediments of wetlands reaches the atmosphere due to oxidation in the water column.

Nitrous Oxide (N2O):

N2O is the most potent GHG, and is approximately 300 times more effective than CO2 at retaining atmospheric energy. Production of this GHG results from denitrification, which is favored by waterlogged soils, and the availability of carbon and nitrogen. Wetlands receiving excessive amounts of nitrate (NO3) as a result of anthropogenic (human) activities have increased rates of denitrification and therefore may be a significant source of N2O to the atmosphere.

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