The Green Technocrat

There have been some surprising discoveries about the carbon sequestering power of wetland areas recently.  Wetlands are also great at sucking carbon out of the atmosphere.  Unfortunately, they also release methane which, as I’m sure you all know by now from reading my post What Is Global Warming Potential? Click Here To Find Out…, has a global warming potential 21 times higher than carbon dioxide.  With this in mind researchers for the USGS set out earlier this year to determine whether the benefits of wetland carbon sequestering out weigh their contribution to global warming from the release of methane.

Earlier this summer the USGS embarked on a $12 million dollar experiment; to build saltwater wetlands on abandoned farmland on islands in California’s Sacramento−San Joaquin River Delta, and to then measure the amount of carbon captured.  The results have been shocking.  According to results published in an article by the ACS the wetlands have captured “an average of 3000 grams of carbon per square meter per year (g-C/m²/yr) over the past 5 years. For comparison, reforested agricultural land, eligible for carbon credits under the Kyoto Protocol on climate change, socks away carbon at a rate much less than 100 g-C/m²/yr, says Gail Chmura, a biogeochemist at McGill University (Canada).”

So if the wetlands are capable of pulling in that much carbon why the worry about their methane emissions?   In order to perform their necessary metabolic activities microbes living in oxygen poor environments (like wetlands) will use other electron acceptors such as sulphates or CO2 in these pathways.  An electron acceptor is a molecule capable of receiving an electron from another molecule.  So that much was probably obvious from the name, but what you might not know is that this process drives the majority of our metabolic activities.  In metabolism electrons move from high energy compounds (electron donors) to low energy compounds (electron acceptors).  The energy that is released in these transfers is then harnessed by the organism.  In many microorganisms (and there are a LOT of these living in wetlands) when CO2 acts an electron acceptor one of the by-products is methane.

Unfortunately it is very difficult to measure the release of methane from these organisms because individually they are releasing very, very small amounts and estimation methods for large areas (like wetlands) are fairly crude.  Scott Bridgham, an ecosystem ecologist at Oregon State University, recently coauthored a chapter in the book The First State of the Carbon Cycle Report, in which he stated that in all likely hood the climate-warming potential of methane cancels out the climate-cooling potential of carbon storage for most North American freshwater wetlands.

What makes the USGS study noteworthy is that they built saltwater wetlands, which are high in sulfates.  These sulfates make better electron acceptors than CO2, thus greatly reducing the amount of methane produced by the organisms living within these marshes.  In the same ACS article I mentioned above Gail Chumra estimates that saltwater marshes in North America sequester an average of 210 g-C/m²/yr, making them an ideal for carbon sequestering projects across the country.  Currently the USGS project is focusing on adding various nutrients to the wetlands to try to maximize carbon storage and reduce methane emissions.