By Rachel Berkowitz
We humans are rather fond of putting carbon dioxide into the atmosphere by burning fossil fuels. But that's not the only way that carbon reaches the atmosphere in the form of a greenhouse gas.
The European Geosciences Union (EGU) General Assembly 2012, held 22–27 April in Vienna, addressed the release of carbon from soil into the atmosphere and ways to enhance soils' ability to sequester carbon.
Permafrost and a neutral emissions cycle
Above latitude 50˚ north, permafrost soil—that is, soil that remains below the freezing point of water—holds about twice as much carbon as does the atmosphere. As temperatures increase, permafrost degrades and could release at least 10% of its trapped carbon into the atmosphere, according to Ko van Huissteden of Amsterdam's Vrije University.
Re-vegetation can actually recover greenhouse gas sinks, as new populations of CO2-hungry plants get busy photosynthesizing. But van Huissteden referred to recolonization of plants in thawed areas and the ensuing recovery of the ecosystem as a “neglected factor in carbon release by permafrost degradation.”
The number of ponds from melted permafrost has tripled in 30 years, and they have submerged and killed vegetation, said van Huissteden. But at the same time, sedges, a family of flowering plants that resemble grass or rushes, are recolonizing the ponds. The plants emit methane (CH4) but take up CO2, which has resulted in carbon-neutral emissions. Another possible recolonist is peat moss. If it becomes widely established, it could lead to an exponential decrease in greenhouse gas emission rates.
The East Siberian Arctic Shelf provides another example of permafrost effects. Supposedly impermeable subsea permafrost traps coal bed methane. But recent data suggest that the permafrost is thawing: No frozen sediments were found in a 53-meter-long drill core from 2011. In a talk at EGU, Natalia Shakhova of the University of Alaska Fairbanks said that the destabilized subsea permafrost is currently leaking a “substantial amount” of methane, which is becoming involved in the modern carbon cycle. That aqueous CH4 may avoid biological oxidation in the relatively shallow water and escape to the atmosphere.
Permafrost degradation is a transient greenhouse gas source whose evolution over time is important. Climate models should include its degradation and ecosystem recovery to predict carbon feedback with more certainty.
Biochar and a negative emissions cycle
Non-permafrost soil also contains carbon that can escape into the atmosphere, especially when treated with fertilizers. Jorge Paz-Ferreiro from the Polytechnic University of Madrid looks to applying charred organic matter, or biochar, to the soil as a way of storing carbon and reducing the need for fertilizers.
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Biochar is made by pyrolysis. Biomass (often agricultural waste) is heated to high temperatures and the oxygen supply closed off so that the biomass’s carbon content cannot combust and is instead safely stored as charcoal. Added to soil, biochar can remain for centuries with potentially positive benefits for plant life, including increased growth and disease resistance, thus reducing the need for standard greenhouse gas–intensive fertilizers.
Microorganisms break down a soil’s organic material, which releases mainly CO2 but also some methane. Biochar's low ratio of both oxygen and hydrogen to carbon gives it a graphite-like structure, which is decomposed by microorganisms much more slowly than other organic matter: It's like moving from a quick carbon cycle to a slower carbon cycle.
“Biochar itself carries no nutrients,” explains Paz-Ferreiro. Rather, it would be used to complement traditional fertilizers and could reduce the fertilizer requirements of a crop as it “retains some of the nutrients in the fertilizer.” Although biochar generally increases crop yields, some studies have reported diminished yields. But scientific trials are optimistic in light of the Amazonian “terra preta,” anthropogenically altered carbon-rich fertile soil that has inspired agricultural use of biochar.
Storing atmospheric CO2 in deep geological formations may be the high-profile way of cleaning up greenhouse gas emissions. But understanding and optimizing the role of soils in the carbon cycle provides important mitigation possibilities as well.
