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Sunday, May 23, 2010

Hermann F. Jungkunst, Nature Geosci., 3 (2010), Soil science: Arctic thaw

Nature Geoscience, 3 (2010) 306-307; doi: 10.1038/ngeo851

Soil science: Arctic thaw

Hermann F. Jungkunst* 
Institute of Geography, Landscape Ecology, University of Gottingen, Goldschmidtstrasse 5, D-37077 Gottingen, Germany
The organic matter stored in frozen Arctic soils could release significant quantities of carbon dioxide and methane on thawing. Now, laboratory experiments show that re-wetting of previously thawed permafrost could increase nitrous oxide production by 20-fold.

Arctic soils store tremendous amounts of organic matter. Over millennia, cold, wet conditions have slowed the breakdown of plant material in the Arctic, and large quantities of carbon and nitrogen have built up in permanently frozen ground — termed permafrost. Global warming threatens to thaw these frozen soils and release large quantities of methane and carbon dioxide to the atmosphere1. Nitrous oxide — another potent greenhouse gas — can also be emitted from permafrost soils, but the relationship to thawing is uncertain2. Writing in Nature Geoscience, Elberling and colleagues3 show that the addition of the original nitrogen- and carbon-rich meltwater to thawed permafrost cores, sampled from Greenland, stimulates nitrous oxide production.

Microbial breakdown of soil organic matter can produce three greenhouse gases: carbon dioxide, methane and nitrous oxide. The magnitude of emissions is not only dependent on temperature, but also on water and oxygen levels. The quality and abundance of soil organic matter — which is heterogeneously distributed in most soils — will also influence gas flux from the soil to the atmosphere.

As the Arctic climate warms, the upper, active layer of permafrost soils, which melts each summer, could thicken, facilitating the breakdown of previously frozen organic matter by soil microbes. Indeed, there is evidence to suggest that newly thawed permafrost releases large volumes of methane and carbon dioxide1, 4. Furthermore, freeze–thaw cycles can promote nitrous oxide emissions5: in the Arctic, permafrost melting creates a mosaic of wet and dry soil conditions — due to small differences in topography and drainage — that favour nitrous oxide production6.

Elberling and colleagues show that the production of nitrous oxide just beneath the active layer can be extraordinarily high when permafrost soils undergo melting and subsequent re-wetting in a laboratory environment3. They examined the effect of thawing on nitrous oxide production in permafrost cores up to three metres in length, collected from a wetland site in northeastern Greenland. To mimic freeze–thaw conditions, cores were thawed, drained and subsequently re-wetted with the original meltwater, which contained high concentrations of ammonium and dissolved organic matter. Rates of nitrous oxide production were low in the frozen and thawed cores. However, there was a 20-fold increase in nitrous oxide production throughout the entire depth of the permafrost soils on re-wetting with the original meltwater; production rose to 18 μg nitrogen per hour per kg of soil. The fact that the addition of carbon- and nitrogen-rich water triggered nitrous oxide production suggests that the carbon and nitrogen cycles in these soils are tightly connected. Measurements of nitrous oxide production in permafrost soils collected from an additional five wetland sites suggest that the high rates of nitrous oxide production observed in the Greenland soils are not unique.

However, not all of the nitrous oxide produced following re-wetting will escape to the atmosphere. Some will be consumed within anoxic micro-zones in the active layer. To gauge the amount of nitrous oxide actually emitted, Elberling et al. measured nitrous oxide emissions from one of the thawed and re-wetted cores from northeast Greenland: only 31% of the nitrous oxide produced was emitted to the atmosphere, although this is still equivalent to 34 mg of nitrous oxide per square metre per day. In fact, emissions of this magnitude exceed those from bare peat patches, match most of those from highly fertilized agricultural sites, and are only beaten by emissions from highly fertilized and compacted potato fields7 (Fig. 1).

Figure 1. High daily nitrous oxide emissions from Arctic, temperate and tropical soils.

Figure 1 : High daily nitrous oxide emissions from Arctic, temperate and tropical soils.
Elberling et al. examined the impact of thawing on nitrous oxide production in permafrost soils3. They found that thawing alone had little impact on nitrous oxide levels, but re-wetting with the original meltwater significantly stimulated production. Experiments on one core suggest that only a third of the nitrous oxide produced following re-wetting is released to the atmosphere. Error bars represent the standard deviation.
Full size image (27 KB)

Of course, their findings need to be verified in the field. A key uncertainty is how plants — which compete with soil microbes for ammonium and nitrate — will influence the production and emission of nitrous oxide. Given that strong nitrous oxide emissions are found in vegetation-free patches of sub-Arctic tundra2, and that plant cover will probably increase in the Arctic region owing to rising temperatures, the impact of vegetation on nitrous oxide emissions deserves examination.

Elberling and colleagues show that nitrous oxide emissions from thawed permafrost can equal those from highly fertilized agricultural soils3. But science is not an Olympic sport, where faster, higher and longer are the only results that count. It is important, too, to understand the more subtle feedbacks, such as those between the Arctic carbon and nitrogen cycles.


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