Long-term Carbon Storage in Cryoturbated Arctic Soils

Soil organic carbon (SOC) stored in the permafrost of the Arctic is one of the largest carbon reservoirs globally and is vulnerable to climate change. Despite its undisputed importance, the amount of arctic SOC remains ambiguous and poorly constrained. A significant proportion of this SOC is stored in the subducted organic matter of Cryosols, suggesting that cryoturbation (i.e., the mixing of soil layers due to freezing and thawing) is one of the most important mechanisms of arctic carbon sequestration. The major objectives of CryoCARB are therefore (i) to advance organic carbon estimates for cryoturbated soils including the carbon stored in the permafrost, (ii) to identify the major SOC stabilization mechanisms, focusing on organic matter quality, microbial community composition and on abiotic factors, and (iii) to assess the vulnerability of arctic carbon stocks in a future climate. CryoCARB is structured in work packages, dealing with carbon storage in cryoturbated soils (WP1), with quality and degradability of SOC (WP2), with microbial processes and community structure (WP3) and with the integrative modelling of SOC dynamics in cryoturbated soils (WP4). These WPs are linked and integrated by a set of joint sampling campaigns in Siberia (three transects), Greenland and Svalbard and by joint experiments that address the sensitivity of SOC to changing environmental conditions in the laboratory and in the field. The work will be supported by the development of a sound theoretical and conceptual framework and by mathematical modelling. CryoCARB represents a multinational collaboration between 8 European countries and Russia, applying an interdisciplinary approach to address critically important issues that link cryoturbated arctic soils to the global carbon cycle. Such a comprehensive undertaking, from molecular microbiology to landscape level carbon inventories and modelling of circum-arctic carbon storage in future climates, is new and has not yet been implemented anywhere for the Arctic.

In Arctic soils, large stocks of organic carbon have accumulated over the past Millenia, as unfavourable (cold and wet) conditions constrain microbial activity in the soil and thereby microbial decomposition of organic matter. This large Arctic soil carbon storage, which holds approximately twice as much carbon as the atmosphere, is currently watched with concern: Rising temperatures are suspected to promote microbial decomposition of soil organic carbon and thus CO2 release from soils, which would further accelerate climate change. A large proportion of arctic soil carbon resides in so called cryoturbated soils, which exhibit a special, climate-related feature: Frequent freezing and thawing has relocated organic-rich topsoil layers or pockets into deeper soil layers, which are usually poor in organic matter. This burying of soil organic matter has additionally delayed its decomposition to a great extent, the reasons for this, however, remained unclear. We explored the mechanisms of soil organic carbon conservation in cryoturbated arctic soils and its vulnerability to climate change in an international research project (www.cryocarb.net). Based on experimental manipulations of soil samples from a large number of field sites across the Eurasian Arctic, we found that it is NOT as commonly believed increasingly harsh environmental conditions in the deep soil, such as lower temperature or higher soil moisture, which lead to the additional slow-down of microbial decomposition in cryoturbated soil horizons. Rather, a mixture of microbial nutrient and energy limitation and a fundamental incompatibility between chemical properties of the buried organic material and its new microbial colonizers in the deep soil are likely to be responsible. Opposed to native topsoil microbes, microbes colonizing the buried organic matter pockets in the deep soil werent able to produce the special suite of extracellular enzymes necessary to effectively break-down topsoil material. Moreover, decomposition of organic matter was strongly limited by energy and nitrogen availability in buried soil layers: the addition of amino-acids as an additional food source dramatically increased organic matter decomposition in the buried horizons (but not in the recent topsoil horizons). Opposed to this, higher temperatures or different soil moistures did not substantially increase microbial decomposition rates in the buried soil pockets. We conclude that buried organic carbon in cryoturbated arctic soils is vulnerable to climate change, but temperature increase alone is unlikely to harm it. Rather, changes in nutrient input or vegetation cover may be crucial. Deeper rooting plants may introduce nutrient- and energy-rich compounds, as well as microbes specialised on degrading topsoil material (f.e. mycorrhizal fungi) into buried soil layers, which may promote decomposition of the long-term conserved buried horizons in cryoturbated arctic soils.