Permafrost soils cover about 22 million km2 in circumarctic areas, and 65% of the land area of the Russian Federation is underlain by permanently frozen soils. Recent estimates on the organic carbon storage in permafrost soils range around 1.700 gigatons. As this is more than twice the amount of carbon that is in the atmosphere in form of CO2, small changes in the stock of organic carbon in permafrost soils will have profound impact on the atmosphere’s CO2 (and CH4) concentration and with that very likely on the Earth’s climate. Knowledge about processes leading to stabilization and destabilization of organic carbon stored in permafrost soils are thus of utmost importance for the prediction of future climate. In my presentation I will address three relevant questions with this respect:
(1) How much organic carbon is stored and what is its distribution in cryoturbated permafrost soils?
(2) How is soil organic matter transformed and is it stabilized by physicochemical processes against decomposition?
(3) What is the link to the soil microbial community composition and activity?
The studies are being carried out at different locations in tundra ecosystems of Siberia. Organic carbon storage shows a high spatial variability of 15-50 kg m-2, most of it located in the subsoil due to cryoturbation processes. The organic matter shows an increasing stage of decomposition with increasing soil depth, as being assessed by the pattern of the carbon-to-nitrogen (C/N) ratio, the δ13C ratio, 13C nuclear magnetic resonance spectra, and X-ray photoelectron spectra. This emphasizes pronounced microbial activity in the soils. Seventy to eighty percent of the organic carbon is associated with the mineral phase, thus possessing a potential stabilization mechanism. Partial least squares regression analysis indicated that preferred interactions between minerals and organic matter occurred by complexation with metal ions as well as associations with poorly crystalline iron oxides and clay minerals. While these minerals harbor much of the organic matter stored in the soil, decomposition kinetics of mineral-associated organic matter as obtained by incubation experiments deviate not much from that of non-protected particulate organic matter. This suggests that the stabilizing effect of soil minerals on the organic carbon is not as pronounced as it is known from temperate and tropical soils. The analysis of the microbial community provided a strong gradient in its composition with soil depth. Of importance with this respect is that the microbial communities in buried topsoil material resembled not those of the topsoil, where it derives from, but the neighboring subsoil horizons. This suggests that the environment is more decisive in the determination of the microbial community composition than the organic substrate. This cryoturbated material also showed reduced protein depolymerization and nitrogen limitation.
It is concluded that stabilization of organic matter by soil minerals is not pronounced due to low concentration and/or fast turnover of reactive minerals. The abundance and activity of different taxa of microorganisms is controlled rather by the soil environment. Changes in environmental conditions (i.e., warmer, drier with more oxygen) may result in the mobilization of a significant amount of the organic carbon stored in permafrost soils. Ongoing research is extends this study into the taiga, and is aiming to regionalize results using remote sensing techniques.