Origin of Organic Carbon Pools in Kolyma Cryosols: Phenomenon of Organic Carbon Impregnation in Mineral Horizons.
Nikita S. Mergelov, Institute of Geography, RAS, Staromonetniy lane, 29, Moscow, 119017, Russia
The genesis of organic carbon profiles of loamy Cryosols formed in tundra and open forests of Northeast Eurasia is still not clear. However as it was many times stated these soils contain significant amount of Organic Carbon (OC). The 130 km2 area of larch open forests in the Kolyma River Lowland has become a key site to study such carbon-rich soils at a local scale. The OC pools have been estimated in various loamy cryohydromorphic soils (Turbic, Turbi-Saprihistic, Gleyi-Turbic & Endogleyi-Turbi-Histic Cryosols) formed at loess-icy complex sediments (edoma). The OC densities were calculated at the depth of active layer. The estimates were conducted separately for mineral and organic parts of profile. The average carbon density in active layer of loamy cryohydromorphic soils is 15,1 kg C/m2. The highest carbon densities have been revealed for Gleyi-Turbic & Gleyi-Turbi-Saprihistic Cryosols (30,1 kg C/m2) formed in accumulative landscape positions and having the most thick organic horizons and also for Gleyi-Turbic Cryosols of the floodplain ridges (34,7 kg C/m2). The important aspect of high-latitude carbon-rich soils is the proportion between OC in mineral and organic horizons. Most of the prior data show that tundra and forest tundra of Northeast Eurasia is an area where processes of organic matter accumulation prevail in the uppermost organic horizons and organic matter is primarily stored at the surface in the form of peat or raw humus. Our data show that in most cases OC density in the mineral part of soil profiles is higher. In average more than 60% of the active layer carbon pool is concentrated in mineral horizons. The highest values of OC in mineral horizons (OCMH) were obtained for watershed soils, e.g. Turbic, Gleyi-Turbic & Oxyaqui-Turbic Cryosols with 78% of OCMH. There is no definite answer what are the mechanisms of mineral profile enrichment with OC. It could possibly be the result of the following processes: a) cryoturbations (CT); b) humus accumulation due to root fall decomposition in situ (HAR); c) cryogenic “retinization” - migration and accumulation of humus near the permafrost border (CR); d) inheriting the high content of OC from the soil-forming material (ISM). The analyses of OC distribution conducted for soils of the Kolyma Lowland and some of the literature data showed that in 45% of cases the OC distribution is nearly accumulative with maximum at the top of mineral profile. More than a half of profiles showed the second OC maximum: 43,3% - immediately above frozen horizon; 11,7% - in a central part of profile. The situation is generally the same for profile distribution of water-soluble organic matter (WOM). The results obtained by UV-spectrophotometry of aqueous extracts from fresh soil samples showed in 36,3% of profiles WOM maximum in over-permafrost layer. The same percent of profiles revealed OC maximum in the central part. The ratio of labile to stable OC fractions is always much higher in contemporary soil (active layer) than in frozen soil-forming material. This fact indicates significant input of labile OC (which could be considered as WOM) into the profile of modern soil. Thus, the cryogenic “retinization” occur. The distribution of root biomass and carbon densities does not show noticeable correlation. The biomass has always accumulative distribution, while OC densities have various distributions. Noticeable correlation between OC distribution and types of soil, types of organic horizons, climatic parameters is absent. The random appearance of each OC distribution types testifies against hypothesis of pure pedogenic (CR, HAR) origin of over-permafrost OC maximum. The most apparent process responsible for the high variety of OC profiles is cryoturbation, which is extremely heterogeneous in space. It is evident that each organic profile is formed by combination of several processes. HAR is responsible only for OC maximum at the upper part of profile. OC maximums in the centre of profile and over permafrost table could possible be explained by combination of CT and CR. The input of OC from soil-forming rock into organic profile does occur, but is not significant. The contemporary processes of OC transformation and distribution are much more rapid. The large carbon pools in mineral profiles of high-latitude soils should become a big issue to be considered in global change studies.