ИСТИНА

Soil in the areas of active volcanism experience extreme and catastrophic stages when pedogenesis interrupts completely and then revive again with incipient stages, governed by microbial communities. The burial of soil profile occurs with the depth of tephra layer exceeding 1-1, 5 cm. Smaller additions of tephra are incorporated in the upper horizons without breaks in pedogenesis. Burial of soil profile is accompanied by the partial or complete collapse of plant and microbial community. The biggest catastrophic eruptions cause complete destruction of al viling organisms in the area of many thousands of square km (Ponomareva, 2010). After such eruptions soils are revived under the influence of microbial biosphere. Microbial communities appear in the fresh tephra already in the first years after eruption (Sidelnikov and Shafranovsky, 1985). Pioneer higher plants have been recorded on fresh ashes already in 9 years, but regeneration of mature plant communities takes much longer time. The development of plant communities could be hampered by considerable input of anions and cations absorbed on ash grains from volcanic gases during the eruption: SO4-, Cl-, HCO3-, Na+, K+, Ca2+, etc. This ensures high content of soluble substances in fresh ash without weathering. For instance, concentration of soluble sulfur in fresh ash may 50 times exceed that in the old ash and chlorine – 63 times, and that could be toxic to plants (Karpachevsky et al., 2009). Input of pyroclastic material alternated or accompanied by pedogenesis resulted in the formation of soil–pyroclastic mantle SPM (Melekestsev et al., 1969) that includes tephra marker layers, ash of small volcanic eruptions and buried soil horizons. Except for soils in the nearest proximity to active volcanoes the depth of SPM is limited to 1-2 m and the age of even the lowest tephra layers is not older than early Holocene because older pyroclastic mantles were destroyed by Pleistocene glaciations. The major part of soil profiles is developed in mid to late Holocene tephra. Owing to a combination of cold climate and com¬paratively young age tephra layers are not deeply transformed by weathering. This, together with low bioturbation intensity, results in a sequence of clearly visible horizontal layers, retaining their morphological, chemical and mineralogical identity. Soils of Kamchatka are mostly presented by Vitric Andosols. Ash layers younger than 1000 years looks like a mix of mineral grains and volcanic glass shards enriched with organo-metallic complexes. There are no cutans, except thin humus films. Soil horizons in tephra between 1,000 and 5,000 years are more transformed. Tephra layers with the age older than 5000 years could meet criteria for andic properties. Small on-going input of pyroclastic material results in the formation of thick Folic horizons (dry-peaty in Russia, Shishov et al., 2004), increased depth of humus horizons and separation of upper horizons with ash lenses. These, together with buried humus horizons make Andosols of the Russian Far-East high-capacious carbon sink reservoir. Organic carbon stock in Andosols is formed from undecomposed plant tissues, humus of buried horizons and mobile fractions of humus, migrating from the upper horizons in the form of organo-metallic complexes. Carbon stock in Kamchatka Andosols is estimated at 40-70 kg TOC/sq. m (Glazovskaya, 2009). Based on the age of tephra this stock was formed for the period of 6000-7.500 years. So the mean annual rate for carbon accumulation in soils varies within 23.10-4 – 47.10-4 TOC/sq. m that substantially exceeds accumulation in other pedolithoformations, including rain-fed bogs of Western Siberia. Research is supported by the Russian Science Foundation, Grant #14-27-00133.