The role of clay fraction organic matter in the dynamics of the qualitative composition of humus burozems of Reineke Island

A high monsoon of island ecosystems climate and active geochemical impact of the sea on their territory creates preconditions for peptization and migration of clay particles of soils and humus bound with it. The development of this process results in formation of dark humus-illuvial burozems by the presence of the illuvial-humic BMhi horizon, with a high level of humus content, in their profile. The aim of this work was to identify the role of humus clay fraction in the dynamics of humus content and composition in these soils. For this, we studied qualitative composition of humus both in the bulk soil mass and in the clay fraction. We investigated dark humus-illuvial burozems occurring under sagebrush (pit 24) and under forest (pit 23) on Reineke Island. These soils are characterized by argillization of the middle part of their profile. It is the most clearly seen in the burozems, forming under active geochemical influence of the sea (pit 24). The content of physical clay and clay fractions in these burozems is higher than in burozems under forest. Despite a lower content of clay fraction in horizon BMhi burozems under forest, it accumulates 45% of the soil humus against 29% in the burozems under sagebrush. In the fractional composition of the humus in the burozems under sagebrush levels of humic acids is higher in the whole profile than that in the burozems under forest. The distinctive trait of the group composition of these burozems humus is fulvate-humate type of humus in humus-illuvial horizon BMhi. In the burozems under forest, humic acid predominated over fulvic acid in humus composition only in AU horizon. The specificity of the profile distribution of black humic acids (HA-2) and brown humic acids (HA-1) in these soils, resulting from different intensity of development of humus-illuvial processes of soil forming. The rates of mobility of all the said fractions accrued from burozems under forest to burozems under sagebrush. It is illustrated by profile distribution of their relative levels. As a result, the humus reserves in the 0-100 s thick in these soils increased to 550t/ha against 383t/ha in the burozems under forest. The feature of the morphological structure of dark humus-illuvial burozems is presence in their profile of tongue-shaped pockets of humus in the upper part of the BMhi horizon. Humus composition of this spot is characterized by increased concentration of nonhydrolyzable residue and a lower content of humic acids compared to the background horizon. Wherein in the composition of humic acids the portion of black humic acids increased and the portion of the first fraction decreased. The composition of humus in the clay fraction is characterized by a considerably higher relative content of fulvic acids (particularly, "aggressive" fraction) and a lower content of humin fraction, as compared with the humus in the bulk soil mass. The content of humic acids is very low, and this reflects a significant decrease in the C _ha:C _fa ratio. Thus, the increase in the content of fulvic acids (fulvatization) in the studied burozems is mainly due to their accumulation in the clay fraction and depends on the contribution of clay fraction humus in the total carbon content in the soil.

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