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DISTRIBUTION OF ELEMENTS IN HUMUSW. De Vos1, V. Gregorauskiene2, I. Salpeteur3, T. Tarvainen4 with contributions by A. Pasieczna5, U. Siewers6, N. Breward7, M. Duris8 1Geological Survey of Belgium, Brussels, Belgium; The term humus is not used in a consistent manner by all scientists. Frequently in soil science the term represents only the humic substances – a series of relatively high-molecular-weight, brown to black coloured amorphous compounds formed by secondary synthesis reactions: humic acid, fulvic acid and humin (Kononova 1966). Other soil scientists use this term to describe the organic material remaining in soil after removal of macroorganic matter, and also include the nonhumic substances, e.g., amino acids, lipids, carbohydrates (Magdoff et al. 1996). At the other extreme are scientists who use the term humus with a much broader meaning and synonymously with the total soil organic matter – non-living heterogeneous components resulting from microbial and chemical transformations of organic debris (Stevenson 1982). In silviculture, soil organic matter defined in this way and deposited on the surface of mineral soil, is called forest litter. In French terminology, litter is limited to the accumulation of organic debris (leaves, twigs, straw, moss, etc.) in the uppermost Ol horizon of the organic-rich soil layer or O layer, whereas an underlying Of horizon represents a biofermented organic horizon with intense biological activity by bacteria and fungi on this debris; finally the lowermost black Oh or humic horizon within the O layer shows no vegetable structure anymore and is considered to be the humus proper (Ignace Salpeteur 2005, pers. comm.). In all definitions given above, humus results from the accumulation of organic matter which is not saturated with water, in contrast to peat. The field manual of the present FOREGS project (Salminen, Tarvainen et al. 1998) takes an intermediate and somewhat generalised position, and defines the humus to be sampled as the decomposed black coloured organic material found in undisturbed forested areas above the normal mineral soil, in an organic layer. Only the uppermost 3 cm of humus should be sampled; the living surface vegetation, fresh litter, big roots and rock fragments should be removed; also the mineral soil layer should be carefully removed. In practice, however, each FOREGS sampling team was confronted with local conditions that deviated from the idealised or generalised situation. Indeed, soil-forming processes on the scale of Europe show much variation, and this is also true for the formation of humus - or its absence. Humus can exist in different morphological varieties depending on terrestrial environment and especially on climatic and geomorphological factors:
In the FOREGS project, humus could be sampled in northern Europe, especially Fennoscandia, and parts of central Europe. In central Europe, the collected humus samples apparently contain some mineral soil, as could be inferred from their geochemical signature. In other parts of central Europe, and in Mediterranean areas, the organic layer is too thin to be sampled, or completely absent, except in some mountainous areas, such as grass-covered alpine meadows in the Alps. The resulting FOREGS geochemical maps for humus necessarily show only the sampled parts of Europe, leaving blank patches in the central part and the whole area of southern Europe. Originally, 48 elements were analysed in humus, but only 12 elements passed the quality control, and were used to draw distribution maps: Ba, Cd, Co, Cu, Ga, Hg, La, Ni, Pb, Rb, Sr and Zn. Acknowledgements Dr Clemens Reimann from the Geological Survey of Norway gave valuable comments on this chapter. | ||||||||||
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