Retention of DOC in Soil

Sorption of DOC on mineral phases is the key geochemical process for carbon preservation in soils (McDowell and Likens 1988). In the broad range of ecosystems, most DOC leached from organic horizons is sorbed and retained in the subsoils (Kaiser and Guggenberger 2000; Kalbitz et al. 2000, 2005). The sorption depends much on the contents of sesquioxides and amount of carbon previously accumulated in soils (Kaiser et al. 2000; Kawahigashi et al. 2006). In general, immobilization

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Fig. 16.3 DOC flux from forest floor in dependence from the amount of seepage water (Prokushkin et al. 2008)

of DOC has been considered an important process in the formation of stabile OC, due to its protection against microbial attack.

Sorption and mineralization of DOC in soil is not uniform, because of the heterogeneity and the complex mixture of organic molecules with different chemical characteristics, including a polymeric structure of major constituents (Schulten and Gleixner 1999). Along with the decrease of DOC concentrations on its passage through mineral soil (Fig. 16.4), there are major biochemical alterations of DOC composition. Hydrophobic compounds of high molecular weight and rich in acidic functional groups and aromatic moieties sorb most strongly (Kaiser et al. 2000; Kawahigashi et al. 2006). However, there is an introduction of "new" substances to soil solution in subsoil due to desorption of humified material and the release of hydrophilic microbial products (Kawahigashi et al. 2004; Prokushkin et al. 2007).

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