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The effect of dissolved organic matter (DOM) on the desorption of polycyclic aromatic hydrocarbons (PAHs) from soil was studied with 14C‐benzo[a]pyrene and 14C‐pyrene in different soil‐aqueous solution systems and after prolonged periods of aging (110 and 216 d). The desorption of 14C‐benzo[a]pyrene and 14C‐pyrene was strongly affected by the composition and properties of the aqueous phase. Desorption linearly increased with increasing DOM concentrations. Distribution coefficients (log Koc′) for the desorption of 14C‐labeled PAHs were about 3.5 (pyrene) and 25 (benzo[a]pyrene) times lower in the presence of DOM. The enhancement of PAH desorption between various types of DOM is controlled by the molecular weight distribution of DOM. Dissolved organic matter with a high proportion of high molecular weight components (>14 000 dalton) has a high affinity for PAHs and thus is more effective in desorbing PAHs from soil. In addition, the desorption of PAHs from soil is controlled by the time elapsed since contamination. In the first 36 d following contamination with 14C‐benzo[a]pyrene, a decrease of the PAH concentration that could be desorbed in the solution phase was observed. The desorbable fraction did not show any further changes for up to 183 d after contamination, suggesting a fraction of PAH that is not or very slowly desorbing from soil. The major proportion (>98%) of the desorbable PAH‐fraction is apparently bound at sites that show a rate‐limited release of PAHs. This is corroborated by the model discrimination conducted with three different models, which favors a two‐site equilibrium‐nonequilibrium model.

We investigated dissolved organic matter (DOM) from soil, sewage sludges, water from waste disposal sites, and composts as sorbents and potential carriers for hydrophobic polycyclic aromatic hydrocarbons (PAHs) in soil. Partition coefficients (expressed log K-)for two 5-ring compounds were 4.8-4.9 for DOM from soil, 4.547 from composts, and 4.34.4 from sewage sludges. The DOM from compost and sewage sludge can influence the transport of non-ionic organic contaminants because of the large concentrations of dissolved organic carbon (DOC) released from these materials. Leachates from waste disposal sites did not sorb PAHs. The DOM from compost contained a large percentage of organic molecules >14 OOO Da (32-46%), whereas DOM from waste disposal leachates contained only 7-lo%, and so bound less PAHs. The percentage of total hydrophobic components, as characterized by XAD-8 chromatography, was 50 2 9% for most of the DOM solutions and did not express the differences in affinity of the organic sorbents to PAHs in the same way as the K D~ values. Isolated molecular-weight fractions of DOM from composts sorbed benzo(k)fluoranthene in each fraction. The log KDOC values were 4.14.3 for both fractions, 14 000 Da.The interaction of PAHs with DOM < 1OOO Da cannot be explained by partitioning within intramolecular nonpolar environments of dissolved macromolecules; rather it seems to be due to the amphoteric properties of DOM. This type of interaction of PAHs with small DOM molecules might affect the mobility of hydrophobic organic chemicals in soils.

The skeletal fraction of forest soils in the Southern Black Forest has a cation exchange capacity (CEC) within the same order of magnitude as fine earth. As the base saturation in the skeleton is much higher than in the fine earth, stones provide a physiologically more favourable environment for nutrient uptake by plants. The fact that 70% of the skeleton grains showed adherent rhizomorphae and/or fine roots suggests that this potential is widely used by nutrient-adsorbing tissues. Therefore the ecological importance of this additional and, until now, unconsidered ion pool available to plants is probably higher than what is reflected by simple ion quantities. The effective CEC of the coarse fraction in forest sites with crystalline bedrock is reasonably predictable considering the water vapour adsorption of the 105°C dried skeleton at a suction of pF 4.6 (vapour pressure equilibrium with an oversaturated K 2 SO 4 solution). The content of exchangeable base cations in the coarse fraction can be estimated using the content of exchangeable Mg in the fine earth. These stochastic relations are causally plausible. The regressions are a first approach to identify pedo-transfer functions predicting the skeleton's nutrient potential with the aid of easily measurable site parameters.

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