Sorption of DOM and DOM fractions to forest soils

The sorption of total, hydrophilic, and hydrophobic dissolved organic matter (DOM) on 125 forest soil samples was studied in batch experiments. The soil samples represented the main soil orders of temperate climatic zones: Entisols, Inceptisols, Alfisols, Mollisols, and Spodosols. In general, sorption was adequately described by the linear initial mass isotherm. At zero addition of DOM, the release of dissolved organic carbon (DOC) into solutions depended on the soil organic C (OC) content. The DOC released was comprised primarily of hydrophilic substances (61–100%). High contents of OC reduced DOC sorption, notably that of hydrophilic DOC. In soils free of carbonates, sorption was related to oxalate-extractable aluminum (Alo) and dithionite-extractable iron (Fed); however, in carbonatec soils DOC retention was correlated with Fed only. Consequently, soils with high OC content and/or low contents of extractable Al and Fe showed low DOC retention (e.g. E horizons), whereas subsoil horizons had high DOC sorption capacities. The majority of the soils preferentially sorbed hydrophobic DOC. This was caused by the higher affinity of hydrophobic DOC to oxide/hydroxide soil components. There was also a strong competition effect between hydrophilic and hydrophobic DOC, especially in soils with a limited number of binding sites. sorption of hydrophobic DOC in some soils was even accompanied by the displacement of indigenous hydrophilic substances. In carbonate-free soils, multiple regression analysis including Alo, Fed, and OC explained 75 and 77% of the variance of the partition coefficient (m) of the initial mass isotherm for the sorption of total and hydrophobic DOC, respectively. Due to the competitive effect of hydrophobic DOC on hydrophilic DOC retention, the relation was weaker for the hydrophilic fraction. In the carbonate-containing soils, multiple regression analysis including Fed and OC explained 67, 58, and 59% of the variance of m for total, hydrophilic, and hydrophobic DOC, respectively.