The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils

Sorption of dissolved organic matter (DOM) is considered to be a major proc ess in the preservation of organic matter (OM) in marine sediments. Evidenc e for this hypothesis includes the close relationship between sediment surf ace area (SA) and organic carbon (OC) concentrations and the strongly reduc ed biological degradability after DOM has sorbed to mineral surfaces. The a im of this study was to discuss the possibility of a similar process in the soil environment. We accomplished this by gathering information from the l iterature, and by an evaluation of our own studies on DOM sorption and accu mulation of OM in soil. We found that in soil a close association of OM wit h the mineral matrix exists. Both the concentration of soil OM associated w ith the mineral matrix, and the sorption of DOM are related to reactive min eral phases such as Al and Fe oxyhydroxides. Sorption of DOM derived from t he oxidative decomposition of lignocellulose to Al and Fe oxyhydroxides inv olves strong complexation bondings between surface metals and acidic organi c ligands, particularly with those associated with aromatic structures. The strength of the sorption relates to the surface area but more importantly to the surface properties of the sorbing mineral phase. The sorption of a l arge part of DOM is hardly reversible under conditions similar to those dur ing sorption (hysteresis). Because sorption of the more labile polysacchari de-derived DOM on mineral surfaces is weaker, adsorptive and desorptive pro cesses strongly favour the accumulation of the more recalcitrant lignin-der ived DOM. In addition, we found the soil OM in an alluvial B horizon and in the clay fraction of a topsoil strongly resembling lignin-derived DOM from the overlying forest floors. Hence, it seems likely that sorption of DOM c ontributes considerably to the accumulation and preservation of OM in soil. However, this does not result in a significant relationship between OC con centration and SA. Reasons for that finding may be the "masking" of mineral surfaces by adsorbed OM, the clustering of OM patches at highly reactive s ites of metal hydroxides, and/or the absence of a relationship between SA a nd the concentration of surface-active Fe and Al oxyhydroxides in some soil types. Overall, we conclude that sorptive preservation of OM in soil is af fected by the chemical structure of the sorbing DOM and the surface propert ies of the mineral matrix. Localisation and conformation of sorbed OM remai ns unclear and therefore should be subject of further research. (C) 2000 El sevier Science Ltd. All rights reserved.