Colloidal matter in water extracts from forest soils

Environmental context. Little is known about the proportion of tiny dispersed particles and true solutions in soil water although the distinction has a major influence on transport processes of organic matter, fertiliser and pollutants in soils and thus, e.g., on carbon storage, and its role in global warming. Our study has found a noticeable amount of tiny particles (range 17 nm to 1.0 ?m) in filtered soil water, that have a different chemical composition and a lower bioavailability of their organic components in comparison to the soluble part. This significant occurrence and the ecological relevance of colloids for the transport and storage of soil constituents highlights the need to partition soil water content into ?particulate' and ?dissolved' since the access to soil pores determines particle transport.  Abstract. Water-extracted organic matter (WEOM) is widely used as a surrogate for natural organic matter in soil water in the investigation of soil carbon dynamics. Information about the dissolved or colloidal nature of the organic matter is scarce since dissolved organic matter (DOM) is simply operationally defined by filtration: ?DOM is what passes through the filter'. Water extracts of two topsoil horizons from both a deciduous (Steinkreuz) and a coniferous (Rotthalmnster) forest, located in Bavaria (Germany), were filtered through a 1-?m quartz filter and analysed regarding the amount of colloids in the range ~17 nm to 1.0 ?m, the chemical composition and the radiocarbon concentration of both the colloidal and the dissolved fraction separated by high-speed centrifugation. Up to 13.9 wt-% of the total charge of the water extracts belongs to the colloidal fraction. The colloidal fraction has a higher relative proportion of metals and older organic C than the dissolved fraction. This demonstrates the dual nature of WEOM and the need for a more careful definition of DOM.