Compositional variety of soil organic matter in mollic floodplain-soil profiles - Also an indicator of pedogenesis

Soils on riverine floodplains in Central Europe are commonly enriched in soil organic matter (SOM). We analyzed the quantity and qualitative aspects of SOM in three soil profiles with mollic horizons along the Elbe River (Germany) after physical fractionation by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to gain deeper insights into the composition of SOM and the formation of these soils. In all horizons, the majority of SOM was recovered in the fraction of particulate OM and in that of sand and aggregates, while SOM in the silt and clay fraction always made up < 20% of soil organic carbon. The C:N ratios of most fractions did not decrease with depth, indicating less decomposed or pyrogenic OM or both. The presence of pyrogenic OM, as a characteristic feature of floodplain soils, was indicated in almost all physical fractions. As derived from DRIFT spectroscopy, SOM was less qualitatively differentiated among the fractions in the soil with the longest duration of flooding, while qualitative differences of SOM were more pronounced in the more aerated ones. The soils have developed from stratified fluviatile sediments and fulfil the criteria of a mollic horizon, irrespective of differences in the composition of SOM. From a soil-genetic point of view, we strongly suggest to classify these soils as Mollic Fluvisols instead of Fluvic Phaeozems, which would accord to the latest WRB classification.