Differentiation of plant derived organic matter in soil, loess and rhizoliths based on n-alkane molecular proxies

Organic matter (OM) in loess-paleosol sequences is used for paleoenvironmental reconstructions, based e.g. on stable carbon isotope composition. Loess OM (LOM) is assumed to derive from synsedimentary vegetation, i.e. predominantly grass. However, low organic C contents make LOM prone to postsedimentary contamination. It was the aim of this study to reveal (1) whether OM of the loess sequence at Nussloch (SW Germany) was altered by postsedimentary input, (2) to which depth, and (3) from which source vegetation this younger OM derives. Therefore, the alkane composition of LOM was compared to that of potential source OM for postsedimentary contamination: recent soil, vegetation growing on the loess sequence, calcified roots (rhizoliths) which derive from postsedimentary deep-rooting plants but not from recent vegetation, and loess in direct vicinity of these former roots. Alkane molecular proxies including carbon preference index and average chain length revealed that grass biomass was the source of soil and LOM. The latter was, except for the uppermost 0.6 m of loess, not affected by pedogenic processes. Further, recent vegetation did not contribute to OM within and loess adjacent to rhizoliths, which were formed under native tree and/or shrub vegetation prior to agricultural use. Strongest degradation of LOM and large amounts of microbial derived OM were found in rhizoloess, indicating former rhizosphere processes. Molecular proxies indicate that overprinting of LOM even in loess distant to former roots cannot be excluded. Therefore, paleoenvironmental reconstructions based on loess delta C-13(org) should be regarded with caution.