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

Abstract

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 d 13 C org should be regarded with caution.

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…This result was of high importance, supporting the largely anecdotal evidence that shrub dominated the areas across the basin study area that is used to be dominated by grasses [Buffington and Herbel, 1965]. At the Los Piños sites, due to the similarities between grama grass and juniper n-alkane signatures, there was a degree of difficulty in determining between sources; based upon n-alkane signature, a problem was previously experienced when using n-alkane signatures alone [Marschner et al, 2008;Gocke et al, 2011]. However, the use of n-alkane signatures from piñon and combined with the δ 13 C values of both piñon and juniper demonstrated that the same trends transcend to the mountainous, grassland to piñon-juniper transition [Wilcox et al, 2003b].…”

Opportunities and limitations related to the application of plant-derived lipid molecular proxies in soil science

“…In addition, with respect to the degree to which wax lipid chain length patterns vary between the leaves and roots of plant species, there appears to be quite some variability. In general, the observed differences between roots and leaves of the same species are reported to be of the same order of magnitude as the differences between leaves of different species (e.g., Jansen et al, 2006;Kirkels et al, 2013;Gocke et al, 2014).…”

“…However, this may be different for the wax lipids, i.e., n-alkanes, n-alcohols, n-fatty acids, and isoprenoids, that have been found to occur in the leaves and roots of species at varying concentrations (Jansen et al, 2007;Huang et al, 2011). Particularly when such wax-derived lipids are applied as molecular proxies for vegetation cover in soil, root input can be an issue for two reasons: (i) roots may contain a different wax lipid composition than leaves qualitatively and quantitatively, thereby clouding the leaf signal Martelanc et al, 2007) and (ii) young root input at depth may disrupt the chronology of a reconstruction in time by overprinting the originally present signal Gocke et al, 2014).…”

Leaf waxes in litter and topsoils along a European transect

“…Several studies reported long-chain n-alkanes not only in leaves but also in other plant parts (roots, stems, blossoms; e.g. Wöstmann, 2006;Jansen et al, 2006;Kirkels et al, 2013;Gocke et al, 2013), but in these studies the patterns show preferential synthesis of shorter chains (< n-C 25 ) with low OEPs, as well as much lower nalkane concentrations (3 to 10 times) than in leaves.…”