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Mendez-Millan et al., 2010). In order to acclimatize to the specific conditions of peatlands (waterlogged, nutrient poor and partly anoxic and acidic), plants develop shallow but flourishing root systems (Rydin and Jeglum, 2006), which can contribute high amount of lipids to peat deposits. To date, only Pancost et al. (2002) noted the root-derived lipids from peat-forming plants.A number of studies have reported the lipid compositions of peatland plant communities (Ficken et al., 1998;Nott et al., 2000;Pancost et al., 2002;Nichols et al., 2006). These communities were distributed in Europe and North America, while the results in other regions were not available in the literature to the best of our knowledge. In addition, except for the peat moss (Sphagnum species) shrubs are abundant in the above bogs. In contrast, the Dajiuhu Peatland is characterized by Sphagnum palustre and various herbaceous plants. It is, therefore, worth assessing the lipid composition of the plant community in such an herb peatland. The n-alkane distributions of several moss species (S. palustre, Aulacomnium palustre, Polytrichum commune and Hypnum revolutum) from the area have been previously reported by Huang et al. (2010 (Received November 16, 2010; Accepted April 30, 2011) The main aim of this study was to assess the free lipid composition of plant community in a Chinese peatland. Twelve plant species from the Dajiuhu Peatland were analyzed for the compositions of n-alkanes, n-fatty alcohols and sterols in leaves and roots. The lipid compositions are significantly different between root and leaf for most plants. In some cases, the roots yield more long chain n-alkanes and n-alkanols than the leaves of the corresponding plant. The long chain nalkanes of the roots in half of the plant species are characterized by a higher C max (homologue with the maximum concentration) relative to the leaves. The root-derived sterols and steroidal ketones may be important sources for corresponding compounds within the peat. Because of the different lipid compositions of roots and leaves, more attention should be paid to root-derived lipids for investigations of the lipid composition and their source assessment in soils and terrestrial sediments, where root-derived organic matter can be an important source. The contribution of root-derived lipids may be especially important in peatlands, where acidic and/or anoxic conditions in the subsoil limit the degradation of organic matter and the contribution of leaf litter.

Abstract. The application of lipids in soils as molecular proxies, also often referred to as biomarkers, has dramatically increased in the last decades. Applications range from inferring changes in past vegetation composition, climate and/or human presence to unraveling input and turnover of soil organic matter (SOM). Molecules used include extractable and ester-bound lipids as well as their carbon or hydrogen isotopic composition. While holding great promise, the application of soil lipids as molecular proxies comes with several constraining factors the most important of which are: i) variability in the molecular composition of plant-derived organic matter plant-internally and in between plant individuals; ii) variability in (relative contribution of) input pathways into the soil; and iii) transformation and/or (selective) degradation of (some of) the molecules once present in the soil. Unfortunately, the information about such constraining factors and their impact on the applicability of molecular proxies is fragmented and scattered. The purpose of this study is to provide a critical review of the current state of knowledge with respect to the applicability of molecular proxies in soil science, specifically focusing on the factors constraining such applicability. Variability in genetic, ontogenetic and environmental factors influence plant n-alkane patterns in the way that no unique compounds or specific molecular proxies pointing to e.g. plant-community differences or environmental influences, exist. Other components such as n-alcohols, n-fatty acids, cutin- and suberin-derived monomers have received far less attention in this respect. Furthermore, there is a high diversity of input pathways offering both opportunities and limitations for the use of molecular proxies at the same time. New modelling approaches might offer a possibility to unravel such mixed input signals. Finally, transformation and turnover of SOM offer opportunities when tracing such processes is the purpose of applying a molecular proxy, whilst posing limitations when they obliterate molecular proxy signals linked to other phenomena. For n-alkanes several modelling approaches have recently been developed to compensate for (selective) degradation. Still such techniques are in their infancy and information about their applicability to other classes of components than n-alkanes is lacking yet. All constraining factors considered can have a significant influence on the applicability of molecular proxies in soil science. The degree of influence strongly depends on the type of molecular proxy as well as the environmental context in which it is applied. However, the potential impact of the constraining factors should always explicitly be addressed whenever molecular proxies are applied in a soil scientific context. More importantly, there is still a serious lack of available information in particular for compound classes other than the n-alkanes. Therefore, we urgently call for the consideration of more holistic approaches determining various parameters during sampling as well as using as many compound classes as possible.

Analysis of lipids and hydrocarbons is performed frequently in recent and ancient plant tissues, soils, sediments, peat deposits, oil, rocks, anthropogenic artifacts (archeological samples), and other materials to trace the contribution of different biological and anthropogenic sources of organic matter as well as environmental changes and the fate of organic matter like degradation. The approaches for the analysis of lipids and hydrocarbons strongly vary from traditional methodologies like thin-layer chromatography to universal approaches like pyrolysis, whereas the preparative separation of lipid fractions based on their polarity enables gas-chromatographic analyses of single fractions and compound-specific analysis of stable (2H/1H, 13C/12C) and radioactive (14C) isotope compositions. Often, lipid extraction operationally defines a subfraction of total lipids. On the one hand, free extractable lipids are obtained by extraction with organic solvents, whereas on the other hand, total samples or extraction residues are extracted for more polar lipid fractions using highly polar organic solvents and water, to release bound lipids. Procedures for extraction of free extractable lipids are diverse and mainly defined by the target of research and availability of instrumentation. In the current protocol, state-of-the-art techniques for the investigation of free extract-able lipids in various materials are explained, which can be applied even in laboratory environments with limited technical equipment. The protocols cover sample preparation, extraction, purification, analysis, as well as a brief overview of the data evaluation using lipid molecular proxies and compound-specific isotopes. Metadata of the chapter that will be visualized online Often, lipid extraction operationally defines a subfraction of total lipids. On the one hand, free extractable lipids are obtained by extraction with organic solvents, whereas on the other hand, total samples or extraction residues are extracted for more polar lipid fractions using highly polar organic solvents and water, to release bound lipids. Also, procedures for extraction of free extractable lipids are diverse and mainly defined by the target of research and availability of instrumentation. In the current protocol, state-of-the-art techniques for the investigation of free extractable lipids in various materials are explained, which can be applied even in laboratory environments with limited technical equipment. The protocols cover sample preparation, extraction, purification, analysis, as well as a brief overview of the data evaluation using lipid molecular proxies and compound-specific isotopes. Keywords (separated by '-') Alkanes -Biomarkers -Fatty acids -Gas chromatography -Lipid extractionLipid fraction -Molecular proxies -Preparative separation -Solid-phase extraction purification, analysis, as well as a brief overview of the data evaluation using lipid molecular proxies and 21 compound-specific isotopes.22

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