Opportunities and limitations related to the application of
plant-derived lipid molecular proxies in soil science
Authors: Jansen Boris , Wiesenberg Guido L. B. 
Published: April 2017

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.