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

Abstract

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 the input and turnover of soil organic matter (SOM). The molecules used are extractable and non-extractable lipids, including ester-bound lipids. In addition, the carbon or hydrogen isotopic composition of such molecules is used. 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 both internally and between individual plants, (ii) variability in (the relative contribution of) input pathways into the soil, and (iii) the 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 influences plant n-alkane patterns in such a way that no unique compounds or specific molecular proxies pointing to, for example, plant community differences or environmental influences, exist. Other components, such as n-alcohols, n-fatty acids, and 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 modeling approaches might offer a possibility to unravel such mixed input signals. Finally, the transformation and turnover of SOM offer opportunities when tracing such processes is the purpose of applying a molecular proxy while imposing limitations when they obliterate the molecular proxy signals linked to other phenomena. For n-alkanes several modeling approaches have recently been developed to compensate for (selective) degradation. Still, such techniques are in their infancy and information about their applicability to classes of components other than n-alkanes is lacking. 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 and 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...

Understanding the effects of early degradation on isotopic tracers: implications for sediment source attribution using compound-specific isotope analysis (CSIA)

“…The lipids from fresh leaves as well as O and A horizons showed even-over-odd predominance in long-chain fatty acids (> n-C 22 : 0 ) and odd-over-even predominance in nalkanes (> n-C 23 ), a property usually ascribed to the lipids derived from higher plant biomass (Eglinton et al, 1962;Kolattukudy et al, 1976;Amblès et al, 1994a). To quantify this pattern of the chain length preference, carbon preference index (CPI) values were also calculated (Eglinton et al, 1962).…”

Leaf wax n‐alkane patterns of six tropical montane tree species show species‐specific environmental response

“…Fully understanding the way in which plant leaf waxes reflect their environment could give an insight into plant resilience to future climate change (Guo, Guo, He, & Gao, ) and is instrumental for reconstructing past environmental conditions (Jansen & Wiesenberg, ). The n‐ alkane fraction of the leaf wax, typically in the range of C 20 and C 37 (Eglinton & Hamilton, ), in particular has been suggested as a proxy for past environments due to their resistance to degradation (Jansen & Wiesenberg, ).…”

“…Fully understanding the way in which plant leaf waxes reflect their environment could give an insight into plant resilience to future climate change (Guo, Guo, He, & Gao, ) and is instrumental for reconstructing past environmental conditions (Jansen & Wiesenberg, ). The n‐ alkane fraction of the leaf wax, typically in the range of C 20 and C 37 (Eglinton & Hamilton, ), in particular has been suggested as a proxy for past environments due to their resistance to degradation (Jansen & Wiesenberg, ). Studies measuring n‐ alkane patterns across plant communities, functional groups, genera, and species have shown that the n‐ alkane pattern seems to be influenced by several environmental factors, notably temperature and precipitation (Bush & McInerney, , ; Feakins et al, ; Hoffmann, Kahmen, Cernusak, Arndt, & Sachse, ; Sachse, Radke, & Gleixner, ; Tipple & Pagani, ).…”

From leaf to soil: <i>n</i>-alkane signal preservation, despite degradation along an environmental gradient in the tropical Andes

“…Our results show that only the soil n-alkane signal reflects changes in CPI, which suggests that the processes leading to changes in CPI are related to soil processes. Potentially, such a process could be the de-novo genesis of n-alkanes without an odd-over-even predominance as a result of microbial alteration (Brittingham et al, 2017;Jansen and Wiesenberg, 2017;Rao et al, 2009;Wu et al, 2019), although this should be studied more extensively. Notably, 280 the CPI does not correlate with the ACL and ratio metrics (Fig.…”