Carbon isotopes of plant waxes as indicators of soil organic matter dynamics

Investigation of soil organic matter (SOM) constitutes an important and pressing challenge due to its value as a resource, its role in the carbon cycle and increasing pressures that soils are experiencing. Particular concerns exist regarding how the large pool of OM that is currently stabilized in soils will respond to the influence of climate change and increasing anthropogenic pressures.

In this project, we seek to utilize bulk SOM, organic matter of density fractions, and plant wax carbon isotope signatures as tools for assessing the dynamics of the bulk and mineral-associated SOM pools, respectively.  We are examining two different soil types (podzol and cambisol) at study sites that form part of the Long-Term Forest Ecosystem Monitoring program (LWF) of the Swiss Federal Institute for Forest, Snow and Landscape research (WSL). To overcome issues of small-scale spatial variability, composite samples derived from 16 subsamples were investigated. Bulk carbon, density fractions and plant wax lipid analyses were carried out for several depth intervals in both soil types.

In both soils fatty acid and n-alkane concentrations decrease with soil depth and the largest fraction is associated with the mineral fraction. Assessment of radiocarbon variations in bulk soils; density fractions and plant waxes shows a decrease of Δ14C values with depth in both soils. Differences between Δ14C values of the density fractions and compounds give indications about the source, transport and degradation of the plant waxes, and allow us to develop an improved understanding of the composition and dynamics of specific SOM components.