Degradation and preservation of vascular plant-derived biomarkers in grassland and forest soils from Western Canada

The total solvent extracts (TSE) of mineral and organic horizons of selected soils and overlying vegetation were analyzed using gas chromatography-mass spectrometry (GC-MS) to determine the composition of solvent-extractable ('free') lipids in soils and to study the degradation and possible preservation of vascular plant-derived molecular markers (biomarkers) in soils. Major compound classes in the TSE of soils and vegetation included a homologous series of aliphatic lipids (alkanoic acids, alkanols, alkanes), steroids, and terpenoids. Characteristic patterns of aliphatic and cyclic biomarkers derived from the overlying, native vegetation were recognized in the associated soil samples indicating the preservation of lipids from the external waxes of vascular plants in the soil organic matter (SOM). The observed biomarker patterns in the grassland soils (Brown Chernozems) were similar to the compounds identified in their major source vegetation, Western Wheatgrass. A similar composition of biomarkers was observed in Aspen leaves and the soil horizons of the forest-grassland transition soil (Dark Gray Chernozem). The Lodgepole Pine needles yielded a characteristic pattern of diterpenoids that was also detected in leaf litter and the O horizon of the associated forest soil (Brunisol). The results demonstrate that solvent extractable biomarkers derived from vascular plants maintain their characteristic pattern of aliphatic and cyclic lipids despite ongoing degradation processes and are thus valuable molecular markers for the determination of the sources of SOM. Furthermore, the abundance of aliphatic wax lipids in plant material and soils decreased at higher rates than the steroids and terpenoids indicating the preferential degradation of aliphatic over cyclic biomarkers. Most of the plant-derived steroids and terpenoids identified in the soils were unaltered, preserved biomolecules as observed in the source vegetation, but minor amounts of their degradation products were also present. Oxidation products of plant sterols are reported here for the first time in soils. The detected alteration products of steroids and diterpenoids are consistent with the oxidative degradation of free cyclic biomarkers in decomposing plant material and soils.