Discrimination of domestic garden soils using plant wax compounds as markers

Abstract

Much of the forensic comparison of soils has focused on mineralogy at the expense of soil organic components. This chapter discusses how the nature of soil organic matter could be a useful tool in forensic comparison of soil. The chemical diversity and resistant nature of most plant-derived wax markers in soil result in the potential to provide robust profiles that vary with the characteristics of the vegetation that contributed to its formation. The epicuticular wax of most plants contains mixtures of long-chain aliphatic lipids, including hydrocarbons (mainly n-alkanes), long-chain fatty alcohols and fatty acids. The composition of these wax compounds differ between plant species, and these patterns persist in the soil. Plant wax markers may have value in characterising soils for forensic investigation where they could be used to indicate vegetation characteristics of an unknown sample, or for evaluative comparison of a 'questioned' evidence sample to that of a known scene of crime/reference site. We conducted a pilot study in North East Scotland to determine whether individual patches of garden planting bed soil could be discriminated from one another based on their plant wax marker profiles. Results indicated that both n-alkane and long-chain fatty alcohol profiles in garden soils were generally specific to the individual planting bed. The soils from the planting beds could be discriminated from one another, with the exception of two beds from within the same garden which differed little in broad-scale vegetation type. These results demonstrate the potential of plant wax markers in discriminating patches of soil at a scale relevant to forensic evaluative comparison. Further studies are required on soils and vegetation from a wider population of gardens and vegetation types, to extend the application of these plant wax markers to facilitate investigative approaches in potentially indicating plant community characteristics associated with an unknown source.