Plant Residues as Direct and Indirect Sources of Hydrocarbons in Soils: Current Issues and Legal Implications

Plant residues are the main source of organic matter in soil; this process takes place naturally in forests and with organic amendments in farmlands. Terrestrial plants also synthesize hydrocarbons. Typically, angiosperms contain hundreds to thousands of milligrams of long chain n-alkanes per kilogram in leaf waxes. However, petroleum pollution is a worldwide issue, and different national regulations set the guideline limit for petroleum hydrocarbons in green areas at 50 mg kg(-1). Focusing on the Italian legislation as a case study, we hypothesized that direct or indirect high inputs of plant residues could lead the level in the soil to exceed this limit, resulting in a false positive petroleum contamination. Therefore, we investigated the occurrence of hydrocarbons in soils with different inputs of natural or farming biomasses. The highest total petroleum hydrocarbon (TPH) concentrations were found in background soils from protected woodlands, remarkably with most samples resulting in levels above the guideline limit. Similarly, the TPH concentrations in agricultural soils amended with compost and digestate were higher than those in samples of soil in which only chemical fertilizers were used. n-Alkane carbon preference indices underlined the role of plant residues as a source of hydrocarbons in these samples, clearly distinguishing spiked petrogenic contamination. Possible revisions of the regulatory and analytical methods are then discussed.