Soil organic matter composition as a factor affecting the accumulation of polycyclic aromatic hydrocarbons

Purpose The objective of this study was to evaluate the mutual relations between the soil organic matter (SOM) fractions: fulvic acids (FA), humic acids (HA), humins (HN), black carbon (BC) and the polycyclic aromatic hydrocarbons (PAH) accumulation level in agricultural soils influenced by historical and actual anthropopressure. The research allowed to indicate which fractions of SOM are the major sequestration sinks for PAHs after they are naturally introduced into the soil. Materials and methods Soil samples were collected from the upper layer of agricultural soils (n=41), and basic physicochemical properties (pH(KCl), total carbon, total nitrogen and clay content) were determined. The SOM was characterised by the total organic carbon content and humic substances, including HA, FA and HN, determined by an adapted method recommended by the International Humic Substances Society. The extracts of HA and FA were analysed for carbon content using a liquid CN analyser, while HN constituted the soil carbon in the residue after FA and HA extraction. The content of BC was analysed by dry combustion at 375 degrees C for 24 h (CTO375); the remaining carbon was determined via combustion in an elemental analyser. Results and discussion In all soils, significant positive relations between PAH concentrations and TOC content (r=0.73, p<0.001) as well as HN (r=0.71, p<0.001) and BC (r=0.86, p<0.001) were noted. Relations among PAHs, HN and BC were statistically significant only in soils with TOC content >= 12 gkg(-1), while they were not observed for soils with TOC content <12 gkg(-1). The links between HN and PAHs were stronger for low-molecular (two and three rings)-weight PAHs (r=0.77, p<0.05) than for high-molecular (>= four rings) PAHs (r=0.68, p<0.05). Contrary to HN, the BC faction showed higher correlations with high-molecular-weight PAHs (r=0.92, p<0.001). FA and HA showed no significant relationship with hydrocarbons. Conclusions Different SOM fractions might possess divergent binding capabilities and dissimilar binding strengths to PAHs. The different proportions and sorption capacities of BC and HN in relation to PAHs of different molecular weight are the result of varying structure, i.e. polar and non-polar functionalities in sorption domains and pore-size structures of these fractions. The high correlation between BC and higher-molecular PAHs may be a result of their co-emission, while the stronger relationships between HN and lower-molecular PAHs are directly related to their higher mobility and easier diffusion to stable SOM fractions.