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 °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 g kg⁻¹, while they were not observed for soils with TOC content < 12 g kg⁻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.

Contamination of agroecosystems with PAHs may affect an ability of a soil to sustain a natural biocenosis or agriculture and create a toxicological risk to plants cultivated in the area. The aim of this study was to evaluate the ecotoxicological activity of soils polluted with PAHs with the respect to their effect on plants. Three different soils (chernozem, rendzina and brown soil) artificially contaminated with the mixture of four PAH compounds (fluorene, anthracene, pyrene and chrysene) at the levels of Σ4PAHs of 1, 10, 50 and 100 mg kg⁻¹ were used in the study (laboratory bioassays). Three of the tested plant species (wheat - Triticum vulgare Vill, oat - Avena sativa L and maize - Zea mays L) were from the monocotyledonous category and three (tomato - Lycopersicon esculentum Miller, bean - Phaseolus vulgaris L and sunflower - Helianthus annus L.) from the dicotyledonous category. Contamination of the soil with PAHs at the level below 10 mg kg⁻¹ stimulated rather than inhibited the growth of the plants at the early stages of their development. The lowest observed level of soil contamination with PAH significantly inhibiting (EC₂₀) plant growth (tomato in sandy soil) was about 20 mg of Σ4PAH per kg of soil while EC₂₀ values for most of the other plants tested exceeded 100 mg kg⁻¹. Phytotoxic activity of soils was negatively related to soil organic matter content regulating concentration of PAHs in the soil water phase.

The objective of this paper was to investigate the molecular characterization of soil organic matter fractions (humic substances (HS): fulvic acids-FAs, humic acids-HAs, and humins-HNs), which are the most reactive soil components. A wide spectrum of spectroscopic (UV–VIS and VIS–nearIR), as well as electrochemical (zeta potential, particle size diameter, and polydispersity index), methods were applied to find the relevant differences in the behavior, formation, composition, and sorption properties of HS fractions derived from various soils. Soil material (n = 30) used for the study were sampled from the surface layer (0–30 cm) of agricultural soils. FAs and HAs were isolated by sequential extraction in alkaline and acidic solutions, according to the International Humic Substances Society method, while HNs was determined in the soil residue (after FAs and HAs extraction) by mineral fraction digestion using a 0.1M HCL/0.3M HF mixture and DMSO. Our study showed that significant differences in the molecular structures of FAs, Has, and HNs occurred. Optical analysis confirmed the lower molecular weight of FAs with high amount of lignin-like compounds and the higher weighted aliphatic–aromatic structure of HAs. The HNs were characterized by a very pronounced and strong condensed structure associated with the highest molecular weight. HAs and HNs molecules exhibited an abundance of acidic, phenolic, and amine functional groups at the aromatic ring and aliphatic chains, while FAs mainly showed the presence of methyl, methylene, ethenyl, and carboxyl reactive groups. HS was characterized by high polydispersity related with their structure. FAs were characterized by ellipsoidal shape as being associated to the long aliphatic chains, while HAs and HNs revealed a smaller particle diameter and a more spherical shape caused by the higher intermolecular forcing between the particles. The observed trends directly indicate that individual HS fractions differ in behavior, formation, composition, and sorption properties, which reflects their binding potential to other molecules depending on soil properties resulting from their type. The determined properties of individual HS fractions are presented as averaged characteristics over the examined soils with different physico-chemical properties.