The subhorizon-Ofh as a prospective geoindicator of temperate soil quality in relation to selected trace elements and PAHs: A review

Soils, next to vegetation, are one of the most popular indicators in environmental quality studies. The response of soils to air pollution is less complex than the response of vegetation. In soils, mainly in their upper layers, the deposition of compounds may occur almost undisturbed, therefore results of chemical analyses are readily used in the assessment of changes caused by natural processes and anthropogenic activities. In a temperate climate zone, the outermost layer of forest soils is formed by an organic horizon (-O), which comprises two basic subhorizons, of which the organic fermentative-humic subhorizon (-Ofh) is recognized as the most noteworthy. The subhorizon-Ofh encompasses partially and fully decomposed organic matter (humic and fulvic acids, humins), as well as Fe, Mn and Al oxides and hydroxides, forming a characteristic tangled mat. The structure pattern of this subhorizon is responsible for the higher concentrations of elements and organic compounds, which are immobilized through sorption, complexation or precipitation. Some previous studies have shown that the subhorizon-Ofh has enhanced accumulative properties in comparison to the remaining subjacent soil horizons. Results derived from soil studies in the Holy Cross Mountains/HCMs (south-central Poland) have also revealed that this subhorizon is mainly enriched in polycyclic aromatic compounds (PAHs) and some trace elements (Hg, Cd, Cu, Pb, Zn). This could make the subhorizon-Ofh a potential geoindicator of soil quality in relation to these compounds over the temperate climate zone. However, further study on this issue is needed to confirm its application in environmental monitoring.