Magnetic susceptibility as indicator of anthropogenic disturbances in forest topsoil: A review of magnetic studies carried out in Central European forests

Forest topsoil is subjected to physical and chemical degradation due to the deposition of urban and industrial dust and landfill, as well as physical disturbances including a relic of former cultivation, clearcutting, and afforestation. Such disturbances are observed in all natural and semi-natural forests across Europe, but most intensively in urban forests surrounding cities and industrial areas. Magnetic susceptibility constitutes a convenient physical parameter that is used for both, determination of levels of industrial and urban dust deposition alongside relevant potentially toxic elements (PTEs), and for the precise localization of polluted areas (so-called hot spots). Deposited on the soil surface, technogenic magnetic particles (TMPs) contained in different kinds of anthropogenic dusts increase the magnetic susceptibility of polluted topsoil. This effect can easily be measured in situ by applying a geophysical (geomagnetic) technique - soil magnetometry - for which magnetic susceptibility is the basic parameter. This technique can be performed inexpensively and rapidly with high spatial resolution at local (e.g., for individual trees, and forest stand areas around pollution sources) and regional (for whole regions or countries) scales. The application of combined magnetic-chemical analyses together with geostatistical methods (especially cokriging methods) can deliver more significant results regarding the spatial distribution of pollution than chemical testing alone. A high degree of correlation between magnetic susceptibility and PTEs content (expressed in the form of Pollution Load Index) significantly improves the level of precision in localizing polluted areas. Knowledge of forest topsoil quality and the use of magnetic susceptibility for the precise delineation of areas with considerable anthropogenic physical and chemical disturbances may facilitate the management application of the ecosystem service concept at local and regional scales.