4. Potentially Harmful Elements in Forest Soils

Forest ecosystems differ from agroecosystems, in the first place by the established vegetation cover of deciduous or needle trees, associated with a specific biochemical cycling of forest organic matter. The presence of a forest floor (litter O and humus-rich A horizons) introduces additional pathways of biogeochemical cycling of potential harmful major and trace elements (PHTE), with respect to soils under agricultural land use. Moreover, unlike agroecosystems, forest soils are predominantly affected by only one way of PHTEs inputs, deriving from atmospheric deposition. Aged and established forests are generally in a state of equilibrium with respect to elemental cycling. In such forest soils, especially in their deep horizons which are most often little affected by any contamination, PHTE contents are fairly close to the initial natural pedo-geochemical background concentrations. Consequently, they can be used as a reference for other soils developed in the same parent material, but under agricultural land use and affected by anthropogenic contaminations. Exceptions are forest soils located in the proximity of industrial or mining areas, which are more exposed to short-range industrial atmospheric fallout.

Forest soils often have a lower buffering capacity against acidification than agricultural soils due to the adding of acid-neutralizing amendments (fertilization, liming, and compost) to the latter. In strongly acid forest soils, the risks of mobility of PHTEs are well-established and migration may occur in soluble, pseudo-soluble or particulate forms. Soil acidity may lead to high levels of Al and Mn, representing additional risks of aluminium or manganese toxicity. The forest floor represents a particular metal trapping medium in soils. When a mor-type humus layer is present at the surface of forest soils, exogenous pollutants accumulate, as a first step, in a fully organic surface horizon. But the fate of contaminants in terms of permanent retention, or subsequent partial or even full release, and times of retention are items that are still under debate. In the particular case of podzols with strongly acidified soil conditions, PHTEs are susceptible to migrate to depth and to a part intercepted (long-term, permanently?) in the B horizons whereas another part may leach out of the soils and possibly transferred to the groundwater.

For a good understanding and a relevant interpretation of PTHE concentrations through the soil profile, in terms of accumulation or impoverishment, the limits of the morphological horizons must be respected during sampling. Such horizons may be of small thickness (for instance Bh horizons in podzols), but they can demonstrate substantially contrasted concentrations, for instance in the case of strongly differentiated soils (cf Table 4.3). Taking into consideration the characteristic processes involved in the formation of soil horizons, is essential for a better insight into mechanisms and pathways of cycling of PTHEs. Hence, for a valid assessment of the presence, distribution and fate of PTHEs in forest soils, and in order to allow an appropriate comparison with anthropogenic contaminated agricultural soils, it is crucial to take account of different soil parameters, such as the nature of the parent material, pedological characteristics and specific physico-chemical conditions. Moreover, and surely, it is essential to adopt a soil sampling strategy that is adapted to the aims and/or different parts of multidisciplinary study programs.