Study of forest soils on an area of magnetic and geochemical anomaly in north-eastern Poland

The Kolno Plateau located in NE Poland is an area of wide soil geochemical and magnetic anomaly whose origin has not been reliably recognized up to now. Field measurements of surface magnetic susceptibility (κ) as well as 34 vertical topsoil profiles (25 cm) were performed in small forests as they were more or less regularly distributed in the whole area of study, where the profiles were not disturbed by current cultivation. Selected topsoil cores of 30 cm were sampled for grain size and chemical (heavy metals) analysis. Additionally, six deep soil pits were excavated for study of soil morphology and soil type identification. The predominant soil type of the study areas was described as rusty soils according to the “Taxonomy of Polish Soils” (1989) (IUSS, 2006: Brunic Arenosols). Such soils have characteristic Bwo horizons and developed from sandy and/or gravelly material. The topsoil magnetic screening exhibited very low values of κ between 2 and 11 × 10− 5 SI units. In soil profiles the maximum κ value as well as the maximum total Fe content (up to 5250 mg.kg− 1) was observed mostly in A (Ap) or Bwo horizons without magnetic enhancement in the uppermost organic horizon typical for polluted soils. The κ value in the topsoil profile is rather stable at depths between 2 and 20 cm, which is commonly observed in arable soils. It suggests the former agricultural use of studied soils which are now afforested. These values together with the pattern of their vertical distribution in soil profile suggest that the measured κ values are of geogenic origin. These soils did not exhibit any increased concentration of heavy metals (max. Pb = 21, Zn = 19, and Cu = 6 mg.kg− 1). The former arable use of rusty soils was also confirmed by morphological observation of the studied profiles, where in many cases the Ap horizon of relatively large thickness with considerable homogenization and a distinct underneath boundary was distinguished. Geochemical and magnetic topsoil anomalies in the area of Kolno Plateau reported in former publications are probably limited to more fertile Haplic Luvisols used as arable soils. In this case the sources of their geochemical and magnetic enrichments are moraine tills occurring in the geological background, which are usually enriched in heavy metals connected with clay mineral fractions.

Research Highlights
► Application of soil magnetometry for interpretation of geochemical and magnetic anomaly.
► Identification of morphological pattern of arable soil as a result of former cultivation on forested areas.
► Characterisation of rusty soil in the area of Kolno Plateau (NE Poland).