Conditions favoring local podzolization in soils developed from flysch regolith – A case study from the Bieszczady Mountains in southeastern Poland

Abstract

Podzolization occurs mainly in soils developed from highly leached and coarse-textured parent materials in areas that remain under the influence of humid climate conditions and where forest or heath vegetation supply enough organic matter to form organic horizons on the surface of mineral soils. However, in certain environmental conditions Podzol development was found also in extremely unfavorable types of parent material, e.g. ultramafic rocks or even regoliths containing carbonates. Flysch rock regolith found in many geographic regions is an example of soil parent material considered to be unfavorable for podzolization in its major part due to its high clay, Ca, and Mg contents. Areas formed of materials considered to be rather unfavorable to Podzol development but characterized by very local occurrence of podzolized soils may be helpful in assessing the factors which favor the occurrence of this process and its thresholds. The main objective of the present study was to determine an optimal combination of conditions that trigger podzolization in soils in the uppermost part of mid-mountain slopes formed of flysch rocks in the Bieszczady Mountains in southeastern Poland. At the same time, an attempt was made to determine pedogenic thresholds of local podzolization in soils in the studied area. The studied soils showed large variability in soil processes and were classified according to the WRB as four different Reference Soil Groups: Podzols, Cambisols, Umbrisols and Leptosols. Two out of the eight soil profiles exhibited well-developed spodic horizons, while the properties of the two other profiles may be explained by either cryptopodzolization and/or formation of andic properties in non-volcanic soil material. Evidence of podzolization was observable in changes in soil morphology (i. e. color and consistence of soil material), vertical distribution of labile C and Fe and Al pedogenic forms, changes in mineral composition (mainly dissolution of chlorite, formation of mixed layered clay minerals and the redistribution of various amorphous phases), and also micromorphological features (presence of monomorphic fine coatings consisting of Fe- and Al-organic compounds). However, the occurrence of Podzols was strongly limited across the study area and determined by a combination of environmental conditions such as the co-occurrence of four key factors: (1) humid and cool climate, (2) sandy texture and mineral composition of parent material dominated by quartz, (3) location on the uppermost part of consequent slopes (ridge crests) and (4) predominance of berry (Vaccinium sp.) thickets in the vegetation cover in tandem with the occurrence of mosses (Polytrichum strictum and Sphagnum sp.). Moss cover facilitated a pedogenic feedback setup by increased water storage which promoted more advanced weathering and increase in dissolved organic carbon content in the soil. In the consequence the pedogenic threshold of podzolization was crossed.