Podzolization in a 150-year chronosequence of soils under pine timber forest on inland dunes in the Toruń Basin (Northern Poland)

Studies on mechanisms and rates of podzolization have mostly been realized in natural landscapes and chronosequences. In this work, an artificial chronosequence of soils managed as a habitat for regularly planted and clear-cut, even-aged, monocultural pine stands (Pinus sylvestris) varying in age between 27 and 150 years has been investigated to determine changes in soil properties and development of genetic horizons with time and growth of pine trees. The soils in five profiles under 27-, 52-, 70-, 105- and 150-year-old pine stands are developed from poor aeolian quartz sands and situated in similar habitat conditions to one another on northern slopes of inland dunes in the Toruń Basin (northern Poland).

The studied soils represent consecutive, early stages of podzolization corresponding to the age of the tree stands. In the organic horizon, the whole sequence of three subhorizons typical of the mor type of humus (Oi-Oe-Oa) develops after ∼100 years. Organic carbon, pedogenic Fe and Al content in the eluvial (AE-E) horizon achieve relative equilibrium after 70 years of forest growth. In the illuvial (Bhs-Bs) horizon, these elements continue to accumulate along the whole chronosequence and its development is certainly not finished after 150 years. Only the soils of more than 100-year-old fulfill the criteria of Podzols according to the WRB (2022) soil classification. Younger soils, although undoubtedly podzolized, have to be classified as Arenosols. The youngest profiles (under 27 and 52-year-old pines) have inherited traces of a former cycle of pedogenesis that ran under the previous generation of pine forest. The soil cover of dune areas in the Torun Basin, which have been managed as productive pine timber forests over the last two centuries, is entirely secondary and polycyclic and does not achieve maximal developmental stage and ecological efficiency, including carbon sequestration.