Ash Composition of Litter Fractions as an Indicator of the Stages of Litter Transformation (by an Example of Swampy Birch Forests)

Highly and roughly decomposed litters formed at 0–30 and 100–140 m from the Elovka River channel within a naturally drained swampy birch (Betula pubescens Ehrh.) forest in Western Siberia (56°23.710′ N, 84°34.043′ E) have been studied. The weighted average metal contents (mg/kg) in the strongly decomposed litter were as follows: Ca11515 > Fe8502> Al8472> Mg1468> K1104 > Sr471 > Na200 > Zn65.3 > Cu15.6 > Pb12.1 ≈ Ni12.2 > Co9.5 > Cr6.2 > Cd0.35. The contents of these elements in the roughly decomposed litter were approximately 1.5–2 times lower. The input of mineral substances to the litter was determined by the composition of the ground plant cover residues in the course of their decay, as well as by the types of the swamp mineral and hydrological regimes and silt deposited during floods and aerosol fallout. The ash composition of morphometric litter fractions varied greatly (Cv – 29–47%) in accordance with aluminum, iron and trace elements content: by 63% in strongly decomposed litter and by 84% in roughly decomposed litter. The sequential accumulation of Fe, Al and trace elements as insoluble organomineral complexes occurs during the comminution of plant fragments simultaneously to the humification process. The gradual (mainly at the fermentation stage) removal of bases as bicarbonates was enhanced by the active CO2 formation. Using clustering algorithms, morphometric fractions of each type of litter were combined by ash composition into three clusters that characterize the stages of the mineral component transformation. The formalized associations conformed to the visual division of the litter subhorizons and characterized their size range: L > 5, F 5–1, H 1–<0.25 mm in the strongly decomposed litter of fern–nettle–meadowsweet birch forests and, respectively, >10, 10–5, 1–<0.25 mm in the roughly decomposed litter of sedge–reed grass birch forests. An objective sign of the stages of litter metamorphosis was the amount of Fe, the rankings of which at each stage of transformation was adequate to approximately double increase relative to the initial content.