Properties of soil organic matter in Podzols under mountain dwarf pine (Pinus mugo Turra.) and Norway spruce (Picea abies (L.) Karst.) in various stages of dieback in the East Sudety Mountains, Poland

Dieback of forests because of industrial air pollution was observed in the East Sudety Mountain, Poland. The properties of soil organic matter in Podzols in the subalpine region under mountain dwarf pine and Norway spruce in various stages of dieback (without degradation, with about 50% brown needles and dead trees) were investigated. The content of carbon, nitrogen, humic substances and characteristics of humic acids (HA) from organic and mineral horizons were analyzed. Forest dieback influenced HA properties depending on the species. Under pine, it led to an increase in alkyl and carboxyl C, while an increase in aromatic C and O- alkyl C was found at spruce sites. Humic acids under Picea abies were characterized by more aliphatic structures, whereas HA under Pinus mugo were more aromatic in nature. HA created in soils under pine stands in the investigated area showed lower solubility, higher stability and a lower tendency to translocate to deeper parts of the soil profile, compared to those under Norway spruce. Organic matter under spruce monoculture can be more susceptible to oxidation and may have less impact on the stability of soil aggregates, than those under pine ecosystems. Conversion of spruce monocultures to mixed forest - more resistant to biotic and abiotic disturbances - or introduction of the pioneer tree species in Norway spruce stands may be an effective tool to prevent forest decline and ensuring the stability of forest ecosystems.