Changes in the nitrogen isotopic composition during the transformation of peat-forming plant litter at dried and post-pyrogenic sites of oligotrophic bogs

Abstract

The aim of the study. Assessment of changes in the nitrogen isotopic composition (δ¹⁵N) during the decomposition of the main peat-forming plants’ litter in peat of oligotrophic bog ecosystems in the southern taiga subzone of West Siberia.

Location and time of the study. The studies were carried out in 2019–2021 in two oligotrophic bogs: “Bakcharskoye” (field station “Vasyuganye” (IMCES SB RAS)) and “Iksinskoye”, which are the north-eastern spurs of the Great Vasyugan mire and located in the Bakcharsky district of the Tomsk region, Russia.

Methods. The decomposition rate of peat-forming plants’ litter was determined by the method of partially isolated samples, which is widely used to study the transformation of plant material and peat. The nitrogen isotopic composition in the original and decomposed samples of the plant material was determined by isotope ratio mass spectrometry using a DELTA V Advantage mass spectrometer combined with a Flash 2000 elemental analyzer (EA-IRMS).

Results. A three-year study of the decomposition of peat-forming plants (Sphagnum fuscum, Chamaedaphne calyculata, Eriophorum vaginatum and a Mixed sample) in four ecosystems (natural - VASnat, drained - VASdry, post-pyrogenic - Iksa1 and Iksa2) showed that melioration and fire influenced the litter decomposition rate, increasing the latter  in the drained bog and slowing down Sph.  fuscum and Mixed sample decomposition by 1.5–9.0% at the site with pyrogenic effects. During the plant litter transformation carbon and nitrogen were released as 24–62% and 21–46%, respectively. In all studied samples, except for Sph. fuscum, a relationship was revealed between the plant litter mass loss and changes in the nitrogen isotopic composition.

Conclusions. The effect of peat fires is manifested by a decrease in the plant residue decomposition rate. The dynamics of changes in the isotopic composition during the transformation of the studied plant residues depended on the plant species, but does not depend on decomposition duration (1, 2, 3 years) and sample location.

Key words: transformation of plant residues; nitrogen isotopes; biogeochemical cycle; pyrogenic peatlands; drained peatlands; West Siberia.