Changes in the abundance of copiotrophic and oligotrophic microorganisms in the Ust-Bakchar raised bog during post-fire restoration

Abstract

The aim of the study was to investigate the population dynamics of copiotrophic and oligotrophic microorganisms in the peat oligotrophic soils of the Ust-Bakchar raised bog during post-fire restoration.

Location and time of the study. The research was conducted in the Ust-Bakchar bog located in the southern taiga subzone of West Siberia in Tomsk region over the period 2022–2024.

Methods. Based on the analysis of archival satellite data, three fire perimeters from 1999, 2003, and 2014 were delineated. Peat samples were collected at two sites: the post-fire UBF1 site, which had experienced fires in both 1999 and 2014, and the conditionally background UB site, affected by the 1999 fire, at depths of 0–15 and 15–30 cm, under microtopographic depressions and elevations. Microbial abundance was determined using dilution of soil suspension and plating it on solid nutrient media with subsequent counting of colony-forming units (CFU). To analyse the functional ecological-trophic structure of the soil microbial community, oligotrophic and mineralisation coefficients were calculated.

Results. The abundance of copiotrophic and oligotrophic microflora in soils of the Ust-Bakchar bog was relatively low, not exceeding 1,7×10⁷ and 6,8×10⁷ CFU g⁻¹ soil, respectively. The study sites, both depressions or elevations, did not exert significant systematic influence on the copiotrophs and oligotrophs abundance, the latter varied primarily due to sampling depth and year of investigation. The presence of fire at the study sites throughout the entire research period predominantly resulted in significant reductions in the abundance of both copiotrophs and oligotrophs (up to 44-fold) compared with the background site, where vegetation cover and upper peat layers were burnt. The abundance of micromycetes was also relatively low (< 2,6×10⁵ CFU g⁻¹ soil). The influence of fires on microbial abundance was ambiguous: in 2022–2023, increases of up to 70 times were observed compared with the background site; however, by 2024, this had declined by 2.6 times at post-fire sites.  Micromycetes in the soils of the Ust-Bakchar raised bog primarily occupied oligotrophic ecological-trophic niches. At the post-fire sites, a decrease in the oligotrophic coefficient (up to 8 times) was typically observed, indicating reduced stability of the bog ecosystem functioning and its increased dependence on fresh organic matter inputs. The mineralization coefficient increased 28 times, indicating intensification of organic matter decomposition processes.

Conclusions. The results demonstrate that fires lead to impoverishment of microbial community composition and reduction in abundance of the main ecological trophic groups. Structural shifts in the soil microflora of the Ust-Bakchar bog at post-fire sites resulted in decreased oligotrophic coefficients and increased mineralization coefficients. Taking into account the relatively low number of microorganisms in the studied area of the Ust-Bacharsky bog in comparison with the number of soil microorganisms in other ecosystems, it is possible to assume that the studied ecosystem is characterized by slow cycling of the main biogenic elements.