We assessed total mercury (THg) concentrations and greenhouse gas (GHG) emissions in pristine and managed hemiboreal peatlands in Latvia, aiming to identify environmental factors that potentially affect their variation. The THg concentrations in soil ranged from <1 µg kg⁻¹ to 194.4 µg kg⁻¹. No significant differences between THg concentrations in disturbed and undisturbed peatlands were found, however, the upper soil layer in the disturbed sites had significantly higher THg concentration. During May–August, the mean CO₂ emissions (autotrophic and heterotrophic respiration) from the soil ranged from 20.1 ± 5.0 to 104.6 ± 22.7 mg CO₂-C m⁻² h⁻¹, N₂O emissions ranged from −0.97 to 13.4 ± 11.6 µg N₂O-N m⁻² h⁻¹, but the highest spatial variation was found for mean CH₄ emissions—ranging from 30.8 ± 0.7 to 3448.9 ± 1087.8 µg CH₄-C m⁻² h⁻¹. No significant differences in CO₂ and N₂O emissions between disturbed and undisturbed peatlands were observed, but CH₄ emissions from undisturbed peatlands were significantly higher. Complex impacts of environmental factors on the variation of THg concentrations and GHG emissions were identified, important for peatland management to minimize the adverse effects of changes in the biogeochemical cycle of the biophilic elements of soil organic matter and contaminants, such as Hg.

Forest management activities in boreal and hemiboreal environments have been found to increase the concentration of carbon, nutrients, and methylmercury (MeHg) in runoff water, thus contributing to environmental quality issues. We evaluated carbon, nutrient, and MeHg concentrations in water at eight small, forested catchments on organic soils in Latvia, subject to ditch cleaning and beaver dam removal. These management-induced disturbances were classified into a major, minor, or no disturbance classes. The concentrations of dissolved organic carbon and total nitrogen were elevated in disturbed catchments (both major and minor) compared to the catchments with no disturbance. The concentrations of MeHg in the water displayed a clear seasonal variation with higher concentrations in spring and summer, but there were no significant differences in MeHg concentrations between catchments with major, minor, and no disturbances. However, the higher concentrations of SO₄²⁻ in the disturbed catchments compared to those undisturbed may promote MeHg formation if the conditions become more reduced further downstream. While most former studies of forest management effects on water quality have focused on forest harvest, our research contributes to the currently rather scarce pool of data on the impact of less-studied management operations, such as ditch cleaning and beaver dam removal, on carbon, nutrient, and MeHg concentrations in runoff water.

Fungal communities can be used as indicators of various environmental processes in forest ecosystems. The diversity of these communities is linked to aboveground plants and soil properties. We assessed fungal diversity at four Norway spruce sampling sites that were growing on fertile mineral soils (Oxalidosa) in northwestern Latvia. Three sites were managed—a three-year-old clear-cut and fifty- and eighty-five-year-old stands; one site was unmanaged—a naturally regenerated site after wind damage in 1969. For metabarcoding, we used a fungal internal transcribed spacer (ITS2) and high throughput sequencing with the Ion Torrent platform. Our results showed high operational taxonomic unit richness in the samples, with notable variation in community composition between individual plots both within and among sites, with the highest being in managed, middle-aged stands and the lowest in unmanaged. Significant differences in the diversity of soil fungal communities were not detected between the sites. Redundancy analysis indicated that pH, soil organic matter, organic carbon, and nitrogen were the most important soil variables that explained the variation in fungal communities. The unmanaged stand differed notably by community composition. This study highlights the importance of monitoring forest soil environmental parameters and fungal communities to gain a more comprehensive assessment of forestry management regimes.

Inorganic mercury (Hg) can be methylated to the highly toxic and bioavailable methylmercury (MeHg) by microorganisms in anaerobic environments. The Hg methylation rate may be affected by forest management activities, which can influence the catchment soils, water, and sediments. Here, we investigate the influence of forest management in the form of ditch cleaning and beaver dam removal, as well as the seasonal variations, on sediment chemistry and microbiota. The relationships between MeHg concentrations in sediment samples and archaeal and bacterial communities assessed by 16S rRNA gene amplicon sequencing were investigated to determine the microbial conditions that facilitated the formation of MeHg. Concentrations of MeHg were highest in undisturbed catchments compared to disturbed or slightly disturbed sites. The undisturbed sites also had the highest microbial diversity, which may have facilitated the formation of MeHg. Low MeHg concentrations and microbial diversity were observed in disturbed sites, which may be due to the removal of organic sediment layers during ditch cleaning and beaver dam removal, resulting in more homogenous, mineral-rich environments with less microbial activity. MeHg concentrations were higher in summer and autumn compared to winter and spring, but the temporal variation in the composition and diversity of the microbial community was less than the spatial variation between sites. Beta diversity was more affected by the environment than alpha diversity. The MeHg concentrations in the sediment were positively correlated to several taxa, including Cyanobacteria, Proteobacteria, Desulfobacterota, Chloroflexi, and Bacteroidota, which could represent either Hg-methylating microbes or the growth substrates of Hg-methylating microbes.

Mycorrhizal fungi contribute to crop growth, yields, and stress tolerance. In forests, common mycorrhizal networks are suggested to function as carbon storage and to transfer substances and signals between trees, thus likely contributing to their resilience. Such properties are crucial under increasing environmental stresses, particularly for clonal forestry. However, mycorrhizal communities in relation to tree field performances have been scarcely studied. In this study, mycorrhizal communities on the roots of clones of silver birch and hybrid aspen growing in distinct trials in deep automorphous mineral soils (podzolic and fluvic) under hemiboreal conditions were assessed using internal transcribed spacer sequencing, bioinformatics, and community analysis. The mycorrhizal communities were moderately rich/diverse and were mostly formed by generalist taxa (prevailingly ectomycorrhizal) common for the region. The differences in communities among the tree clones were estimated for silver birch, while for hybrid aspen, the productivity of clones was inversely related to the richness and diversity of the communities, suggesting a top-down effect of the host. Accordingly, some mycorrhizal taxa (e.g., Hyaloscypha sp.) showed clone-specific abundances indicating a preference for a specific host. These findings prompt further functional studies and highlight the need to consider genetic differences of forest regenerative material for maximizing mycorrhizal diversity, as well as for more effective inoculation.