Long-term relationship between soil carbon dynamics, hydrology, and microbiome in peatlands around North America’s largest historical point source pollutant of toxic metals, metalloids, and sulfur (Sudbury, Ontario, Canada)

Abstract

Peatlands are major long-term carbon (C) sinks whose stability depends on tightly coupled soil carbon dynamics, hydrology, and microbial communities, yet these relationships can be profoundly disrupted by industrial pollution. In the 1970s, Sudbury (Ontario) was North America’s largest point source of sulfur dioxide (SO₂) and toxic metal and metalloid (TMM) emissions, generating a deposition gradient that provides a natural experiment for examining the long-term consequences of atmospheric contamination on peatland ecosystems. Using radiocarbon-constrained peat cores from three poor fen sites - two proximal to the smelter centre (Laurentian, Transplant), one distal (Cartier) - this thesis integrates high-resolution geochemical analyses and paleoecological reconstruction to evaluate how industrial disturbance altered carbon accumulation and restructured microbial assemblages. Geochemical profiles (Corg, Cinorg, N, S, Ca, P, and nine key metals/metalloids) reveal distinct smelter-derived signatures, including pronounced enrichments of Cu, Ni, Pb, As, Cd, Zn, and S at or below the Industrial Isochron (1880–1975 CE), accompanied by Ca depletion and coincident increases in N and P. Long-term apparent rates of carbon accumulation (LARCA) show a paradoxical response: heavily polluted fens exhibit industrial-era peaks in apparent C accumulation but lower Holocene-scale mean LARCA relative to a minimally impacted site with intact Sphagnum cover, indicating a cumulative long-term carbon deficit attributable to enhanced decomposition and export from older catotelm strata. Stratigraphic and geochemical evidence suggests vertically divergent effects of pollution, including suppressed microbial decomposition and enhanced apparent C preservation in shallow horizons under extreme metal–acid stress, coupled with enhanced decomposition and C loss in deeper peat driven by acidification, sulfate migration, and destabilization of humic-Fe-S complexes. Stratigraphically constrained cluster analysis (CONISS), canonical correspondence analysis (CCA), variation partitioning, permutational multivariate analysis of variance (PERMANOVA), and indicator species analysis were used to characterise microbial community change relative to geochemical gradients. Pre-industrial testate amoebae (TA) assemblages at all three sites were dominated by sphagnophilous taxa, particularly Hyalosphenia subflava, indicating long-term hydrological stability. Pronounced community restructuring coincident with the Industrial Isochron was evident at the two smelter-proximal sites, where disturbance-tolerant TA taxa (Cyclopyxis arcelloides type, Centropyxis cassis type, Phryganella acropodia type) displaced wet-affinity forms. The distal reference site exhibited comparatively muted change. Geochemical variables explained a significant proportion of total community variance (20%; CCA p < 0.001), with carbon composition emerging as the strongest unique predictor (adj. R² = 0.057), exceeding the independent contribution of toxic trace metals (adj. R² = 0.019). Critically, no discrete post-industrial recovery assemblage was detected at either proximal site: industrial-era taxa persist in surface samples and CONISS does not resolve a recovery zone distinct from the disturbance interval. These findings indicate that passive recovery following emission reductions has not reversed the microbial legacy of industrial contamination and that active restoration intervention may be required to re-establish pre-disturbance ecological conditions. Keywords: Peatland degradation, Carbon dynamics, Industrial pollution, Testate amoebae, Community ecology, Paleoecology, Sudbury, Heavy metals, Ecosystem recovery, Smelter emissions