Natural abundance stable isotope values as a tool to link
\nmicrobial processes and geochemistry applied to boreal
\nforest soils and serpentinization associated springs

Naturally occurring variations in stable isotope ratios provide a tool to study biogeochemical \ncycles in the absence of disturbance, including amended labelled substrates. This thesis \nfollowed two such approaches to study microbial processes in two contrasting ecosystems, \nultra-basic, reducing springs and boreal forest soils along a climate transect. \nFirst, the fact that the carbon stable isotope ratios of biomass represent the stable isotope \nratio of the carbon source of an organism ("You are what you eat") was exploited to \ninvestigate substrate use pattern of microorganisms in undisturbed forest soils. On a pedon \nscale, fungal and bacterial specific fatty acid biomarkers were isotopically distinct but exhibited \nno variation with depth in contrast to the increasing δ¹³C of the bulk soil C. This \nsuggests a common substrate supporting microorganisms across soil horizons. Further, the \ncarbon isotope ratio of the total microbial community, determined using ubiquitous fatty \nacid biomarkers, appeared to be dictated by the proportion of ¹³C-enriched bacterial relative \nto ¹³C-depleted fungal biomass in the community. This suggests that the decrease of \nfungi:bacteria ratio with depth likely contributes to the widely observed increase in δ¹³C of \nbulk soil organic carbon with depth. \nGeochemical analyses of soils from across a boreal forest climate transect indicate that \nclimate history and diagenesis can influence distinct chemical properties of soil organic matter. \nPatterns in these distinct properties across climates indicate that distinct litter inputs \nrather than soil organic matter diagenesis controls the climate effects on soil chemistry in \nthese forests. First, a warmer climate led to a decrease of moss litter relative to coniferous \nlitter, thus changing the composition of the total litter inputs. Second, a warmer climate led \nto an increase in nitrogen concentration, which in turn led to a shift in microbial substrate \nuse patterns and to distinct changes of litter chemistry during early litter decomposition. \nTogether, these indirect climate effects led to the establishment of less bioreactive organic \nmatter in warmer climate soils. \nSecond, the investigation of ultra-basic, reducing springs, in this thesis provides the \nfirst microcosm experimental evidence of microbial methanogenesis in this environment. \nFurthermore, this thesis exploited the distinct isotope fractionation factors associated with \nthe two main metabolic pathways of methanogenesis demonstrating that carbonate, rather \nthen acetate, was the main precursor of microbial methane in this environment. This result \nsuggests that CO₂ injections for carbon capture and storage, which have been proposed for \nsimilar sites, might stimulate methanogenesis. \nOverall, this thesis made contributions to the understanding of each of the studied ecosystems, \nand demonstrates the great potential and flexibility of the application of natural abundance \nstable isotope analysis for reconstructing microbial processes in undisturbed or hardto- \naccess environments.

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