Natural abundance stable isotope values as a tool to link microbial processes and geochemistry applied to boreal forest soils and serpentinization associated springs

Kohl, Lukas (2016) Natural abundance stable isotope values as a tool to link microbial processes and geochemistry applied to boreal forest soils and serpentinization associated springs. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Naturally occurring variations in stable isotope ratios provide a tool to study biogeochemical cycles in the absence of disturbance, including amended labelled substrates. This thesis followed two such approaches to study microbial processes in two contrasting ecosystems, ultra-basic, reducing springs and boreal forest soils along a climate transect. First, the fact that the carbon stable isotope ratios of biomass represent the stable isotope ratio of the carbon source of an organism ("You are what you eat") was exploited to investigate substrate use pattern of microorganisms in undisturbed forest soils. On a pedon scale, fungal and bacterial specific fatty acid biomarkers were isotopically distinct but exhibited no variation with depth in contrast to the increasing δ¹³C of the bulk soil C. This suggests a common substrate supporting microorganisms across soil horizons. Further, the carbon isotope ratio of the total microbial community, determined using ubiquitous fatty acid biomarkers, appeared to be dictated by the proportion of ¹³C-enriched bacterial relative to ¹³C-depleted fungal biomass in the community. This suggests that the decrease of fungi:bacteria ratio with depth likely contributes to the widely observed increase in δ¹³C of bulk soil organic carbon with depth. Geochemical analyses of soils from across a boreal forest climate transect indicate that climate history and diagenesis can influence distinct chemical properties of soil organic matter. Patterns in these distinct properties across climates indicate that distinct litter inputs rather than soil organic matter diagenesis controls the climate effects on soil chemistry in these forests. First, a warmer climate led to a decrease of moss litter relative to coniferous litter, thus changing the composition of the total litter inputs. Second, a warmer climate led to an increase in nitrogen concentration, which in turn led to a shift in microbial substrate use patterns and to distinct changes of litter chemistry during early litter decomposition. Together, these indirect climate effects led to the establishment of less bioreactive organic matter in warmer climate soils. Second, the investigation of ultra-basic, reducing springs, in this thesis provides the first microcosm experimental evidence of microbial methanogenesis in this environment. Furthermore, this thesis exploited the distinct isotope fractionation factors associated with the two main metabolic pathways of methanogenesis demonstrating that carbonate, rather then acetate, was the main precursor of microbial methane in this environment. This result suggests that CO₂ injections for carbon capture and storage, which have been proposed for similar sites, might stimulate methanogenesis. Overall, this thesis made contributions to the understanding of each of the studied ecosystems, and demonstrates the great potential and flexibility of the application of natural abundance stable isotope analysis for reconstructing microbial processes in undisturbed or hardto- access environments.

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