Metabolic pathways in natural systems: A tracer study of carbon isotopes

Anthropogenic contributions of greenhouse gasses to the atmosphere have initiated changes in environmental conditions around the world. The response of climate to these anthropogenic emissions is complex and global climate models are often used to predict future environmental conditions. Response of ecosystem-level net atmospheric flux to changing environmental conditions can be complex and is often the net result of two large opposing fluxes such as ecosystem assimilation and respiration. Bulk-flux measurements can be insufficient to resolve the two flux endmembers. The use of stable isotope tracers coupled to mass-balance equations can provide the higher resolution necessary to determine greenhouse gas flux rates and stable isotopic concentrations.;The delta13C value of foliage respiration has been considered a constant in the past and modeling efforts have assumed that the delta 13C value of foliage respiration is constant and is directly related to substrate without any fractionation. Consecutive delta13C measurements of foliage dark-respired CO2 (delta13C r) for slash pine trees (Pinus elliottii) over several diel cycles were used to test the hypothesis that significant variation in delta 13Cr would be observed. delta13Cr values collected in daylight from all time series showed mid-day 13C enrichment (5--10‰) relative to bulk biomass, but values become more 13C depleted following shading and at night and approach bulk-biomass delta 13C values by dawn. Assimilation model results suggest that respiration during daylight has the potential to significantly affect Delta13 C by as much as 1.6‰, but night dark respiration has little impact on 24-hour integrated Delta13C (0.1‰).;Net greenhouse gas flux from permafrost peatlands in the Discontinuous Permafrost Zone of northern Canada is small relative to the two larger component fluxes: carbon assimilation and methane production. These permafrost peatlands are sensitive to changes in global temperature because of the direct link between increased temperatures and permafrost thaw. Permafrost thaw will undoubtedly change the ecological conditions and alter the CO2/CH4 flux balance. We sampled methane and CO2 from collapse scar bogs (transient permafrost degradation features in permafrost peatlands) to test the hypotheses that microbial respiration and methane production are stimulated by permafrost degradation and collapse and that the fen-like vegetation (i.e. Carex and Eriophorum) found in collapse scar bogs near the collapsing edge stimulates acetate fermentation. Our results show that collapse scar bogs have an evolution of spatial variation in methanogenic pathways that is related to surface vegetation cover type. (Abstract shortened by UMI.)...