Metabolic Pathways in Natural Systems

The δ13C value of foliage respiration has been considered a constant in the past and modeling efforts have assumed that the δ13C value of foliage respiration is constant and is directly related to substrate without any fractionation. Consecutive δ13C measurements of foliage dark-respired CO2 (δ13Cr) for slash pine trees (Pinus elliottii) over several diel cycles were used to test the hypothesis that significant variation in δ13Cr would be observed. δ13Cr 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 δ13C values by dawn. Assimilation model results suggest that respiration during daylight has the potential to significantly affect ∆13C by as much as 1.6‰, but night dark respiration has little impact on 24-hour integrated ∆13C (0.1‰). We also 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 andxi 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. We also demonstrate that changes in stable-isotope fractionation caused by shifts from acetate fermentation and CO2 reduction occur over long time scales (> annual) and are dependent on changes in wetland morphology and surface vegetation cover. We also used radiocarbon as a tracer to test the hypothesis that melting permafrost surrounding the collapse scar bog provides nutrients to the bog-moat location, stimulating the production of radiocarbon-depleted methane. Our results show that the radiocarbon content of methane and DIC at these sites is highly variable and may depend on groundwater input, surface vegetation, and morphological factors associated with the melting permafrost plateau. We conclude that the younger, more labile, carbon stimulating acetate fermentation at one of the sites is supplied by the fen-like surface vegetation, while the older, more recalcitrant, carbon stimulating CO2 reduction at the other site may be supplied by melting permafrost plateau..