Doc Reactivity in a Northern Minnesota Peatland

• Other Authors: Corbett, J. Elizabeth (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Earth sciences; Oceanography; Atmospheric sciences; Geophysics
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-6899

The DOC reactivity of a northern peatland was investigated by measuring and modeling concentrations, stable isotopes, and natural abundance radiocarbon of solid phase peat, DOC, DIC, and CH4 in field and lab studies. We tested the hypotheses that 1) fen DOC is more labile than bog DOC, 2) modern DOC is advected downward from surface peat and respiration in deep peat is controlled by organic matter production at the surface, 3) the presence or absence of respiration pathways can be distinguished with stable isotopes, 4) isotope-mass balance calculations can be used to partition fractionating and non-fractionating CO2 production pathways, 5) and a DIR (depth integrated rate) model can be used to estimate production rates of CO2 and CH4 and to better constrain advection rates within the peat column. Fieldwork was done in Glacial Lake Agassiz Peatlands (GLAP) in Northern Minnesota. The GLAP terrain is flat which inhibits water drainage and results in carbon accumulation. Raised bogs and fens comprise the major ecosystems in this area. Bogs have lower pH values, lower water tables, and lower nutrient content than fens. Bogs vegetation is characterized by Sphagnum moss while fen vegetation is characterized by Carex plants. Because of the physical and biological differences in these two main environments, the DOC characteristics, pathways of CO2 and CH4 production, and rates of CO2 and CH4 production vary as well. DOC concentrations and DOC:DON values were lower in fens than bogs which suggests that fen DOC was more labile and subjected to more degradation than bog DOC. Fens contained twice as much LMW (low molecular weight) DOC (kDa) as bogs. The LMW DOC was determined to be older and more recalcitrant (because of higher DOC:DON values) than the HMW (high molecular weight) size fraction in both bogs and fens. This suggests that the presence of LMW DOC was a result of degradation processes which were more rapid in fens because of the greater lability of the bulk DOC. Bog DOC was shown to have higher fluorescence and aromaticity than fen DOC. Since fen DOC has been found to be more labile, higher fluorescence in a sample may be a sign of recalcitrance. Radiocarbon analysis was consistent with the hypothesis that fen DOC was more labile than bog DOC since the radiocarbon values of respiration products in the fen were comparable to values of fen DOC. In bogs, the radiocarbon values of respiration products were in between the radiocarbon values of the solid phase peat and the DOC which suggests that both the peat and DOC were substrates for bacteria in the bog. Both bogs and fens contained radiocarbon enriched DOC at depth while deep peat was radiocarbon depleted. Incubations of rinsed peat yielded DOC radiocarbon values that were similar to the peat. These results indicate that the modern DOC found in the peat column is most likely advected downward from more surficial layers and must come from another source other than the peat from the same layer. Stable isotope analysis of δ13C-DIC, δ13C-CH4, and δD-CH4 showed that bogs utilized CO2 reduction at all depths while fens utilized acetate fermentation at surface depths and CO2 reduction at subsequent depths. Based on the stable isotope analysis, methane oxidation was not found to be a pathway that was utilized in the peat column. The evidence of methanogenic pathways and the lack of methane oxidation in the peat column in both bogs and fens imply that a significant amount of methane is emitted from these peatlands. CO2 concentrations were found to be ten times higher than CH4 concentrations in both bogs and fens because of the low solubility of CH4, ebullition and plant transport, and additional CO2 production from higher fermentation and respiration of electron acceptors such as oxygen, humics, and sulfate. Using an isotope-mass balance model, bogs were found to have higher amounts of CO2 from methanogenesis than fens, but methanogenesis was the predominate pathway at depth in both bogs and fens. Fens had a greater amount of methane loss than bogs, but both systems showed a methane loss of 80-90%. An inverse pore water model was developed to compare production rates calculated from pore water concentrations with those obtained from incubation and chamber measurements. Production rates from the model were calculated with advection rates measured in the GLAP. Measured advection rates are variable in the GLAP, so a variety of advection rates, within the range of those measured, were used in the model. The calculated production rates from the model were found to be similar to those measured in field and incubation studies but were sensitive to the advection rates used so that if advection rates doubled so did production rates. This model can help to constrain overall advection rates in a system where advection rates can be highly variable by modeling pore water production and comparing calculated production rates to those measured in the field. Fen advection rates measured in the field of (126 cm y-1) produced CO2 production rates (20-25 mmol m-2 d-1) that matched those from other studies which used alternate methods (chamber measurements and land deformation). Measured bog advection rates (2.2 cm y-1) were much lower than fen rates and needed to be increased at least 10 times in order for the model to produce similar production rates (2.6 mmol m-2 d-1) as those observed from alternate methods ( 1.7-31 mmol m-2 d-1). However, it is not unreasonable that bog advection rates may actually be higher than the ones measured since the measured advection rates had been reported as minimal values. The results from this dissertation show that fens may be higher risk environments for global climate change than bogs due to the greater lability of the DOC. Much of the respiration within the peat column was dominated by surficial DOC production and brought to depth by downward advection. Most of the CO2 produced in the peat column was from methanogenic pathways as opposed to HMW fermentation which shows that methane is produced throughout the peat column. However, low concentrations of CH4 in pore water samples and the lack of isotopic evidence for methane oxidation suggest that the produced CH4 is rapidly lost to the atmosphere. The DIR model can help to constrain physical processes, such as advection, in these systems when calculated production rates are compared to those determined from alternate methods. These findings further emphasize that peatlands are significant contributors to atmospheric greenhouse gases and may become more so in the future. This work has increased the understanding of DOC characteristics, respiration, and water movement in northern wetlands and the methods used have applications for wetlands worldwide. === A Dissertation submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester, 2012. === October 11, 2012. === Climate Change, Dissolved Organic Carbon, Methane === Includes bibliographical references. === Jeffrey P. Chanton, Professor Directing Dissertation; Yang Wang, University Representative; William T. Cooper, Committee Member; Markus Huettel, Committee Member; Bill Burnett, Committee Member.