Search

The fate of carbon stored in permafrost-zone peatlands represents a significant uncertainty in global climate modeling. Given that the breakdown of dissolved organic matter (DOM) is often a major p ...

We have made measurements of the 13 C 12 C fractionation of methane (CH4) during microbial oxidation by an upland temperate soil from College Woods, New Hampshire, using both in situ and laboratory incubation measurements. Uptake rates of 1–4.8 mg CH4/m2/d were measured during the active season in New Hampshire while rates of uptake were 2.6–6.8 mg CH4/m2/d in jars used for incubation studies. The fractionation factor, calculated from field measurements, was α = 0.978 ± .004. This corresponds to a kinetic isotope effect (KIE) of ki 2 k 13 = 1.022 ± .004 . Only a small dependence on temperature was noted for air temperatures between 281 and 296 K. Our results indicate that the KIE of soil CH4 oxidation is controlled by physical parameters based on gaseous diffusion into the soil. The implications of these results are discussed with respect to the global CH4 budget and balancing CH4 sources and sinks through the use of δ13CH4 measurements.

Methane production from C-14 labeled bicarbonate and acetate was measured over the top 28 cm of anoxic Cape Lookout Bight sediments during the summer of 1983. The depth distribution and magnitude of summed radioisotopically determined rates compare well with previous measurements of total methane production and the sediment-water methane flux. Methane production from CO2 reduction and acetate fermentation accounts for greater than 80 percent of the total production rate and sediment-water flux. Methane production from bicarbonate was found to occur in all depth intervals sampled except those in the top 2 cm, whereas significant methane production from acetate only occurred at depths below 10 cm where sulfate was exhausted. Acetate provided 20 to 29 percent of the measured methane production integrated over the top 30 cm of the sediments.

Rates of sulfate reduction were measured over a 3 year period in the anoxic nearshore sediments of Cape Lookout Bight, North Carolina, using both a tube incubation method and a 35S-sulfate direct injection technique. The methods yielded similar depth-integrated rates over the upper 30 cm ranging from less than 10 mol SO=4 · m−2 · y−1 in winter to greater than 50 mol SO=4 · m−2 · y−1 in summer. There were also seasonal changes in the Arrhenius activation energies for the sulfate reduction rates indicating that the assumption that Ea is constant with temperature is not always valid. The time averaged annual turnover rate for all three years was 20.4 (±11.4) mol SO=4 · m−2 · y−1. Surface rates ranged seasonally from less than 0.01 to over 3 mM SO=4 · d−1 between winter and summer, respectively. A subsurface rate maximum was observed to develop during the summer months which accounted for 28 percent of the annual depth integrated sulfate reduction rate. Subsurface rate maxima are the result of changes in the chemistry (substrate type and/or concentration) and the microbiology in the sediments. The possibility of the subsurface maximum being an artifact of the 35S method is also discussed. However, the sulfate reduction rates compare well with previous measurements of the carbon sediment-water plus burial fluxes and with a depth integrated CO2 production rate modelled from a ΣCO2 concentration profile from the same site.