CO2 Production Potentials Research Articles

Response of anaerobic carbon cycling to water table manipulation in an Alaskan rich fen

To test the effects of altered hydrology on organic soil decomposition, we investigated CO2 and CH4 production potential of rich-fen peat (mean surface pH = 6.3) collected from a field water table manipulation experiment including control, raised and lowered water table treatments. Mean anaerobic CO2 production potential at 10 cm depth (14.1 ± 0.9 μmol C g−1 d−1) was as high as aerobic CO2 production potential (10.6 ± 1.5 μmol C g−1 d−1), while CH4 production was low (mean of 7.8 ± 1.5 nmol C g−1 d−1). Denitrification enzyme activity indicated a very high denitrification potential (197 ± 23 μg N g−1 d−1), but net NO3− reduction suggested this was a relatively minor pathway for anaerobic CO2 production. Abundances of denitrifier genes (nirK and nosZ) did not change across water table treatments. SO42− reduction also did not appear to be an important pathway for anaerobic CO2 production. The net accumulation of acetate and formate as decomposition end products in the raised water table treatment suggested that fermentation was a significant pathway for carbon mineralization, even in the presence of NO3−. Dissolved organic carbon (DOC) concentrations were the strongest predictors of potential anaerobic and aerobic CO2 production. Across all water table treatments, the CO2:CH4 ratio increased with initial DOC leachate concentrations. While the field water table treatment did not have a significant effect on mean CO2 or CH4 production potential, the CO2:CH4 ratio was highest in shallow peat incubations from the drained treatment. These data suggest that with continued drying or with a more variable water table, anaerobic CO2 production may be favored over CH4 production in this rich fen. Future research examining the potential for dissolved organic substances to facilitate anaerobic respiration, or alternative redox processes that limit the effectiveness of organic acids as substrates in anaerobic metabolism, would help explain additional uncertainty concerning carbon mineralization in this system.

Toward restoring the net carbon sink function of degraded peatlands: Short‐term response in CO2 exchange to ecosystem‐scale restoration

Northern peatlands represent a globally important stock of soil carbon and have acted as a net sink of atmospheric CO2 throughout the Holocene. Disturbance for horticultural peat extraction disrupts ecosystem function and converts these ecosystems to large, persistent sources of carbon dioxide (CO2). This study investigates the effect of ecosystem‐scale restoration on growing season CO2 exchange in a peatland by comparing a restored site to a neighboring nonrestored section for 1 year prerestoration (1999) and 3 years postrestoration (2000–2002). Prior to restoration, less than 23% of the site was vegetated, and it was a source of 245 g C m−2 to the atmosphere during the growing season (May to early October). Following restoration, the water table remained deep, and soil moisture was significantly higher than the nonrestored section. By the third year postrestoration, vegetation covered 50% of the restored peatland. Moss covered 90% of this vegetated area. Vegetation productivity at the restored site was also enhanced with gross ecosystem photosynthesis under full light conditions significantly higher at the restored site at both moss and herbaceous plots by 2002. While this increase in vegetation productivity provided fresh substrate and resulted in higher CO2 production potential for restored site peat, ecosystem respiration was similar to or lower than that at the nonrestored site for both bare peat and vegetated areas because of the generally wetter site conditions resulting from restoration. By upscaling chamber CO2 exchange measurements to the ecosystem level, on the basis of the relative proportion of each surface cover type, we determined the site was a net sink of ∼20 ± 5 g C m−2 during the growing season only 2 years postrestoration. Combining our results with previous work on CH4 emissions and dissolved organic carbon export, we suggest that this degraded peatland ecosystem will likely return to a net carbon sink in 6 to 10 years postrestoration.

Nutrient Input and Carbon and Microbial Dynamics in an Ombrotrophic Bog

Slow rates of plant production and decomposition in ombrotrophic bogs are believed to be partially the result of low nutrient availability. To test the effect of nutrient availability on decomposition, carbon dioxide (CO2) flux dynamics, microbial biomass, and nutrients, we added nitrogen (N) with phosphorus (P) and potassium (K), to prevent limitation of the latter 2 nutrients, over 2 growing seasons to plots at Mer Bleue peatland, Ontario, Canada. After the first growing season, increasing N fertilization (with constant P and K) decreased in vitro CO2 production potential and increased microbial biomass measured with a chloroform fumigation-extraction technique in the upper peat profile, while by the end of the second season, CO2 production potential was increased in response to N plus PK treatment, presumably due to more easily decomposable newly formed plant material. In situ CO2 fluxes measured using chamber-techniques over the second year corroborated this presumption, with greater photosynthetic CO2 uptake and ecosystem respiration (ER) during high N plus PK treatments. The more efficient microbial community, with slower CO2 production potential and larger biomass, after the first year was characterized by larger fungal biomass measured with signature phospholipid fatty acids. The majority of N was likely quickly sequestered by the vegetation and transferred to dissolved organic forms and microbial biomass in the upper parts of the peat profile, while additional P relative to controls was distributed throughout the profile, implying that the vegetation at the site was N limited. However, in situ CO2 flux data suggested the possibility of P or NPK limitation. We hypothesize that nutrient deposition may lead to enhanced C uptake by altering the microbial community and decomposition, however this pattern disappears through subsequent changes in the vegetation and production of more readily decomposable plant tissues.

SEASONAL AND INTER-ANNUAL DECOMPOSITION, MICROBIAL BIOMASS, AND NITROGEN DYNAMICS IN A CANADIAN BOG

To establish the temporal and spatial variability of substrate contribution to ecosystem respiration (ER), we measured the seasonal and inter-annual microbial carbon dioxide (CO2) production potential, microbial biomass, and nitrogen dynamics over a period of 2 years in the upper 30 cm of a peat bog in southern Ontario. Samples collected during a warmer year with lower average summer water table position had larger inorganic and organic nitrogen (N) concentrations and microbial CO2 production potentials. Across all sampling dates, the distance of the water table beneath the surface was significantly and positively correlated with N availability, and in turn N availability was significantly and positively correlated with CO2 production, although direct correlation between water table position and CO2 production was only significant at P = 0.1. Inter-seasonal variability in CO2 production, microbial biomass, or N did not follow consistent patterns between years, and inorganic N species, particularly nitrate, concentrations varied relatively the most between sampling dates, although concentrations were always small relative to microbial biomass N and potassium sulfateextractable organic N. Microbial CO2 production from the surface peat profile was calculated to be between 2.5 and 5.7 g CO2 m j2 day j1 . Data extrapolation showed that microbial production of CO2 can be between 41 and 67% of the CO2 emitted as ER with the larger value falling in a warmer, drier year and that inter-annual changes in production potentials may partially explain increased ER in warmer, drier years. These results suggest that changes in microbial CO2 production and microbial community and nutrient characteristics may play an important role in controlling the emission of CO2 from terrestrial ecosystems such as peatlands. (Soil Science 2005;170:902–912)

Carbon dioxide and methane production potentials of peats from natural, harvested and restored sites, eastern Québec, Canada

Drainage, vegetation removal and harvesting, and vegetation restoration have a profound effect on carbon cycling in peatlands. Through laboratory incubations of 114 peat samples collected from the surface layer and just above and just below the water table at 13 sites, we examined the potential for carbon dioxide (CO2) production under aerobic and anaerobic conditions and methane (CH4) production under anaerobic conditions. CO2 production rates ranged from 0.04 to 1.05 mg g 21 d 21 under aerobic conditions and 0.01 to 0.29 mg g 21 d 21 under anaerobic conditions. Rates of CO2 production were generally smallest in the lower parts of the profiles and at the recently restored sites where deep peat was exposed at the surface; they were largest in the freshly-formed surface peat at the undisturbed bog and older restoration sites where a strong cover of vegetation had developed. The CO2 production potentials were negatively correlated with the Von Post Index of decomposition, and aerobic:anaerobic production ratios averaged 4.3: 1. Largest rates of anaerobic CH4 production occurred in samples close to the soil surface with fresh peat accumulation and a high water table, and smallest rates were in samples from the subsurface of sites with a low water table. Anaerobic CH4 production was significantly positively correlated with aerobic and anaer- obic CO2 production. These production potentials show that drainage, harvesting, and restoration change the ability of the peat profile to produce and emit CO 2 and CH4.

Microbial responses to fluctuation of soil aeration status and redox conditions

The response of the microbial community to changes in aeration status, from oxic to anoxic and from anoxic to oxic, was determined in arable soil incubated in a continuous flow incubation apparatus. Soil incubated in permanently oxic (air) and/or anoxic (O2-free N2) conditions was used as the control. Before experiments soil was preincubated for 6 days, then aeration status was changed and glucose added. Glucose concentration, extractable C, CO2 production, microbial biomass, pH and redox potential were determined 0, 4, 8, 12, 16, 24, 36 and 48 h after change of aeration status. If oxic conditions were changed to anoxic, the amount of glucose consumed was reduced by about 60%, and CO2 production was 10 times lower at the end of incubation compared to the control (permanently oxic conditions). Microbial biomass increased by 114% in glucose-amended soil but did not change in unamended soil. C immobilization prevailed over C mineralization. Redox potential decreased from +627 mV to –306 mV. If anoxic conditions were changed to oxic, consumption of glucose and CO2 evolution significantly increased, compared to permanently anoxic conditions. Microbial biomass did not change in glucose-amended soil, but decreased by 78% in unamended soil. C mineralization was accelerated. Redox potential increased from +238 to +541 mV. The rate of glucose consumption was low in anoxic conditions if soil was incubated in pure N2 but increased significantly when incubation was carried out in a CO2/N2 mixture.

Microbial production of CO2 in unsaturated geologic media in a mesoscale model

The production of microbially derived CO2 was investigated in the 3.6 m thick unsaturated geologic media of a mesoscale model system. The mesoscale model, which was 2.4 m diameter × 4.6 m high, was filled with a medium grained lacustrine sand, capped with an Ap (0.11 m) and B (0.20 m) soil profile, and maintained under steady state moisture conditions for 343 days. A near steady state soil gas CO2 concentration profile (>100 times greater than atmospheric) developed after 130 days implying that geochemical stability was rapidly attained. The presence of metabolically active microorganisms, consumption of O2 and dissolved organic carbon, and carbon 13 data indicated that the CO2 was microbially produced. At steady state, the calculated and modeled CO2 flux to the atmosphere (about 0.9 mol m−2 day−1) were greater than the measured CO2 flux (0.1 mol m−2 day−1) and greater than the calculated CO2 flux to the water table (0.02 mol m−2 day−1). The difference between the calculated and measured fluxes of CO2 to the atmosphere was attributed to spatial variability in the gas diffusion coefficient and reflected the difficulty in estimating CO2 production, even under steady state conditions. Laboratory estimates of microbial CO2 fluxes (1.8 and 2.9×103 mol m−2 day−1 ) were much greater than those determined from measured and calculated fluxes. Measured microbial respiration rates suggested that between 50 and 70% of the respired CO2 was produced in the unsaturated geologic medium beneath the Ap and B, and that there was sufficient CO2 production potential by bacteria to account for the respired CO2 determined from the flux calculations. These observations indicated that long‐term experimentation in mesoscale models can overcome some of the difficulties encountered in field studies and provide insight into biogeochemical processes in natural systems.

The influence of clay minerals, oxides, and humic matter on the methylation and demethylation of mercury by micro‐organisms in freshwater sediments

AbstractLaboratory experiments and analysis of field samples showed that clay minerals, Fe and Mn oxides, and humic matter (colloids) have complex and often dramatic effects on the microbial methylation and demethylation of Hg, and on other microbial activities, in lake sediments. Depending on the nature, abundance, and surface chemistry of the colloids, the source of the sediment, the nature of the microbes, and synergistic/antagonistic effects of environmental variables, the colloids either strongly inhibited or stimulated the Hg transformations, had little or no effect, or alternated in their effects. Most of the results suggest specific effects on particular kinds of microbes, and are not attributable to general inhibition or stimulation of microbial growth or to effects due to the binding of Hg by the colloids. The colloids probably alter the species composition of the microbial community and affect the course of ecological succession, upsetting the dynamic balance between methylation and demethylation and causing alternate increases and decreases in methyl mercury (CH3Hg+) levels along with changes in other indicators of microbial activity [CO2 and CH4 production and oxidationreduction potential (Eh)].The role of clays was critically dependent on surface coatings. Clays often interfered with methylation (while in some cases strongly promoting subsequent demethylation); but iron oxide (FeOOH) often promoted methylation, and FeOOH coatings on clay tended to counterbalance the negative influence of the clay. Removal of oxide coatings depressed both methylation and demethylation. Manganese oxide (MnOOH) coatings sometimes promoted methylation, but larger amounts of MnOOH (unlike FeOOH) strongly suppressed methylation. On addition of organic nutrients, oxide coatings enhanced methylation and impeded demethylation; without nutrient enrichment, the reverse tended to occur. Humic matter in solution tended to stimulate methylation; but humic coatings on clay impeded methylation and fostered demethylation. Thus, the effects of natural colloids on Hg speciation are vitally important but variable, inconsistent, and not altogether predictable.