Stephen C. Whalen

Interannual variations in tundra methane emission: A 4‐year time series at fixed sites

Abstract. This paper summarizes 4 years (1987-1990) of weekly net CH4 flux measurements at permanent sites representing important plant components of Arctic tundra. The data coincide with variations in precipitation and temperature of interest in regional and global modeling efforts and are useful in placing bounds on the role of tundra in the global CH4 budget. Precipitation in the study area during the summer emission period ranged from twice to half the long-term mean, and air temperature anomalies were about +2 °C. This data set also permits consideration of temporal (seasonal to interannual) and spatial variability in CH4 flux. We studied the relationship between the net CH4 flux and subsurface properties (water table depth, thaw depth, soil temperature, /pCH4 distributions) at these permanent sites during the 1988 and 1989 emission periods. Net CH4 emission and subsurface properties are largely unrelated. Relationships between soil temperature (or any single variable) and emission are site specific and are of little value as flux predictors. Parameters that integrate conditions influencing flux appear to be the best flux predictors over the emission period. We estimate that Arctic wet meadow and tussock:shrub tundra presently emit about 42 ± 26 Tg CH4 yr−1 to the atmosphere. This estimate has a North American bias, but it is supported by measurements in a range of locations, transect studies, and model calculations.

Black Sea methane geochemistry

Methane concentrations and oxidation rates were measured in the water column and sediments of the Black Sea at a central station during leg 5 of the 1988 U.S.—Turkey Black Sea Expedition. Methane concentrations were 10 nM in the upper 100 m, increased to 11 μM at 550 m, and were uniform to the bottom. Water column methane oxidation rates were measured using two independent radiotracer techniques: tracer level additions of 3H−CH4, and non-tracer level additions of 14C−CH4. The methods agree within a factor of two. Methane oxidation rates were low in the surface 100 m and increased to relatively uniform values of 0.6μM y−1 below 500 m. Sediment methane concentration and oxidation rate distributions showed that shelf and slope sediments were methane sources, while deep basin sediments were methane sinks. These measurements were used to construct a methane budget for Black Sea waters. Microbially mediated anaerobic methane oxidation is the dominant water column methane sink, followed by evasion to the atmosphere, abyssal plain sediment consumption and outflow at the Bosporus. The source of methane appears to be anoxic, high deposition rate shelf and slope sediments. The water column oxidation rate measurements suggest a short (5–20 year) residence time for methane in the Black Sea, indicating a higher geochemical activity than previously believed. The quantity of carbon participating in the Black Sea methane cycle is equivalent to about 0.5% of the primary production.

Moisture and temperature sensitivity of CH4 oxidation in boreal soils

Abstract We used laboratory experiments to evaluate CH4 uptake kinetics and the influence of soil moisture and temperature on rates of CH4-oxidation by boreal soils at in situ CH4 concentrations. Two upland forest sites (AS2 and BS2) were atmospheric CH4 sinks; a bog site (LB) was an atmospheric CH4 source characterized by distinct depth zonation of CH4 production and consumption. Apparent half-saturation constants (Ks) for CH4-oxidation showed relatively well-adapted communities. The Ks for the high CH4-source soil (LB) was 1.1 μ m , about 10-fold higher than values for CH4-sink soils (AS2 and BS2), 37 and 124 n m . Experiments assessing the individual effects of moisture and temperature on CH4-oxidation indicated that moisture was the primary control in CH4-sink soils at AS2 and BS2, while temperature was more important in CH4-source soil at LB. A combination of the highest moisture content and lowest temperature for each soil gave the lowest CH4-oxidation rates in experiments evaluating the interactive effects of these two variables. Conversely, a soil moisture content close to the optimum identified in moisture dependence experiments combined with the highest soil temperature consistently gave the highest CH4-oxidation rate.

Oxidation of methane in boreal forest soils: a comparison of seven measures

Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH4 oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH4) headspace, (b) estimating CH4 oxidation rates from soil CH4 distributions and (c)222Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH4) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH4) headspaces, and (g) jar experiments on core sections involving tracer additions of14CH4. Short-term unamended chamber measurements,222Rn-calibrated flux measurements, and soil CH4 distributions show independently that the soils are capable of oxidizing atmospheric CH4 at rates ranging to < 2 mg m−2 d−1. Jar experiments with free-air headspaces and soil CH4 profiles show that CH4 oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH4 oxidation occurs at low (< 0.9 ppm) thresholds. The14CH4-amended jar experiments show the distribution of end products of CH4 oxidation; 60% is transformed to CO2 and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH4 oxidation and indicate potential CH4 oxidation rates as high as 867 mg m−2 d−1. Methane oxidation in moist soils modulates CH4 emission and can serve as a negative feedback on atmospheric CH4 increases.

Methane consumption and emission by Taiga

Taiga or boreal forest environments are a poorly understood component of the global CH4 budget. Results from a 1-year study of CH4 fluxes at a range of representative floodplain and upland taiga sites in the Bonanza Creek long term ecological research area show that soil consumption of atmospheric CH4 was the dominant process. Methane emission occurred only sporadically in the earliest successional stages in the floodplain system; all other floodplain and upland sites were net CH4 consumers. Our results suggest that upland and floodplain taiga soils are an atmospheric CH4 sink of up to 0.8 Tg yr-1. Point-source bogs and fens are the only important CH4-emitting sites in taiga. -Authors

Nutrient limitation of phytoplankton production in Alaskan Arctic foothill lakes

We used 54 enrichment bioassays to assess nutrient limitation (N, P) of 14C uptake by natural phytoplankton assemblages in 39 lakes and ponds in the Arctic Foothills region of Alaska. Our purpose was to categorize phytoplankton nutrient status in this under-represented region of North America and to improve our ability to predict the response of primary production to anticipated anthropogenically mediated increases in nutrient loading. Experiments were performed across several watersheds and included assays on terminal lakes and lakes occupying various positions in chains (lakes in series within a watershed and connected by streams). In total, 89% (48 of 54) of the bioassays showed significant stimulation of 14C primary production by some form of nutrient addition relative to unamended controls. A significant response was observed following enrichment with N and P, N alone and P alone in 83, 35 and 22% of the bioassays, respectively. In experiments where N and P proved stimulatory, the influence of N alone was significantly greater than the influence of P alone. Overall, the data point to a greater importance for N than P in regulating phytoplankton production in this region. The degree of response to N and P enrichment declined as the summer progressed and showed no relationship to irradiance or water temperature, suggesting secondary limitation by some micronutrient such as iron as the summer advanced. Phytoplankton nutrient status was often consistent across lakes within a watershed, suggesting that watershed characteristics influence nutrient availability. Lakes in this region will clearly show increased phytoplankton production in response to anthropogenic activities and anticipated changes in climate that will increase nutrient loading.

Methane Oxidation, Production, and Emission at Contrasting Sites in a Boreal Bog

Boreal peatlands, a major source of atmospheric CH4, are characterized by a rapidly fluctuating water table position and meter-scale variations in relief. Regional and ecosystem-based studies show that water table position generally controls CH4 emission from boreal peatlands by influencing the relative extent of the zones of CH4 oxidation and production within the peat profile. We used a combined field and laboratory study to assess the influence of local hydrology on the short-term dynamics of CH4 production, oxidation, and emission from sites in an Alaskan boreal peatland that were characterized by temporarily (site LB1A) and permanently (LB2) water-saturated subsurface peat during the thaw season. The two sites contrasted sharply with respect to the dynamics of CH4 cycling. Site LB1A, which showed low CH4Boreal peatlands, a major source of atmospheric CH4, are characterized by a rapidly fluctuating water table position and meter-scale variations in relief. Regional and ecosystem-based studies show that water table position generally controls CH4 emission from boreal peatlands by influencing the relative extent of the zones of CH4 oxidation and production within the peat profile. We used a combined field and laboratory study to assess the influence of local hydrology on the short-term dynamics of CH4 production, oxidation, and emission from sites in an Alaskan boreal peatland that were characterized by temporarily (site LB1A) and permanently (LB2) water-saturated subsurface peat during the thaw season. The two sites contrasted sharply with respect to the dynamics of CH4 cycling. Site LB1A, which showed low CH4

Influence of N and non-N salts on atmospheric methane oxidation by upland boreal forest and tundra soils.

Abstract The short-term (24 h) and medium-term (30 day) influence of N salts (NH4Cl, NaNO3 and NaNO2) and a non-N salt (NaCl) on first-order rate constants, k (h–1) and thresholds (CTh) for atmospheric CH4 oxidation by homogenized composites of upland boreal forest and tundra soils was assessed at salt additions ranging to 20 μmol g–1 dry weight (dw) soil. Additions of NH4Cl, NaNO3 and NaCl to 0.5 μmol g–1 dw soil did not significantly decrease k relative to watered controls in the short term. Higher concentrations significantly reduced k, with the degree of inhibition increasing with increasing dose. Similar doses of NH4Cl and NaCl gave comparable decreases in k relative to controls and both soils showed low native concentrations of NH4+-N (≤1 μmol g–1dw soil), suggesting that the reduction in k was due primarily to a salt influence rather than competitive inhibition of CH4 oxidation by exogenous NH4+-N or NH4+-N released through cation exchange. The decrease in k was consistently less for NaNO3 than for NH4Cl and NaCl at similar doses, pointing to a strong inhibitory effect of the Cl– counter-anion. Thresholds for CH4 oxidation were less sensitive to salt addition than k for these three salts, as significant increases in CTh relative to controls were only observed at concentrations ≥1.0 μmol g–1 dw soil. Both soils were more sensitive to NaNO2 than to other salts in the short term, showing a significant decrease in k at an addition of 0.25 μmol NaNO2 g–1 dw soil that was clearly attributable to NO2–. Soils showed no recovery from NaCl, NH4+-N or NaNO3 addition with respect to atmospheric CH4 oxidation after 30 days. However, soils amended with NaNO2 to 1.0 μmol NaNO2 g–1 dw showed values of k that were not significantly different from controls. Recovery of CH4-oxidizing ability was due to complete oxidation of NO2–-N to NO3–-N. Analysis of soil concentrations of N salts necessary to inhibit atmospheric CH4 oxidation and regional rates of N deposition suggest that N deposition will not decrease the future sink strength of upland high-latitude soils in the atmospheric CH4 budget.

Effect of temperature on methane dynamics and evaluation of methane oxidation kinetics in shallow Arctic Alaskan lakes

Large uncertainties exist regarding the influence of ongoing climate change to microbially mediated methane cycling in arctic lakes. Specifically, the coupled response of methanogenesis (MG) and methane oxidation (Mox) to increased temperature is poorly understood. Therefore, the effect of temperature on rates of sediment MG and water column Mox in two shallow Arctic Alaskan lakes were evaluated in 2010. To understand the capacity of Mox to offset potential increases in dissolved methane concentrations, kinetics of water column Mox were also determined. Rates of MG responded positively to increased temperature with a greater influence exerted at higher incubation temperatures. Substrate-saturated Mox significantly increased with temperature and was controlled by substrate and temperature interactions. In contrast, substrate-limited Mox was not influenced by temperature and was controlled by substrate supply. Analysis of Mox kinetics pointed to a community of water column dwelling methane oxidizing bacteria that are capable of oxidizing dissolved methane concentrations far in excess of observed levels. Assuming no diffusion limitation, our results suggest that Mox will likely offset increased MG in response to elevated temperature regimes as a function of ongoing climate change.

SPATIAL VARIABILITY OF DENITRIFICATION POTENTIAL AND RELATED SOIL PROPERTIES IN CREATED, RESTORED, AND PAIRED NATURAL WETLANDS

To gain a better understanding of the spatial patterns of denitrification potential and related soil properties in created (CW), restored (RW), and natural wetlands (NWs), four CW/RW-NW pairs in North Carolina, USA were sampled. These site pairs spanned a range of hydrogeomorphic (HGM) settings common in the Coastal Plain. It was hypothesized that denitrification enzyme activities (DEAs) and related soil properties of CW/RWs would have less spatial variability than DEAs and soil properties of NWs, as prior land-use and mitigation activities tend to homogenize soil properties. Cochran’s C tests indicated that variances were significantly lower in CW/RWs than in NWs for most soil properties, and that for nitrate (NO3-N), variances were significantly lower in CW/RWs across all HGM settings. Interpolated maps of the soil properties revealed homogeneous distributions of NO3-N across the CW/ RW plots compared to much more heterogeneous distributions of NO3-N across the NW plots. Multiple stepwise regressions confirmed that either NO3-N or soluble organic carbon were significant predictors of the DEA at each plot. Interpolated maps of predicted DEA generally showed similar patterns to those of NO3-N. While some nitrate and DEA hotspots were observed in the CW/RWs, more were present in the NWs. These results indicated that spatial distributions of soil chemical properties and DEAs were considerably different in CW/RWs than in paired NWs. This is the first study to document such differences, suggesting that CWs and RWs with homogeneous soil chemical distributions may not develop the full range of soil biogeochemical processes that occur in NWs.