Soil Methane Uptake Research Articles

Snow depth, soil frost and nutrient loss in a northern hardwood forest

We have initiated a long-term experiment to examine the consequences of decreases in snowpack accumulation at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood dominated forest located in the White Mountains of New Hampshire. We are quantifying the effects of decreases in snowpack accumulation on root dynamics of two key tree species in this forest (sugar maple, yellow birch), microbial biomass and activity, NO 3 - and cation loss, the acid-base chemistry of drainage water, and soil--atmosphere trace gas fluxes. We are calibrating an existing model (SNTHERM) to depict snow depth and soil frost dynamics given past or future climate scenarios for our site. In this paper, we describe the methods we are using for the manipulation studies that began in the winter of 1997/1998 and present preliminary results from our first full year of treatment. Results from our methods development efforts show that it is possible to keep plots snow free by shovelling without disturbing the forest floor. Preliminary test plot work showed that the SNTHERM model is capable of depicting snow depth and soil temperatures in both control and manipulated plots at our site. Results from our first full year of treatment showed that a relatively mild freezing event induced significant increases in nitrogen (N) mineralization and nitrification rates, solute leaching and soil nitrous oxide production and caused significant decreases in soil methane uptake. These results suggest that soil freezing events may be major regulators of soil biogeochemical processes and solute delivery to streams in forested watersheds.

Effect of the organic horizon on methane oxidation and uptake in soil of a boreal Scots pine forest

We studied in situ and in the laboratory whether the organic horizon (O horizon) could act as a physical `buffer system' against soil dryness and water stress for the CH 4 oxidizing microbes in boreal coniferous forest soil. The laboratory experiments with samples from a Scots pine forest soil showed that the CH 4 oxidation took place in the uppermost mineral soil and was not affected by removal of the O horizon (peeling) and irrigation. The removal of a thin O horizon (2–3 cm) in early June increased the mean summertime uptake of CH 4 by 50% and decreased the release of CO 2 by 50%. Thus, the O horizon acts as a diffusion barrier, lowering the uptake rate of atmospheric CH 4. The experimental 50% increase in precipitation by irrigation (from 58 to 84 mm month −1) had only a marginal effect on the gas fluxes in both control plots and peeled plots. During the study summer two dry periods with low matric potentials (<−1 MPa) in mineral soil occurred without there being any reduction in CH 4 uptake. In laboratory experiments, the gravimetric water content of 12% (−9 kPa) led to the maximum CH 4 oxidation. The CH 4 oxidation was still 60% of the maximum at a water content of 4% (<−1.5 MPa) and totally inhibited at a water content of 1%. The results reveal that a low soil water content is not an important factor in restricting CH 4 oxidation in mineral soil of boreal forests.

Effect of the organic horizon on methane oxidation and uptake in soil of a boreal Scots pine forest

We studied in situ and in the laboratory whether the organic horizon (O horizon) could act as a physical ‘buffer system’ against soil dryness and water stress for the CH4 oxidizing microbes in boreal coniferous forest soil. The laboratory experiments with samples from a Scots pine forest soil showed that the CH4 oxidation took place in the uppermost mineral soil and was not affected by removal of the O horizon (peeling) and irrigation. The removal of a thin O horizon (2–3 cm) in early June increased the mean summertime uptake of CH4 by 50% and decreased the release of CO2 by 50%. Thus, the O horizon acts as a diffusion barrier, lowering the uptake rate of atmospheric CH4. The experimental 50% increase in precipitation by irrigation (from 58 to 84 mm month−1) had only a marginal effect on the gas fluxes in both control plots and peeled plots. During the study summer two dry periods with low matric potentials (<−1 MPa) in mineral soil occurred without there being any reduction in CH4 uptake. In laboratory experiments, the gravimetric water content of 12% (−9 kPa) led to the maximum CH4 oxidation. The CH4 oxidation was still 60% of the maximum at a water content of 4% (<−1.5 MPa) and totally inhibited at a water content of 1%. The results reveal that a low soil water content is not an important factor in restricting CH4 oxidation in mineral soil of boreal forests.

Methane uptake in Swedish forest soil in relation to liming and extra N-deposition

Methane uptake to soil was examined in individual chambers at three small forest catchments with different treatments, Control, Limed and Nitrex sites, where N-deposition was experimentally increased. The catchments consisted of both well-drained forest and wet sphagnum areas, and showed uptake of CH4 from the ambient air. The lowest CH4 uptakes were observed in the wet areas, where the different treatments did not influence the uptake rate. In the well-drained areas the CH4 uptakes were 1.6, 1.4 and 0.6 kg ha–1 year–1 for the Limed, Control and Nitrex sites, respectively. The uptake of methane at the well-drained Nitrex site was statistically smaller than at the other well-drained catchments. Both acidification and increase in nitrogen in the soil, caused by the air-borne deposition, are the probable cause for the reduction in the methane uptake potential. Uptake of methane was correlated to soil water content or temperature for individual chambers at the well-drained sites. The uptake rate of methane in soil cores was largest in the 0- to 10-cm upper soil layer. The concentration of CH4 in the soil was lower than the atmospheric concentration up to 30 cm depth, where methane production occurred. Besides acting as a sink for atmospheric methane, the oxidizing process in soil prevents the release of produced methane from deeper soil layers reaching the atmosphere.

Methane uptake in soils of Southern Spain estimated by two different techniques: Static chamber and 222radon flux and soil air concentration profiles

Measurements of methane fluxes from four differing soils in the surroundings of Málaga (Spain) were carried out by two methods: a direct method using a static accumulation chamber at the soil surface and an indirect method obtained from simultaneously measured 222Rn flux from the soil surface in parallel with the concentration profile measurements of 222Rn and CH 4 in the air of soil. The directly measured methane flux at all the investigated soils was higher than the methane fluxes derived from the indirect method. Atmospheric methane was consumed by all the soils and mean direct flux to the atmosphere were 3.25 μmol m −2 h −1. The study showed no correlation between methane uptake and soil temperature, which might be due to the dry conditions prevailing during the study period.

Effect of increasing atmospheric methane concentration on ammonium inhibition of soil methane consumption

SOILS currently consume about 30–40 Tg methane per year1,2, which is comparable to the net annual increase in atmospheric methane concentration from 1980 to 19903. Most soils consume methane2,4–9, but the extent varies with soil water content, land use and ammonium inputs5,10–13. Ammonium concentrations in many soils have increased in recent years as a result of land-use changes and increases in ammonium concentration in precipitation14,15. Ammonium strongly inhibits soil methane consumption, but the mechanism is uncertain. Even if enhanced ammonium concentrations are subsequently reduced, inhibition can still persist for months to years12,13. Here we show, from field and laboratory experiments, that the extent of ammonium inhibition increases with increasing methane concentration. We propose that nitrite formation from methanotrophic ammonium oxidation accounts for much of the observed inhibition, and that the persistence of inhibition with reduced ammonium concentrations is due to the limited capacity of methanotrophs to grow or recover in present concentrations of atmospheric methane. We suggest that past increases in atmospheric methane concentration may have increased the inhibitory effect of ammonium, thereby decreasing soil methane uptake capacity, and that this mechanism could also provide a positive feedback on future atmospheric methane concentrations.

Influence of nitrogen fertilization on methane uptake in temperate forest soils

METHANE, a long-lived gas (8–10 years residence time), is important in the chemistry of the atmosphere and the Earth's radiation balance1–3. The tropospheric abundance of CH4 has been increasing by ˜1.1% yr–1 over the past decade4,5. The cause of this increase may be due to either increases in global sources or decreases in global sinks1,6,7. Although considerable research has focused on measuring CH4 emissions from major biological sources7,8, much less is known about the magnitude of, and factors controlling, biological sinks of CH4. The largest biological sinks for methane are microorganisms in aerobic soils7. Here we report a study of CH4 uptake by aerobic temperate-forest soils. We measured CH4consumption rates (up to 3.17 mg CH4–C m–1 day–1) that were higher than reported previously. Globally, soils of temperate and boreal forests may consume up to 9.3 Tg CH4–C yr–1. We also found that the CH4 uptake rates of these soils were decreased significantly by elevated soil moisture (14%) and nitrogen additions (33%), implying that nitrogen fertilization may reduce this CH4 sink.

Notches and edges as reference flaws in ultrasonic testing

Aerobic oxidation of methane at (circum-)atmospheric concentrations (<40 ppmv) has long been assumed to be catalyzed by the as-yet-uncultured high-affinity methanotrophs in well-aerated, non-wetland (upland) soils, the only known biological methane sink globally. Although the low-affinity canonical methanotrophs with cultured representatives have been detected along with the high-affinity ones, their role as a methane sink in upland soils remains enigmatic. Here, we show that canonical methanotrophs can contribute to (circum-)atmospheric methane uptake in agricultural soils. We performed a stable-isotope 13CCH4 labelling incubation in the presence and absence of bio-based residues that were added to the soil to track the flow of methane. Residue amendment transiently stimulated methane uptake rate (<50 days). Soil methane uptake was sustained throughout the incubation (130 days), concomitant to the enrichment of 13CCO2. The 13C-enriched phospholipid fatty acids (PLFAs) were distinct in both soils, irrespective of amendments, and were unambiguously assigned almost exclusively to canonical alphaproteobacterial methanotrophs with cultured representatives. 16S rRNA and pmoA gene sequence analyses revealed that the as-yet-uncultured high-affinity methanotrophs were virtually absent in these soils. The stable-isotope labelling approach allowed to attribute soil methane uptake to canonical methanotrophs, whereas these were not expected to consume (circum-)atmospheric methane. Our findings thus revealed an overlooked reservoir of high-affinity methane-oxidizers represented by the canonical methanotrophs in agriculture-impacted upland soils. Given that upland agricultural soils have been thought to marginally or do not contribute to atmospheric methane consumption due to the vulnerability of the high-affinity methanotrophs, our findings suggest a thorough revisiting of the contribution of agricultural soils, and the role of agricultural management to mitigation of climate change.