Atmospheric Methane Consumption by Forest Soils and Extracted Bacteria at Different pH Values

Abstract

The effect of pH on atmospheric methane (CH4) consumption was studied with slurries of forest soils and with bacteria extracted from the same soils. Soil samples were collected from a mixed hardwood stand in New Hampshire, from jackpine and aspen stands at the BOREAS (Boreal Ecosystem Atmosphere Study) site near Thompson, northern Manitoba, from sites in southern Québec, including a beech stand and a meadow, and from a site in Ontario (cultivated humisol). Consumption of atmospheric CH4 (concentration, approximately 1.8 ppm) occurred at depths of >5 cm in both acidic (pH 4.5 to 5.2) and alkaline (pH 7.2 to 7.8) soils. In slurries of acidic soils, maximum activity occurred at different pH values (pH 4.0 to 6.5). Bacteria extracted from these soils by high-speed blending and density gradient centrifugation showed pH responses different from the pH responses of the slurries. In all cases, these bacteria had a methanotrophy pH optimum of 5.8 and exhibited no activity at pH 6.8 to 7.0, the pH optimum range for known methanotrophs. This difference in pH responses could be useful in modifying media currently used for isolation of these organisms. Methanotrophic activity was induced in previously non-CH4-consuming soils by preincubation with 5% (vol/vol) CH4 (50,000 μl of CH4 per liter) or by liquid enrichment with 20% CH4. The bacteria showed pH responses typical of known methanotrophs and not typical of preexisting consumers of ambient CH4. Furthermore, methanotrophs induced by high CH4 levels were more readily extracted from soil than preexisting ambient CH4 consumers were. In the alkaline soils, preexisting activity either was destroyed or resisted extraction by the procedure used. The results support the hypothesis that consumers of ambient CH4 in soils are physiologically distinct from the known methanotrophs.

The effect of soil acidification on atmospheric methane uptake by a Maine forest soil

“…In addition, we have examined interactions between ammonium toxicity and acidi¢cation in order to determine the extent to which pH stresses might amplify (or limit) ammonium stresses. Results from sieved soils in this study di¡er from those of Sitaula [12], but con¢rm patterns reported by Amaral et al [13] based on slurries and bacterial extracts. However, we show that at a given pH nitric acid results in greater inhibition of methane consumption than sulfuric acid, which suggests that pH responses in situ may vary among sites as a function of the speci¢c mix of acids deposited in precipitation.…”

“…Fig. 2), indicate that pH optima for atmospheric methane consumption range between about 4.5 and 6.5, in some instances coinciding with ambient pH regimes [13]. These optima may vary somewhat as a function of the acid used to adjust pH, since nitric and sulfuric acids yield di¡erent methane consumption rates (e.g.…”

“…Thus, use of HCl (e.g. [13,15]) may overestimate the sensitivity of atmospheric methane consumption to pH changes. Since acid deposition is comprised primarily of nitric and sulfuric acids, and is often dominated by the latter, use of these acids likely provides a better indication of the potential e¡ects of acid-i¢cation.…”

“…In contrast, atmospheric methane consumption in aspen forest soils at native pH values of 7^8 maintained signi¢cant activity over a relatively broad pH range. In addition activity increased with increasing acid-i¢cation for soils adjusted to pH 8^9 relative to soils adjusted to pH 6^7 [13]. Atmospheric methane consumption in naturally acidic forest soils has also shown signi¢cant pH tolerance, although acidi¢cation typically decreases activity [10,13].…”

“…In addition activity increased with increasing acid-i¢cation for soils adjusted to pH 8^9 relative to soils adjusted to pH 6^7 [13]. Atmospheric methane consumption in naturally acidic forest soils has also shown signi¢cant pH tolerance, although acidi¢cation typically decreases activity [10,13]. Nevertheless, Sitaula et al [12] reported an increase in forest soil methane consumption after irrigation with sulfuric acid and a decrease in pH from an original value of about 5 to 3.…”