Mechanistic Analysis of Ammonium Inhibition of Atmospheric Methane Consumption in Forest Soils

Abstract

Methane consumption by forest soil was studied in situ and in vitro with respect to responses to nitrogen additions at atmospheric and elevated methane concentrations. Methane concentrations in intact soil decreased continuously from atmospheric levels at the surface to 0.5 ppm at a depth of 14 cm. The consumption rate of atmospheric methane in soils, however, was highest in the 4- to 8-cm depth interval (2.9 nmol per g of dry soil per day), with much lower activities below and above this zone. In contrast, extractable ammonium and nitrate concentrations were highest in the surface layer (0 to 2 cm; 22 and 1.6 μmol per g of dry soil, respectively), as was potential ammonium-oxidizing activity (19 nmol per g of dry soil per day). The difference in zonation between ammonium oxidation and methane consumption suggested that ammonia-oxidizing bacteria did not contribute significantly to atmospheric methane consumption. Exogenous ammonium inhibited methane consumption in situ and in vitro, but the pattern of inhibition did not conform to expectations based on simple competition between ammonia and methane for methane monooxygenase. The extent of ammonium inhibition increased with increasing methane concentration. Inhibition by a single ammonium addition remained constant over a period of 39 days. In addition, nitrite, the end product of methanotrophic ammonia oxidation, was a more effective inhibitor of methane consumption than ammonium. Factors that stimulated ammonium oxidation in soil, e.g., elevated methane concentrations and the availability of cosubstrates such as formate, methanol, or β-hydroxybutyrate, enhanced ammonium inhibition of methane oxidation, probably as a result of enhanced nitrite production.

Stability of methane oxidation capacity to variations in methane and nutrient concentrations

“…However, the lack of any stimulation of methane consumption by nitrate additions suggests that assimilable nitrogen is not a major limiting factor. In contrast the response of these soils to ammonium was inhibitory, as described elsewhere [16,24]. Copper availability also did not appear to limit growth or activity, since addition of copper sulfate did not affect methane consumption, even at relativ high concentrations (10 pmol gfw soill ' 1.…”

“…For methanotrophs in culture, approximately 50% of the methane consumed is dissimilated while the rest is assimilated [2,12]. From previous work it has been evident that soil methanotrophs have a limited capacity to grow when using atmospheric methane [16,24]. One reason could be that at low substrate concentrations the energy de-mands for maintenance may require a high level of methane dissimilation, leaving only a small amount of carbon for cell mass syntheses.…”

“…The soil from the 4-6 cm depth interval used in this study had a low extractable nitrogen content (6.5 pmol ammonium gdw soil-', 0.3 pmol nitrate gdw soil-'; [24]) that might be growth limiting for methanotrophs. However, the lack of any stimulation of methane consumption by nitrate additions suggests that assimilable nitrogen is not a major limiting factor.…”

“…Ammonium-oxidizing bacteria have been suggested as the organisms responsible for atmospheric methane consumption [19,25]. However, the location of maximum potential nitrification rates at the soil surface where concentrations of ammonium and nitrate are highest [24] is inconsistent with significant methane consumption.…”

A review of nitrogen enrichment effects on three biogenic GHGs: the CO2 sink may be largely offset by stimulated N2O and CH4 emission

“…KNO 3 ,NH 4 Cl and NH 4 NO 3 ) have been shown to suppress the activity of methanotrophic bacteria by increasing osmotic pressure (King & Schnell 1998;Bodelier & Laanbroek 2004). Other mechanisms, such as toxicity of nitrite (NO 2 ) ) produced by nitrification or denitrification processes, may also involve in the inhibition of CH 4 oxidation by N addition (Schnell & King 1994).…”