MECHANISTIC ANALYSIS OF AMMONIUM INHIBITION OF ATMOSPHERIC METHANE CONSUMPTION IN FOREST SOILS

Methane consumption by forest soil was studied in situ and in vitro wi th respect to responses to nitrogen additions at atmospheric and eleva ted methane concentrations. Methane concentrations in intact soil decr eased continuously from atmospheric levels at the surface to 0.5 ppm a t a depth of 14 cm. The consumption rate of atmospheric methane in soi ls, however, was highest in the 4- to 8-cm depth interval (2.9 nmol pe r g of dry soil per day), with much lower activities below and above t his zone. In contrast, extractable ammonium and nitrate concentrations were highest in the surface layer (0 to 2 cm; 22 and 1.6 mu mol per g of dry soil, respectively), as was potential ammonium-oxidizing activ ity (19 nmol per g of dry soil per day). The difference in zonation be tween ammonium oxidation and methane consumption suggested that ammoni a-oxidizing bacteria did not contribute significantly to atmospheric m ethane consumption. Exogenous ammonium inhibited methane consumption i n situ and in vitro, but the pattern of inhibition did not conform to expectations based on simple competition between ammonia and methane f or 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, nit rite, 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 concent rations and the availability of cosubstrates such as formate, methanol , or beta-hydroxybutyrate, enhanced ammonium inhibition of methane oxi dation, probably as a result of enhanced nitrite production.