Different NH4+-inhibition patterns of soil CH4 consumption: A result of distinct CH4-oxidizer populations across sites?

The short- and long-term effects of NH4+-fertilization on soil CH4 oxidation were examined in two upland taiga forests in interior Alaska. Both sites were fertilized for five consecutive snow-free seasons. A paper birch (Betula papyrifera) stand exhibited a delayed inhibition response that became severe (60–70%) by the third season. CH4 flux was not affected in a white spruce (Picea glauca) stand. In laboratory incubations, (NH4)2SO4 additions had no effect on extant CH4 oxidation rates in either soil, indicating that neither direct enzymatic inhibition nor acute toxicity were responsible for inhibition in the field. In both sites, maximal CH4 oxidation rates occurred within the upper 20 cm of the mineral soil profile. After the deeper birch soil (20–40 cm) was exposed to ambient atmospheric CH4 (1.8 μl l−1) for 10 d; however, oxidation increased to rates similar to shallower soils, suggesting that methanotrophs in this soil experienced a physiological upshift in response to enhanced CH4 supply. When (NH4)2SO4 was added, however, methanotrophic activity did not increase. A similar upshift did not occur in the spruce soil. The CH4 oxidizers in the two soils differed with respect to NH4+-sensitivity, salt-sensitivity, response to atmospheric CH4 and maximum CH4 oxidation capacities. Thus, the two different responses of CH4 consumption in the field to NH4+-fertilization may have resulted from physiologically distinct CH4 oxidizer communities in the two study sites.