Pergamon Soil Biol. Biochem. Vol. 29, No. 1, 13-21, PP. 1997 0 1997 Elsevier ScienceLtd. All rights reserved Printed in Great Britain PII: SOO38-0717(96)00265-9 0038-0717/97 $17.00 + 0.00 DIFFERENT NH,+-INHIBITION PATTERNS OF SOIL CH4 CONSUMPTION: A RESULT OF DISTINCT CH4-OXIDIZER POPULATIONS ACROSS SITES? JAY GULLEDGE,* ALLEN P. DOYLE and JOSHUA P. SCHIMELt Institute of Arctic Biology, University of Alaska, Fairbanks, AK 99775, U.S.A. (Accepted 2 September 1996) Summary-The short- and long-term effects of NH:-fertilization on soil CH4 oxidation were examined in two upland taiga forests in interior Alaska. Both sites were fertilized for five consecutive snow-free seasons. A paper birch (Bern/a pupyrifera) stand exhibited a delayed inhibition response that became severe (60-70%) by the third season. CH4 flux was not affected in a white spruce (Picea gluucn) stand. In laboratory incubations, (NH&SO4 additions had no effect on extant CH4 oxidation rates in either soil, indicating that neither direct enzymatic inhibition nor acute toxicity were responsible for inhibition in the field. In both sites, maximal CH4 oxidation rates occurred within the upper 20 cm of the mineral soil profile. After the deeper birch soil (20-40 cm) was exposed to ambient atmospheric CH4 (1.8 ~1 I-‘) for 10 d; however, oxidation increased to rates similar to shallower soils, suggesting that methanotrophs in this soil experienced a physiological upshift in response to enhanced CH4 supply. When (NHd)$Od 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 NH:-sensitivity, salt-sensitivity, response to atmospheric CH4 and maximum CH4 oxidation capacities. Thus, the two different re- sponses of CH4 consumption in the field to NH:-fertilization may have resulted from physiologically distinct CH4 oxidizer communities in the two study sites. 0 1997 Elsevier Science Ltd INTRODUmION Biological CH4 consumption directly from the at- mosphere in well-drained soils is estimated to com- prise 3-9% of the global atmospheric CH4 sink (Prather et al., 1995). Although this figure may seemsmall, the magnitude of the terrestrial CH4 sink is similar to the estimated annual increase in the atmospheric CH4 pool, making soil CH4 con- sumption a key determinant of the rate of change in atmosphericCH4 concentration (Reeburgh et al., 1993; Prather et al., 1995). NHf-fertilization inhi- bits CH4 consumption in a variety of ecosystems and representsa means by which human activity could reduce the atmospheric CI& sink (Keller et al., 1990;King, 1992; Ojima et al., 1993;Schimel et al., 1993). NH:-inhibition in the field was first reported by Steudler et al. (1989) in temperate for- ests. Since then there have been reports of inhi- bition in temperateand taiga forests (Whalen et al., 1991; Adamsen and King, 1993; Schnell and King, 1994;Castro et al., 1995), grasslands (Mosier et al., 1991),alpine tundra (Neff et al., 1994), agricultural soils (Conrad and Rothfuss, 1991; Hiitsch et al., *Author for correspondence. tPresent address: Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106, U.S.A. 1993), and aquatic sediments (King, 1990; Conrad and Rothfuss, 1991; Bosseet al., 1993). As global land-useand N deposition increase, the importance of NH:-inhibition of soil CH4 consumption to the global CH4 cycle is likely to increase (Ojima et al., 1993). The direct physiological cause of inhibition is unclear and may vary (Dunfield and Knowles, 1995). Methanotrophic bacteria, thought to carry out methane oxidation in most soils (King, 1992; Bender and Conrad, 1994), oxidize methane via monooxygenase enzymes (Holmes et al., 1995). Physical similarities between CH4 and NH3 permit both compounds to compete for enzyme binding sites,so that fortuitous NH3 oxidation can competi- tively inhibit CH4 oxidation (Bidard and Knowles, 1989; Carlsen et al., 1991; Dunfield and Knowles, 1995). Another possible inhibition mechanism is toxicity of NO; produced from NH3 oxidation (King and Schnell, 1994a,b; Schnell and King, 1994). In the field, inhibition dynamics due to fertiliza- tion vary. In some cases CH.t consumption rates decrease immediately (within hours or days) follow- ing fertilization (Adamsen and King, 1993; Crill et al., 1994; Schnell and King, 1994).This short-term inhibition could be explained by substrate compe- tition, NO; toxicity, or both simultaneously 13