Conversion of natural ecosystems to agriculture usually reduces soil methane-oxidizing activity. When adjacent cultivated and pristine ecosystems are compared, methane oxidation rates are up to an order of magnitude lower in the cultivated sites (Keller et al., 1990; Mosier et al., 1991; Hütsch et al., 1994; Jensen and Olsen, 1998; Priemé and Christensen, 1999; Knief et al., 2005a). Microbial community analysis of a cultivated field and adjacent forests in Thailand indicated that dramatic changes in the methanotrophic species composition had occurred in the agricultural soil (Knief et al., 2005a). Nitrogen fertilization is often postulated as the major cause of the reduced methane sink in agricultural soils (Section 2.5), but reduced methane oxidation has been noted in unfertilized arable land as well (Mosier et al., 1991).
Other anthropogenic disturbances destroy or reduce the atmospheric methane sink. These include application of a variety of agrochemicals such as pesticides and herbicides (Topp, 1993; Syamsul Arif et al., 1996; Boeckx et al., 1998; Priemé and Ekelund, 2001). Soil acidification via acid rain can inhibit methanotrophs through direct pH effects or aluminium toxicity (Nanba and King, 2000; Bradford et al., 2001b; Sitaula and Bakken, 2001). Even sample handling for scientific purposes can decrease metha-notrophic activity, indicating that the bacteria are very sensitive to physical disturbance of natural chemical gradients (Whalen et al., 1992; Amaral and Knowles, 1997).
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