For evaluating N 2 fixation of diazotrophic bacteria, nitrogen-poor liquid media supplemented with at least 0.5% sugar and 0.2% agar are widely used for acetylene reduction assays. In such a soft gel medium, however, many N 2 -fixing soil bacteria generally show only trace acetylene reduction activity. Here, we report that use of a N 2 fixation medium solidified with gellan gum instead of agar promoted growth of some gellan-preferring soil bacteria. In a soft gel medium solidified with 0.3% gellan gum under appropriate culture conditions, bacterial microbiota from boreal forest bed soils and some free-living N 2 -fixing soil bacteria isolated from the microbiota exhibited 10-to 200-fold-higher acetylene reduction than those cultured in 0.2% agar medium. To determine the N 2 fixation-activating mechanism of gellan gum medium, qualitative differences in the colony-forming bacterial components from tested soil microbiota were investigated in plate cultures solidified with either agar or gellan gum for use with modified Winogradsky's medium. On 1.5% agar plates, apparently cryophilic bacterial microbiota showed strictly distinguishable microbiota according to the depth of soil in samples from an eastern Siberian Taiga forest bed. Some pure cultures of proteobacteria, such as Pseudomonas fluorescens and Burkholderia xenovorans, showed remarkable acetylene reduction. On plates solidified with 1.0% gellan gum, some soil bacteria, including Luteibacter sp., Janthinobacterium sp., Paenibacillus sp., and Arthrobacter sp., uniquely grew that had not grown in the presence of the same inoculants on agar plates. In contrast, Pseudomonas spp. and Burkholderia spp. were apparent only as minor colonies on the gellan gum plates. Moreover, only gellan gum plates allowed some bacteria, particularly those isolated from the shallow organic soil layer, to actively swarm. In consequence, gellan gum is a useful gel matrix to bring out growth potential capabilities of many soil diazotrophs and their consortia in communities of soil bacteria.In 1967, Schöllhorn and Burris discovered that nitrogenase from an N 2 -fixing rhizobium of soybean can reduce acetylene to produce ethylene (C 2 H 4 ) (32), a reaction analogous to the conversion of the natural substrate N 2 into ammonia. Shortly afterwards, it was shown that this acetylene reduction activity parallels N 2 reduction by nitrogenase (13), and since then, acetylene reduction assays have been widely used in the evaluation of biological N 2 fixation. An acetylene reduction assay is generally performed under the following conditions: precultured bacterial cells are suspended into N-free or -deficient liquid medium containing a carbon source, usually D-glucose or D-mannitol (35) at 0.5 to 2.0%, and exposed for 24 h or less at a representative room temperature, e.g., 25°C (2). However, this method is not applicable to free-living, microaerobic N 2 -fixing bacteria, which have been regarded as notoriously difficult to culture. To solve this problem, Döbereiner and her group developed a soft g...

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[1] 'Alases' are mature thermokarst depressions covered by grassland distributed in taiga forests in central Yakutia, eastern Siberia, following thermokarst formation initiated in early Holocene. Alases are important land-cover class in the central Yakutia lowland occupying 17% of the total land area. CH 4 and N 2 O fluxes were measured temporally in a typical alas which have a pond at the center during the growing season of two years. Seven monitoring plots represented various vegetation types: a larch forest (F), a dry grassland (DG), four wet grasslands (WG) flooded temporarily or continuously, and a pond surface (P) flooded continuously without vegetation. The pond flooding area reached its maximum just after snow-melting and decreased during the summer. The F and DG plots were small CH 4 sinks. The wet plots (including WG and P) were large CH 4 sources (cumulative value of growing season: 17 to 864 kg CH 4 -C ha À1 ), and emission rates vary drastically depending on flooding conditions. All plots were slight sources or sinks of N 2 O except for the WG plots (0.16 to 1.7 kg N 2 O-N ha À1 ) where peak emissions were observed after the flooding ended. The global warming potentials (time horizon of 100 years) of the cumulative N 2 O emissions from the plots were lower than those of each CH 4 emissions. Estimated total CH 4 emission from the whole WG area (7.93 ha) accounted for 88% of the emission from the whole alas (28.9 ha). The current permafrosttaiga region in central Yakutia could be a net CH 4 source owing to the presence of alases.