Pupuk Organik dan Pupuk Hayati 113 6. BAKTERI PENAMBAT NITROGEN R.D.M. Simanungkalit, Rasti Saraswati, Ratih Dewi Hastuti, dan Edi Husen Summary Nitrogen-fixing bacteria. The chapter deals with taxonomy, distribution, mechanism of nitrogen fixation, inoculant technology and utilization of inoculant. There are two groups of nitrogen-fixing bacteria, e.i free-living nitrogen-fixing bacteria and symbiotic nitrogen-fixing bacteria. Though of the latest development in the taxonomy of symbiotic nitrogen fixing bacteria in the world, very little progress made in identifying indigenous bacteria, so it is very difficult to know how rich is the diversity of rhizobia in Indonesia. Rhizobial inoculant has been produced commercially in the 1980’s in order to support the national intensification program of soybean. Methods of inoculant production has followed the existing standards of production technology. Successful inoculation has been shown in some soybean growing areas. Currently, rhizobial inoculant in the form of compound biofertilizer, in which soybean–infecting rhizobia are mixed with other functional group microbes such as phosphate-solubilizing microbes and free-living nitrogen- fixing bacteria. However, production technology of compound biofertilizer has not developed yet. Kebutuhan bakteri akan unsur N dapat dipenuhi dari sumber N yang terdapat dalam berbagai senyawa organik maupun dari N 2 udara. Beragam jenis bakteri bertanggung jawab pada penambatan N 2 secara hayati, mulai dari Sianobakter (ganggang hijau biru) dan bakteri fotosintetik pada air tergenang dan permukaan tanah sampai pada bakteri heterotrofik dalam tanah dan zona akar (Ladha and Reddy, 1995; Boddey et al., 1995; Kyuma, 2004). Bakteri mampu melakukan penambatan nitrogen udara, baik melalui nonsimbiosis (free-living nitrogen-fixing bacteria) maupun simbiosis (root- nodulating bacteria). Bakteri penambat N 2 hidup bebas (nonsimbiosis) Berbagai jenis bakteri penambat N 2 yang hidup-bebas (non-simbiotik) di tanah sawah tercantum dalam Tabel 1.