Characterization and ecological role of free-living nitrogen-fixing bacteria isolated from the rhizoplane of Melastoma malabathricum inhabiting acidic plain lands in Kalimantan

Abstract

The rhizoplane microorganisms of Melastoma malabathricum, distributing throughout the acid sulfate and peat soils of South Kalimantan, Indonesia, were investigated, using a soft gel media of N-free Winogradsky’s mineral solution solidified with 0.3 % gellan gum. Two bacteria, Sphingomonas rosa EC-K013 and Burkholderia cepacia EC-K014 belonging to the subclasses of α- and β-Proteobacteria, respectively, emerged in the nitrogen-free medium. Inoculation of S. rosa EC-K013 in the M. malabathricum seedlings grown in N-free, 1⁄4 Hoagland’s No. 2 medium (pH 3.0) resulted in better growth of the seedlings. In the soft gel medium, EC-K013 also stimulated the growth of a root-associating fungus, tentatively identified as Nectria sp. and a nitrogen-fixer, Mesorhizobium sp., suggesting that S. rosa plays an important role in promotion of microfloral devlopment around the rhizosphere of some acidic soil-adapting plants.