Effect of root exudates on growth of newly isolated nitrifying bacteria from barley rhizoplane

An ammonia-oxidizing bacterium, strain PJA1, and nitrite-oxidizing bacterium, strain PJN1, were newly isolated from the rhizoplane of barley. The cells of strain PJA1 were lobate, compartmentalized, and showed characteristics of the genus Nitrosolobus. The similarity of the 16S rRNA gene to Nitrosolobus multiformis ATCC25196 was 99.04%. The cells of strain PJN1 were rod-shaped. The similarity of the 16S rRNA gene to Nitrobacter agilis ATCC14123 was 98.57%. These newly isolated bacteria were identified as Nitrosolobus sp. PJA1 and Nitrobacter sp. PJN1, respectively. The exudates prepared from barley roots of both the pre-heading and post-heading stages promoted the growth of strain PJA1 (15–20% increase). The growth of the non-rhizoplane strains like Nitrosomonas europaea ATCC25978 was remarkably inhibited by the exudates. The growth of strain PJN1 was also remarkably promoted by the root exudates at the pre-heading stage (6 time increase), and by the root exudates at the post-heading stage (2.5 time increase). The root exudates did not effect the growth of Nitrobacter winogradskyi IFO14297, isolated from the non-rhizoplane.