Ecological studies of root-nodule bacteria introduced into field environments—6. Antigenic and symbiotic stability in Lotononis rhizobia over a 12-year period

Rhizobia suited to Lotononis bainesii do not occur naturally in Australia. Two serologically distinct strains of African origin were introduced into an isolated field station where perennial Lotononis-pangola grass pastures are grazed. Paddocks varied in the period of rhizobial occupancy from 5–12 years. Stability of four rhizobial characters: colony colour, effectiveness, cell antigens and antibiotic sensitivity were assessed from nodules on persisting Lotononis. Both strains became established throughout the 782 ha. No changes were observed in colony colour or serological reaction. Symbiotically, 8.5% of isolates had a lower N fixing capacity than stock cultures held in the laboratory although there were no ineffective isolates. Change in effectiveness was unrelated to serotype or to time of occupancy. There was also little change in antibiotic sensitivity although one isolate was tolerant to 15 parts/10⁶ streptomycin. The red bacterium Protaminobacter rubra was recorded for the first time as a cohabitant of Lotononis nodules. It is concluded that colony colour, serological and symbiotic properties are sufficiently stable to be of use in long-term ecological studies although antibiotic sensitivity appears less stable.     

Ecological studies on coccoid bacteria in a pine forest soil—II. growth of bacteria introduced into soil

The growth of five typical but phenetically distinct cocci and Arthrobacter strains, isolated from a pine forest soil, has been investigated. Soil reaction was found to have a marked effect on growth and unless naturally acidic soils were made more alkaline they did not support growth, even in the presence of added nutrients. However, in the presence of fungi which could use the added nutrients, e.g. chitin, mycelial fragments etc., bacterial growth was possible and could be correlated with a decrease in acidity, especially around particles of organic matter. Where the pH rose above 7.9, bacterial growth again decreased. All the bacterial strains tested reacted in the same way, suggesting that they occupied similar microenvironments in both the acidic and alkaline soil horizons examined. Some explanations for the occurrence of nonsporing bacteria in soils in which apparently they cannot grow are suggested.     

Rhizobia in tropical legumes—IX. pot and field trials with inoculants for Psophocarpus tetragonolobus (L.) DC.

Antigenically identifiable inoculants for Psophocarpus tetragonolobus were evaluated in three non-sterile soils contained in pots (sandy-clay, Renggam series; a loamy-sand, Sungei Buloh series; silty-clay, Munchong series). Most-probable-numbers of indigenous rhizobia ranged from 4 (Renggam series) to 13 (Munchong series) g^?1. Only two (RRIM 56 and 968) of the eight rhizobia tested formed > 50% of the nodules in all soils. Recovery of two strains (RRIM 968 and UMKL 12) was significantly poorer from the Munchong series soil which had the most indigenous rhizobia and the highest silt plus clay content. In a field trial using a Sungei Buloh series soil containing 700 rhizobia g^?1 capable of nodulating P. tetragonolobus, none of the applied strains formed > 18% of the nodules; two formed no nodules. There were no significant increases in plant yield in response to inoculation in the field trial and in two soils in the pot trials. In Sungei Buloh series soil, RRIM 56 formed 90% of the nodules when the indigenous rhizobia were 5 cells g^?, and 14% when the population was 700 g^?1. This raises the question of the need to inoculate seed sown into soils with high indigenous rhizobial populations, but there was some indication of increasing representation of inoculant strains in nodules with time.