Abstract
Survival of antibiotic resistant Bradyrhizobium japonicum (streptomycin, 200μg ml-1; spectinomycin, 200μg ml-1), Rhizobium leguminosarum (streptomycin, 100μg ml-1; spectinomycin, 100μg ml-1; rifampicin, 100μg ml-1) and Rhizobium meliloti (streptomycin, 200μg ml-1; spectinomycin, 200μg ml-1; rifampicin, 200μg ml-1) associated with the application of two commercially prepared composts, Bark and Tenporon, in an acid soil was investigated. Population changes were monitored using antibiotic media and the rhizobial number was enumerated by the plate count method. Populations of the three rhizobium strains generally decreased between 1 and 3 weeks and thereafter remained constant. The soil carrying capacity (asymptotic number of rhizobia reached) for the three rhizobium strains was significantly (p<0.05) elevated by Bark compost, but a significant (p<0.05) decrease for R. leguminosarum and R. meliloti was noted in the soils treated with Tenporon.
The effects of these composts on extractable NH4+ and NO3-, fluorescein diacetic acid (FDA) hydrolytic activity and amount of Ca released were also examined. Addition of Tenporon compost resulted in a high amount of extractable NH4+ and NO3-. Bark did not affect them, although FDA hydrolytic activity was high. Both composts increased the amount of exchangeable Ca, however, the amount in the Bark-treated soil was higher than that of Tenporon. Investigation of the survival of the three rhizobium strains indicated that application of some organic materials can increase soil carrying capacity for rhizobia in acid soil for subsequent nodulation without liming.
