Field persistence of Bradyrhizobium sp. (Lupinus) inoculant for serradella (Ornithopus compressus L.)

Abstract

Experiments were conducted, in situ in north-western New South Wales and in the laboratory, to evaluate the longevity of Bradyrhizobium sp. (Lupinus) inoculated as a peat culture onto seed of yellow serradella (Ornithopus compressus L.). Observations were made on the field persistence of the inoculant strain over 2 years. Two categories of seed were examined: seed encased in a segment of pod coat (hulled), and seed from which the pod coat had been removed (dehulled). The use of adhesive (methyl cellulose or gum arabic) ensured that the inoculant adhered to the seed. At a constant temperature of l8¦C, inoculant applied to hulled seed survived for >8 months without loss of viability. Type of adhesive did not affect survival. At the 2 field sites, inoculated seed was sown into dry soil. Inoculant was sufficiently viable for 77-159 days to colonise the rhizospheres of young serradella and to form nodules on a significant (P<0.05) proportion of the plants. Rhizosphere colonisation and nodulation were better with hulled seed than with dehulled seed but were not affected by type of adhesive. Evidence is presented that the inoculant was able to establish in these soils as a permanent component of the soil microflora and to persist as a reservoir of inoculum for annually regenerating serradella.