Nodulation and growth of pasture legumes with naturalised soil rhizobia. 3. Lucerne (Medicago sativa L.)

An assessment was made of the ability of 2 genetically diverse cultivars of lucerne (Trifecta and Sceptre) to nodulate and form effective symbioses with the strains of Sinorhizobium meliloti naturalised in 50 soils. Soils were collected from 42 dryland lucerne pastures and 8 irrigated multiple-use lucerne stands in the south-east of South Australia. The density of lucerne in the paddock, age of the lucerne stand and rhizobial inoculation practice were recorded at sampling. The lucerne swards were on average 5.6 years old (range 1-28) and had a mean plant density of 25 plants/m2 (range 3-66). The lucerne in 39 of the paddocks had been inoculated with rhizobia at sowing. The most probable number of rhizobia (S. meliloti) able to nodulate lucerne was determined for each soil. The most probable number of lucerne rhizobia exceeded 1.0 × 103/g in 23 of the soils. The most probable number of rhizobia in the soil was significantly correlated (r = 0.64) with soil pH. All 13 soils that contained less than 1.0 × 102 rhizobia/g had a pH of less than 6.3. The effectiveness at nitrogen fixation of each population of soil rhizobia was determined in a glasshouse experiment. There was no significant interaction between lucerne cultivar and soil inoculant. Generally, the soil rhizobia were highly effective, with 35 of the 50 soil inoculants producing more than 70% of the shoot dry weight associated with an effective inoculant strain (WSM826). Decreased lucerne shoot dry weights resulting from the remaining soil inoculants were associated with a lower most probable number of rhizobia in the soil, rather than poor symbiotic effectiveness of those rhizobia. This study highlighted the ability of both Trifecta and Sceptre lucernes to form an effective symbiosis with the rhizobia that have naturalised in the soils across a 25 000 km2 region of South Australia. It also showed that soil acidity is detrimental to rhizobial survival even in a perennial legume system and suggests that rhizobia that persist and form nodules in acidic soils should be a focus in the selection of new inoculant strains for lucerne.