Symbiotic effectiveness and tolerance to early season nitrate in indigenous populations of subterranean clover rhizobia from S.W. Australian pastures

The effects of indigenous rhizobial communities on the establishment and functioning of subterranean clover-Rhizobium symbioses was examined using inocula obtained from soils of nine clover pastures and applied as dilute soil suspensions to pot-cultured subterranean clover (Trifolium subterranean cv. Trikkala). The clover was grown without added N (minus N) or in pots fed with 5 mM KNO3 (+NO3− treatment) for the first 4 weeks of a 12 week growth period. Symbiotic performances were compared with that of the recommended commercial inoculant, Rhizobium leguminosarum bv. trifolii strain WU95. Harvests involved separation of the plants into shoot, root and nodules and counting and weighing of nodules. Using NO3− of known δ15N, the proportions of plant N derived from NO3− and N2 were determined. Total N accumulated in the +NO3− treatment inoculated with WU95 was close to the average of that of the 9 soil-based inocula, but this Rhizobium performed poorly in the minus N treatment (viz. 67 for WU95 versus a mean of 81 mg N plant−1 for soil inocula). Some inocula, including WU95, proved to be relatively tolerant of early season NO3−, with fixation in +NO3− treatments up to 80% of those of corresponding minus N cultures. In other associations fixation under +NO3− treatment was less than half that of matching minus N plants. Highly variable performances were recorded between associations in respect of nodule number, mean nodule size, nodule biomass per plant and nodule fixation rate. Several instances were recorded where total N accumulations in the presence of nitrate were more than 30% less than that of fully symbiotic plants. The reasons for variation in nodulation and N2 fixation under early season NO3− are not clear, but suggest complex Rhizobium-directed influences shaping host plant response. The possibility of selecting NO3− tolerant rhizobia as alternative inoculants, for situations in which early season nitrate availability is restrictive to nodulation, is discussed.