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 a nd 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 (Trifo lium subterranean cv. Trikkala). The clover was grown without added N (minus N) or in pots fed with 5 mM KNO3 (+NO3- treatment) for the firs t 4 weeks of a 12 week growth period. Symbiotic performances were comp ared with that of the recommended commercial inoculant, Rhizobium legu minosarum bv. trifolii strain WU95. Harvests involved separation of th e plants into shoot, root and nodules and counting and weighing of nod ules. Using NO3- of known delta(15)N the proportions of plant N derive d from NO3- and N-2 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 minu s 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 mi nus N plants. Highly variable performances were recorded between assoc iations in respect of nodule number, mean nodule size, nodule biomass per plant and nodule fixation rate. Several instances were recorded wh ere 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 N-2 fixation under early season NO3- are not clear, but suggest complex Rhizobium-directed influences shaping host plant r esponse. The possibility of selecting NOS tolerant rhizobia as alterna tive inoculants, for situations in which early season nitrate availabi lity is restrictive to nodulation, is discussed. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.