Effect of rhizobia from caucasian clover (Trifolium ambiguum) on nodulation and nitrogen fixation of white clover (Trifolium repens)

Three glasshouse trials investigated N-2 fixation in white clover following inoculation with rhizobia from caucasian clover. We grew white clover in agar, vermiculite/perlite and soils low in available-N and with no detectable rhizobia. Media were subsequently treated with inoculum containing rhizobia effective on either (i) caucasian clover, (ii) white clover, or (iii) mixtures of the two. The percentage of ineffective nodules on white clover increased as the proportion of rhizobia from caucasian clover in the inoculant mix increased. However, the percentages of ineffective nodules on white clover were less than the proportion of ineffective rhizobia in the inoculant, indicating that the ineffective rhizobia were less competitive in nodule formation. Foliage yield and symbiotic N-2 fixation in white clover were not significantly affected when ineffective rhizobia from caucasian clover made up 67% or less of the inoculant. However, when ineffective rhizobia made up a high proportion of the inoculant mix (>92%), both foliage yield and N-2 fixation were significantly reduced compared with plants inoculated with only white clover rhizobia. Extrapolation of the present glasshouse results to the field is difficult and further field trials are required to determine if rhizobia from caucasian clover have any effect on symbiotic N-2 fixation of white clover in pastoral situations.