Caucasian clover rhizobia are not a threat to nitrogen fixation by white clover

Abstract

Caucasian clover has specific rhizobial requirements, and inoculation with a recommended strain is essential for nodulation and symbiotic N2 fixation. However, these rhizobia form nodules on white clover which do not fix nitrogen i.e. are ineffective. At the Nelson NZGA conference, Elliot and co-workers cautioned that caucasian clover rhizobia may over time reduce N2 fixation by white clover in New Zealand pastures. We have shown that high populations of caucasian clover rhizobia build up in the soil under swards of caucasian clover with numbers ranging from 103 to 106/g soil. A field trial was established on a site containing a naturalised population of rhizobia nodulating white clover (105 rhizobia/g soil). Treatments were imposed by mixing caucasian clover rhizobia into the soil, so that they made up 0, 50% or 99% of the clover rhizobia population. The trial was then sown with uninoculated white clover seed. The planned levels of caucasian clover rhizobia were present after 1 month and there was only a small decline in numbers up to 22 months after sowing. White clover rhizobia increased over the first 14 months at least 10-fold, to 106/g soil, even in the presence of caucasian clover rhizobia. There were no significant effects on white clover forage yield or on the calculated amount of N2 fixed, even when soil populations of caucasian clover rhizobia made up 99% of the population of clover rhizobia at sowing. This lack of effect could be explained by the small percentage (up to 5%) of nodules on white clover formed by caucasian clover rhizobia. There is no doubt that high populations of caucasian clover rhizobia will develop in New Zealand soils with the increased use of caucasian clover and its associated rhizobia, but the presence of these rhizobia did not reduce symbiotic N2 fixation of white clover in this field experiment. Keywords: caucasian clover, competition, nitrogen fixation, nodulation, rhizobia, Trifolium ambiguum, Trifolium repens, white clover