Caucasian clover (Trifolium ambiguum) rhizobia persists in New Zealand soils for over 42 years

Abstract

Most legume species can fix atmospheric N2 via bacteria (rhizobia) in root nodules, thus allowing them to colonise low nitrogen soils. The South Island of New Zealand’s high country is made up of highly weathered low fertility soil that experiences frequent snow events in winter thus providing an unsuitable climate for most pasture species. Caucasian clover a winter dormant perennial clover from the Caucasus region of Eurasia has been shown to have potential in New Zealand soils, especially the high country. However, Caucasian clover has a specific rhizobia requirement, it only forms functional nodules with rhizobia from its native sites. These rhizobia do not occur naturally in New Zealand and must be supplied by inoculum. The first objective of this study was to determine if the Caucasian clover rhizobia applied as inoculum over a period of 2-42 years has persisted in New Zealand soils. Caucasian clover plants grown at the field sites were nodulated. When grown in a controlled environment, Caucasian clover plants were nodulated in soil from each of the field sites. The ERIC PCR banding patterns indicated similar genotypes over all field sites except for two isolates, as expected from Caucasian clover inoculant. Using phylogenetic analysis trees of 16S rRNA the genus of the isolates was confirmed as Rhizobium. The nodA phylogenetic tree confirmed the species as Rhizobium leguminosarum and the nifH tree confirmed the biovar as Rhizobium leguminosarum biovar trifolii. The second objective of this study was to determine if the Caucasian clover plants in high country soils are deficient in nutrients other than nitrogen thus limiting nodule and biomass production. Full nutrient (no nitrogen) fertiliser was applied to Caucasian clover plants at Craigieburn site in autumn and visually assessed in spring, resulting in an increased biomass and nodule production in fertilised plants. In conclusion, the Caucasian clover planted with specific rhizobia at different sites, aged 2-42 years, were shown by the phylogenetic analysis and ERIC-PCR to have persisted in the soil. Caucasian clover plants fertilised at the Craigieburn site were limited in non-nitrogen nutrients and showed an increased biomass and nodule production after fertiliser was applied.