Populations of Rhizobia in New Zealand Soils

Abstract

ALTHOUGH LEGUMES are of prime importance in New Zealand pastures, the number of species involved is small, and is restricted almost entirely to the genera Trifolium (the clovers), Medicago (lucerne and other medics), and Lotus. These three genera form nodules with three separate groups of rhizobia (root nodule bacteria) with no effective crossnodulation between them. Also, within any one of these groups of rhizobia, there are numerous strains. These may differ in a number of respects, but in particular they differ in the range of species with which they are effective in fixing nitrogen. Thus, among clover rhizobia, one strain may be effective on white (Trifolium repens L.) and red clovers (T. pratense L.) but not on subterranean clover (T. subterraneum L.), a second strain effective on subterranean, weakly effective on red and ineffective on white clover, and a third strain effective on all three species.

White clover seed inoculation and coating in New Zealand

“…Clover rhizobia were absent from large areas of the tussock grasslands but were present in some soils, particularly low altitude dry soils where haresfoot clover (Trifolium arvense) was present. Rhizobia that nodulate haresfoot clover also nodulate white clover (Greenwood 1965). Clover rhizobia populations of up to 4000 per gram of soil were detected in 40-60% of tussock grasslands soils, even in the absence of naturalised clovers, which confirms the ability of clover rhizobia to colonise and persist in the soil even in absence of a host legume (Greenwood 1965).…”

“…This occurs through a process where soil bacteria, rhizobia, invade the roots of the legume plant and form root nodules in which the nitrogen conversion occurs. Rhizobia that nodulate clovers are not native to New Zealand, and were introduced accidentally with the onset of European settlement (Greenwood 1965). It is presumed that they were introduced by chance in soil and dust attached to equipment, plant material, seed and stock hooves.…”

“…Rhizobia were undetected in undisturbed gumland soils by Greenwood (1961), detected in low levels in soils cultivated out of scrub by Gould et al (1968), and found to be patchy and variable in numbers in soils of the North Island hill country (Greenwood 1965;MacFarlane & Bonish 1986).…”

“…Rhizobia that nodulate haresfoot clover also nodulate white clover (Greenwood 1965). Clover rhizobia populations of up to 4000 per gram of soil were detected in 40-60% of tussock grasslands soils, even in the absence of naturalised clovers, which confirms the ability of clover rhizobia to colonise and persist in the soil even in absence of a host legume (Greenwood 1965). Survey results of Gaur and Lowther (1980) were the basis for the recommendation to inoculated clover seed oversown onto tussock grassland soils unless there was a uniform cover of haresfoot or other clovers (Lowther & Patrick 1994).…”

“…Populations of >100,000 clover rhizobia per gram of soil have been shown to occur in pastoral soils (Hale 1980(Hale , 1981Gaur & Lowther 1980;Patel & Lambert 1985;Elliot 1997), and once established in the soil, rhizobia can survive in the absence of clover as long as soil conditions, particularly soil pH, are suitable (Greenwood 1965). Populations of clover rhizobia have been shown to be higher in the rhizosphere of clover roots that in the surrounding soil (Pryor & Crush 2006).…”