Symbiotic performance of Mediterranean Trifolium spp. with naturalised soil rhizobia

Abstract

Naturalised soil rhizobia that nodulate clover occur in high number and are known to vary in their symbiotic performance (SP) with subterranean clover (Trifolium subterraneum L.). However, the extent of suboptimal fixation across a range of other clover species is not well understood. T. subterraneum and nine other annual clover species of Mediterranean origin were evaluated for their SP in combination with the naturalised clover rhizobia in 71 Australian soils and five strains of Rhizobium leguminosarum bv. trifolii that have been used in the inoculants produced for clovers. The most probable number method, using subterranean clover as the trap plant was used to estimate the number of clover rhizobia in the soils. Ninety-two percent of soils tested contained more than 1000 rhizobia/g. An extract of each soil, or strain of rhizobia was used to inoculate plants growing in N-deficient media in the greenhouse. Plants were grown for 4 weeks after inoculation and shoot dry matter determined and expressed as a percentage of the ‘best’ soil rhizobia treatment, to provide a proportional measure of SP for each clover species. SP (mean of clover species) ranged from 96% with the current inoculant strain for annual clovers (WSM1325) down to 48% with former inoculant strain WU95. When inoculated with soils predominantly from mainland Australia, SP (mean of soil treatments) of the different Trifolium spp. was 55% (resupinatum), 53–47% (subterraneum), 50% (nigrescens), 49% (michelianum), 48% (isthmocarpum), 38% (hirtum), 35% (purpureum), 32% (vesiculosum), 25% (spumosum) and 21% (glanduliferum). Within each of the clover species, SP resulting from individual soil treatments ranged from 100% (by definition for the best soil treatment) down to close to zero. Trifolium glanduliferum formed nodules readily with the inoculant strains but nodulation was erratic with the rhizobia in many soils. It is therefore proposed that the naturalised rhizobia in many soils are unlikely to be inoculant strains. This research demonstrates symbiotic efficiency across annual clover species is compromised where diverse populations of clover rhizobia have naturalised in soils.

The Effect of Inoculation on Growth, Nodulation and Nitrogen in Annual Clover Cultivars Grown at Two Mediterranean Locations of Sardinia, Italy

“…Importantly, this is supported mostly by nodulation data and shoot N content and may explain the poor results of the selected inoculant with T. subterraneum plants in Bolotana. Moreover, this is in agreement with recent findings indicating a large diversity in the symbiotic capacity of clovers with naturalised soil rhizobia in Spain (Ramirez-Bahena et al, 2009) and Australia (Drew et al, 2011). Overall, the different response to inoculation recorded at the two sites do not allow us to indicate an univocal effect of inoculation which mostly appeared species and cv.…”

“…Our research contributes to clarify the role, in a southern European area, of a commercial rhizobial strain selected in Australia, indicating a sub-optimal performance of the selected strain with many of the tested clover cultivars. Interestingly, the plant selection is being considered as a potential tool to overcome sub-optimal N fixation associated with poorly effective soil rhizobia (Drew et al, 2011).…”

Partner diversity and identity impacts on plant productivity in Acacia–rhizobial interactions

“…; Drew et al . ; Thrall et al . ) and the physical environment within which the interaction takes place (Graham ).…”

Delayed inoculation of alfalfa with Sinorhizobium meliloti and Penicillium bilaiae

“…Reports of a lack of yield response in legumes to inoculation are common in the scientific literature and are typically thought to be the result of high initial populations of indigenous or naturalized rhizobia in the soil competing with the strains introduced in the inoculant (Thies et al 1991;Drew et al 2011). However, successful nodulation does not necessarily mean that the resulting nodulated legume is fixing N 2 optimally, as levels of BNF by these naturalized strains were often lower that those obtained with commercial inoculant strains (Drew and Ballard 2010;Drew et al 2011). Contrast analysis for year-of-seeding Rhizobium and Rhizobium plus P. bilaiae vs. delayed Rhizobium and Rhizobium plus P. bilaiae.…”

“…Although inoculation at seeding is a widely recommended practice, discussions with producers reveal that some do not inoculate seed and instead rely on existing soil populations of rhizobia to nodulate plants. These indigenous rhizobia are often ineffective strains, which are less efficient in fixing N 2 than introduced strains (MarquesPinto et al 1974;Thies et al 1991;Drew and Ballard 2010;Drew et al 2011). Little scientific information is available addressing situations where producers fail to inoculate the forage legume at seeding, or where inoculation fails and biological N fixation (BNF) is limited.…”