High spatial variation in population size and symbiotic performance of Rhizobium leguminosarum bv. trifolii with white clover in New Zealand pasture soils

Abstract

Biological nitrogen fixation through the legume-rhizobia symbiosis is important for sustainable pastoral production. In New Zealand, the most widespread and valuable symbiosis occurs between white clover (Trifolium repens L.) and Rhizobium leguminosarum bv. trifolii (Rlt). As variation in the population size (determined by most probable number assays; MPN) and effectiveness of N-fixation (symbiotic potential; SP) of Rlt in soils may affect white clover performance, the extent in variation in these properties was examined at three different spatial scales: (1) From 26 sites across New Zealand, (2) at farm-wide scale, and (3) within single fields. Overall, Rlt populations ranged from 95 to >1 x 108 per g soil, with variation similar at the three spatial scales assessed. For almost all samples, there was no relationship between rhizobia population size and ability of the population to fix N during legume symbiosis (SP). When compared with the commercial inoculant strain, the SP of soils ranged between 14 to 143% efficacy. The N-fixing ability of rhizobia populations varied more between samples collected from within a single hill country field (0.8 ha) than between 26 samples collected from diverse locations across New Zealand. Correlations between SP and calcium and aluminium content were found in all sites, except within a dairy farm field. Given the general lack of association between SP and MPN, and high spatial variability of SP at single field scale, provision of advice for treating legume seed with rhizobia based on field-average MPN counts needs to be carefully considered.

Host‐specific competitiveness to form nodules in Rhizobium leguminosarum symbiovar viciae

“…Indeed, local variation of the relative composition of Rlv communities between soil plots in each site is large and in the same range as the variation between geographic locations. Systematic analyses have already revealed high local variation of Rlt population sizes and their symbiotic potentials (Wakelin et al , ). Whether these variations are the consequences of independent evolution trends of the bacteria, agronomic practices, the presence of different hosts, environmental parameters or soil composition favouring specific groups of isolates remain to be investigated.…”

“…The nodM , N , T and O genes encode, respectively, glucosamine synthases (likely involved in the synthesis of the GlcNAc Nod factor moiety), a putative outer membrane translocator, and a cation‐specific channel. These are not essential for nodulation but may modulate the process (Economou et al , ; Baev et al , ; Wakelin et al , ). Consistent with a role in modulating nodulation capacities in a host‐specific manner, the nodM and nodT genes have been implicated in cultivar‐specific nodulation of Rlt (Lewis‐Henderson & Djordjevic, ,b), and nodO was found to complement loss of nodE , an essential determinant of host specificity in Rlv , which is required for the synthesis of NFs with unsaturated acyl moieties (Downie & Surin, ; Demont et al , ).…”

Amplicons and isolates:Rhizobiumdiversity in fields under conventional and organic management

“…The UK site, which had a uniform and unique composition of only gsB, has the highest phosphorus (P) content. P content has been shown to be correlated with rhizobium population size in a previous study, but no genotyping was done (Wakelin et al, 2018). The high nutrient content might enhance the fitness difference between fast-and slow-growing Rlt strains (Leff et al, 2015), thereby driving the population differences between high and low P sites.…”

“…A study of spatial variation of Rhizobium symbiotic performance used a field sampling layout to test the effect at different spatial scales (Wakelin et al, 2018). Similar study designs, with neighbouring fields managed in different ways, would be appropriate for a more in-depth assessment of the effect of management on Rhizobium populations while disentangling it from geographical and soil chemical variation.…”

Partner preference in the legume-rhizobia symbiosis and impact on legume inoculation strategies

“…In soil and rhizosphere, bacteria form complex populations which composition vary depending on environmental conditions (Fierer & Jackson, 2006;Kaiser et al, 2016;Rousk et al, 2010). Rhizobia represent a little part of soil bacteria (<1%) but high local variations in population size and symbiotic performances are often observed (Wakelin et al, 2018). Although different protocols were designed to study rhizobia populations from soil samples, their identification and direct quantification remain difficult, mainly because of their low density in soils combined with quality issues of the DNA extracted from soil samples (Laguerre, Bardin, & Amarger, 1993;Sullivan, Eardly, van Berkum, & Ronson, 1996;Zézé, Mutch, & Young, 2001).…”