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

Steven Wakelin,Maureen O’Callaghan,Rene Geurts,Ross Ballard,Matthew Brown,Elizabeth Farquharson,Emily Gerard,Robert Van Ham,Hayley Ridgway,Guyléne Tillard,Tzen-Yuh Chiang

doi:10.1371/journal.pone.0192607

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.

Highlights

Legumes, in symbiosis with rhizobia bacteria, can fix atmospheric nitrogen into plant available forms [1]
The mean values for Rhizobium leguminosarum bv. trifolii (Rlt) populations collected at farm or field spatial scales did not significantly vary (P>0.05; Fig 2A)
Across the NZ wide and Whangara farm sample sets, strong and significant correlations between most probable number (MPN) and soil NO3-N content were determined. While this was negative for Whangara (r = -0.605), i.e. MPNs increased as NO3-N declined, the reverse was true for the NZ wide sampling (r = 0.789; Table 1)

Summary

Introduction

In symbiosis with rhizobia bacteria, can fix atmospheric nitrogen into plant available forms [1]. Constraints can include hostile soil conditions (e.g. pH, high Al3+ content, P-infertility), adverse climatic conditions (soil moisture deficit, extremes of temperature), through to pests, diseases, and weeds (see review by Tozer et al 2013 [5] and references therein) In many cases, these can be addressed through careful selection of legume species that match the soil and environmental conditions, and effective management of the farming system such as timing of over-sowing and controlling of grazing pressure [5]. The success of legumes in agricultural systems is affected by the availability of sufficient population sizes of nodule-forming (nod+) and nitrogen-fixing (fix+) rhizobia in soils [6,7]. These determine if legumes can enter nodule-forming symbioses, and how effectively the symbiosis can fix nitrogen

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