Selection of Competitive and Efficient Rhizobia Strains for White Clover.

Pilar Irisarri,Gerónimo Cardozo,Pamela Gutiérrez,Carolina Tartaglia,Jorge Monza,Rafael Reyno,Fernando A Lattanzi,Mónica Rebuffo

doi:10.3389/fmicb.2019.00768

Abstract

The practice of inoculating forage legumes with rhizobia strains is widespread. It is assumed that the inoculated strain determines the performance of the symbiosis and nitrogen fixation rates. However, native-naturalized strains can be competitive, and actual nodule occupancy is often scarcely investigated. In consequence, failures in establishment, and low productivity attributed to poor performance of the inoculant may merely reflect the absence of the inoculated strain in the nodules. This study lays out a strategy followed for selecting a Rhizobium leguminosarum sv. trifolii strain for white clover (Trifolium repens) with competitive nodule occupancy. First, the competitiveness of native-naturalized rhizobia strains selected for their efficiency to fix N2 in clover and tagged with gusA was evaluated in controlled conditions with different soils. Second, three of these experimental strains with superior nodule occupancy plus the currently recommended commercial inoculant, an introduced strain, were tested in the field in 2 years and at two sites. Plant establishment, herbage productivity, fixation of atmospheric N2 (15N natural abundance), and nodule occupancy (ERIC-PCR genomic fingerprinting) were measured. In both years and sites, nodule occupancy of the native-naturalized experimental strains was either higher or similar to that of the commercial inoculant in both primary and secondary roots. The difference was even greater in stolon roots nodules, where nodule occupancy of the native-naturalized experimental strains was at least five times greater. The amount of N fixed per unit plant mass was consistently higher with native-naturalized experimental strains, although the proportion of N derived from atmospheric fixation was similar for all strains. Plant establishment and herbage production, as well as clover contribution in oversown native grasslands, were either similar or higher in white clover inoculated with the native-naturalized experimental strains. These results support the use of our implemented strategy for developing a competitive inoculant from native-naturalized strains.

Highlights

Most legumes establish symbiosis with rhizobia, bacteria capable of fixing atmospheric nitrogen in a process referred to as biological nitrogen fixation (BNF)
Red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) are inoculated in Uruguay since 1967 with Rhizobium leguminosarum sv. trifolii strain U204 introduced from the United States (Labandera et al, 1982)
This practice is done because the soils have native-naturalized R. leguminosarum sv. trifolii strains without appropriate symbiotic efficiency

Summary

Introduction

Most legumes establish symbiosis with rhizobia, bacteria capable of fixing atmospheric nitrogen in a process referred to as biological nitrogen fixation (BNF). Trifolii strain U204 introduced from the United States (Labandera et al, 1982) In clovers, this practice is done because the soils have native-naturalized R. leguminosarum sv. This practice is done because the soils have native-naturalized R. leguminosarum sv. These less efficient indigenous rhizobia strains can be more competitive than the inoculant and occupy a significant portion of the nodules, reducing the impact of the inoculant strain on herbage production (Rodríguez et al, 2010; Yates et al, 2011). In Uruguay white clover is a widely used forage legume, both in ley farming pastures and oversown into native grasslands (Wilman et al, 2005), contributing substantially to the nitrogen budget of these systems (Mallarino and Wedin, 1990)

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