Rhizobial populations, diversity, nodulation, and its relevance to legume cropping

Slides presented during "How legume science is enabling industry "

Webinar 6: European legumes in transition 4th May 2021

"Rhizobial populations, diversity, nodulation, and its relevance to legume cropping"

Presented by: Dr Euan James - Researcher - James Hutton Institute- Nitrogen fixation

Faba bean (Vicia faba L.) and pea (Pisum sativum L.) form nitrogen (N)-fixing symbioses with Rhizobium leguminosarum sv. viciae (Rlv), but little is known about their capacity for biological N fixation (BNF) in northern Europe, especially after prolonged periods without legume cropping, and hence with potentially low populations of effective Rlv genotypes. This was tested on faba bean over a four-year period using the 15N natural abundance method in an experimental field-scale rotation with no history of legume cropping for more than 50 years. Soil rhizobial populations before crop sowing were determined using qPCR, and the nodulating rhizobia were typed via their 16S rRNA, nodA and nodD genes. Faba bean obtained most of its N through BNF (>80%) regardless of plant genotype or year (2012–2015). Soil rhizobial populations were c. 106 R. leguminosarum cells and c. 105 Rlv nodD gene copy numbers g-1 soil dry weight regardless of field or treatment. The nodA and nodD genes of 157 strains of rhizobia grouped them into two large clades of Rlv, one consisting mainly of V. faba strains and the other of strains isolated from various hosts (V. faba, P. sativum, Lens culinaris, Vicia and Lathyrus spp.). It is concluded that faba bean (and pea) has a high reliance on BNF, but this is only possible due to the presence in the soil of sufficient numbers of effective Rlv strains; in the prolonged absence of legume cropping these may be derived from adjacent legume-cropped fields, and possibly also from native legumes in the field margins.