Nitrogen contributions from faba bean (Vicia faba L.) reliant on soil rhizobia or inoculation

Understanding the impact of soil rhizobial populations and inoculant rhizobia in supplying sufficient nodulation is crucial to optimising N-2 fixation by legume crops. This study explored the impact of different rates of inoculant rhizobia and contrasting soil rhizobia on nodulation and N-2 fixation in faba bean (Vicia faba L.). Faba beans were inoculated with one of seven rates of rhizobial inoculation, from no inoculant to 100 times the normal rate of inoculation, sown at two field sites, with or without soil rhizobia present, and their nodulation and N-2 fixation assessed. At the site without soil rhizobia, inoculation increased nodule number and increased N-2 fixation from 21 to 129 kg shoot N ha(-1), while N-2 fixation increased from 132 to 218 kg shoot N ha(-1) at the site with high background soil rhizobia. At the site without soil rhizobia, inoculation increased concentrations of shoot N from 14 to 24 mg g(-1), grain N from 32 to 45 mg g(-1), and grain yields by 1.0 Mg (metric tonne) ha(-1). Differences in nodulation influenced the contributions of fixed N to the system, which varied from the net removal of 20 kg N ha(-1) from the system in the absence of rhizobia, to a net maximum input of 199 kg N ha(-1) from legume shoot and root residues, after accounting for removal of N in grain harvest. The impact of inoculation and soil rhizobia strongly influenced grain yield, grain N concentration and the potential contributions of legume cropping to soil N fertility. In soil with resident rhizobia, N-2 fixation was improved only with the highest inoculation rate.