Legume inoculant application methods: effects on nodulation patterns, nitrogen fixation, crop growth and yield in narrow-leaf lupin and faba bean

AIMS: Liquid and granular rhizobial inoculants have some practical advantages for delivering rhizobial inoculants to legume crops in terms of ease-of-use and in separating rhizobia from potentially harmful seed-applied pesticides. The aim of this research was to determine whether inoculant application methodologies altered the patterns of nodulation on roots, inputs of symbiotic nitrogen (N₂) fixation, the accumulation of legume shoot dry matter (DM), grain yield, and grain nitrogen (N). METHODS: Eight field experiments were established at four different locations in south-eastern Australia to quantify the response of lupin (Lupinus angustifolius L.) and faba bean (Vicia faba L.) to three inoculant application methods (on-seed application as a peat slurry, in-furrow peat inoculant delivered as a liquid suspension at seeding, in-furrow peat granules delivered at seeding) compared with uninoculated treatments. N₂ fixation was assessed using the ¹⁵N natural abundance method and canola was included as a non-legume reference. RESULTS: Inoculation significantly improved crown nodulation, from 0.05 to 13 nodules plant⁻¹ in lupin at two sites and from 0.17 to 21.3 nodules plant⁻¹ in faba bean at three sites. Nodulation responses were decreased for faba bean treatments at sites with low pH, and for both lupin and faba bean at sites where soils contained large populations of naturally-occurring rhizobia. Inoculation increased grain yield from 0.48 to 1.94 t ha⁻¹ in faba bean relative to uninoculated treatments at two sites; N₂ fixation increased by 175 kg N ha⁻¹ in lupin at one site and by 46 to 280 kg N ha⁻¹ in faba bean at two sites. The different inoculant application methods led to minor differences in crown and lateral root nodulation patterns but only impacted N₂ fixation and grain yield at one site with faba bean, where peat slurry treatments had 186 to 195 kg N ha⁻¹ more N₂ fixation than other treatments and peat slurry and granules provided 0.8 to 1.0 t ha⁻¹ more grain yield than liquid inoculants. CONCLUSION: On-seed application of peat slurry always provided the best nodulation, grain yield and N₂ fixation. Small changes in nodulation patterns using in-furrow inoculants only resulted in reduced N₂ fixation in faba bean at one site. At that site faba bean grain yield was reduced by 1.0 t ha⁻¹ in liquid inoculant treatments, compared with on-seed peat slurry treatments.