Effect Of Inoculation On Nodule Occupation and Rhizobia Diversity Of Cover Crop Legumes.

Commercial rhizobia inoculant is often added to increase legume biological nitrogen fixation (BNF) and is of particular importance in organic farming systems where cover crop BNF is a leading fertility source.  To maximize nitrogen contribution, a better understanding of inoculation effectiveness in the presence of established soil rhizobia populations is needed.  The winter annual legume cover crops Trifolium incarnatum L. (crimson clover), Vicia villosa Roth (hairy vetch), Pisum sativum subsp. Arvense L.  (Austrian winter pea), and Vicia villosa Roth (woolypod vetch) were planted in a randomized split plot design with and without commercial seed inoculation on three organically managed farms. Legume biomass, biomass nitrogen, nodule number, and nodule dry weight were measured in spring 2011, and Most Probable Number (MPN) assays carried out to determine rhizobia population sizes in sampled field soils.  A total of 576 rhizobia strains isolated from surface sterilized nodules of subsampled plants from all treatments were fingerprinted using rep-PCR to determine origin and diversity of nodule rhizobia strains.  Across both inoculated and un-inoculated treatments, plant biomass nitrogen ranged from 80 to 206 kg ha^-1. At three field sites, legume inoculation did not result in an increase in plant biomass, biomass nitrogen, nodule number, or mass.  A majority of rhizobia isolates belong to 13 DNA fingerprint clusters whose occupants were over 70% similar to other isolates in that cluster, typically not grouping by cover crop host species, farm location, or inoculation treatment.  Surprisingly, as few as 8% of strains isolated from inoculated nodules had DNA fingerprints closely related to the commercial inoculant.  Results suggest a complex rhizobia ecology may be present in organic farm soils resulting from high levels of competitive native rhizobia populations established through diverse cultivation histories.