Nitrogen Fixation by U.S. and Middle Eastern Chickpeas with Commercial or Wild Middle Eastern Inoculum.

 

Chickpeas (Cicer arietinum L.) are native to the Middle East (ME), and must be inoculated with symbiotic bacteria in order to fix nitrogen (N) in North American soils. Commercial inocula for chickpea contain several strains of the known N-fixing symbiont Mesorhizobium ciceri.  It is not known whether the available M. ciceri strains are more or less effective than other wild strains from ME soils when paired with various chickpea variety hosts.  This study was conducted to isolate N fixing bacteria symbiotic with chickpea from ME soils and to compare their effectiveness against commercial strains on U.S. and ME varieties of chickpeas. Chickpeas were inoculated with individual strains and grown in growth chambers for 8 weeks. Plants received 2 mM (¹⁵NH₄)₂ SO₄ (5% atom excess) to allow measurement of N fixation by isotope dilution. Below and above ground biomass and the proportion of plant N provided by fixation (PNF) were determined. Commercial and wild ME strains were characterized by sequencing the 16S rDNA region. The PNF was significantly influenced by different inoculant strains, and by the variety of chickpea. Varieties Sierra, Troy and Almaz had the greatest PNF. Among strains, Jord-M1 contributed to greater PNF than Syr-M1. Overall, chickpea varieties had a greater effect on PNF than did strain selection. This finding supports efforts to increase agricultural N fixation that focus on varietal breeding and selection. Wild ME symbionts belonged to the genus Burkholderia, and this discovery warrants the study of N-fixing bacteria beyond the currently known symbionts.