Nitrogen Fixation by US and Middle Eastern Chickpeas with Commercial and Wild Middle Eastern Inocula

Abstract

Chickpeas (Cicer arietinum L.) are native to the Middle East (ME) and must be inoculated with symbiotic bacteria (Mesorhizobium ciceri) to fix nitrogen (N) in North American soils. Whether commercial M. ciceri strains are more or less effective than wild strains from ME soils when paired with various chickpea hosts must be elucidated. Wild N-fixing bacterial strains were isolated from ME soils, and their effectiveness was compared against commercial strains on US and ME chickpea varieties. Chickpeas were inoculated with individual strains and grown in chambers for 8 weeks. Plants received 2 mM (15NH4)2 SO4 (5% atom excess) to measure N fixation by isotope dilution. Plant below- and above-ground biomass and proportion of N fixed (PNF) were determined. Commercial and wild ME strains were examined for genetic diversity by sequencing their 16 S rDNA region. The PNF was significantly influenced by inoculant strains and chickpea varieties. Among varieties, Sierra, Troy, and Almaz had the highest PNF of 86.7%, 85.3%, and 85.2%, respectively. Among strains, Jord-M1 contributed to greater PNF (84.7%) compared to Syr-M1 (81.4%). Overall, chickpea varieties had greater effect on PNF than strain selection. These findings support efforts focusing on varietal breeding and strain selection to increase agricultural N fixation.