Increased competitiveness and efficiency of biological nitrogen fixation in peanut via in-furrow inoculation of rhizobia

Abstract

Inoculation of legume crop plants with effective rhizobial strains provides an environmentally-friendly alternative to chemical fertilization, to increase productivity. Viable rhizobia can be introduced into the agricultural ecosystem via " on-seed inoculation" , or by direct application into the soil, termed " in-furrow inoculation" Peanut (Arachis hypogaea L.) is a major legume crop grown in central Argentina. Rhizobial inoculation of peanut is a controversial practice because nodulation by native bacteria is often assumed to be sufficient. We used lab models simulating on-seed and in-furrow inoculation methods to compare effects of these methods on efficiency of nitrogen fixation, and competitiveness, for introduced, recommended Bradyrhizobium strain C-145 vs. native strains found in soil. In-furrow inoculation, compared to on-seed inoculation, gave higher values for most of the efficiency parameters, and higher biomass of peanut plants. Competitiveness assays based on nodule occupancy clearly demonstrated interference by native strains on nodulation of C-145 introduced by either method. The competitiveness of native strains was more pronounced when C-145 was inoculated on-seed; i.e., C-145 occupied <10% of nodules even when inoculated in a 99:1 ratio to native strains. For in-furrow inoculation, nodule occupancy by C-145 increased from ~4% to ~60% as its ratio to native strains increased from 1:1 to 999:1. These findings clearly indicate the superiority of in-furrow inoculation over on-seed inoculation, in terms of nitrogen-fixing efficiency, competitiveness, and legume crop production.