Native Rhizobia Improve Plant Growth, Fix N(2), and Reduce Greenhouse Emissions of Sunnhemp More than Commercial Rhizobia Inoculants in Florida Citrus Orchards

Sunnhemp (Crotalaria juncea L.) is an important legume cover crop used in tree cropping systems, where there is increased interest by growers to identify rhizobia to maximize soil nitrogen (N) inputs. We aimed to isolate and identify native rhizobia and compare their capabilities with non-native rhizobia from commercial inoculants to fix atmospheric dinitrogen (N(2)), produce and reduce nitrous oxide (N(2)O), and improve plant growth. Phylogenetic analyses of sequences of the 16S rRNA and recA, atpD, and glnII genes showed native rhizobial strains belonged to Rhizobium tropici and the non-native strain to Bradyrhizobium japonicum. Plant nodulation tests, sequencing of nodC and nifH genes, and the acetylene-dependent ethylene production assay confirmed the capacity of all strains to nodulate sunnhemp and fix N(2). Inoculation with native rhizobial strains resulted in significant increases in root and shoot weight and total C and N contents in the shoots, and showed greater N(2)-fixation rates and lower emissions of N(2)O compared to the non-native rhizobium. Our results suggest that native rhizobia improve plant growth, fix N(2), and reduce greenhouse emissions of sunnhemp more than commercial rhizobia inoculants in Florida citrus orchards.