Competitive fitness and stability of ammonium-excreting Azotobacter vinelandii strains in the soil

Abstract

Non-symbiotic N2-fixation would greatly increase the versatility of N-biofertilizers for sustainable agriculture. Geneticmodification of diazotrophic bacteria has successfully enhanced NH4+ release. In this study, we compared the competitivefitness of A. vinelandii mutant strains, which allowed us to analyze the burden of NH4+ release under a broad dynamic range.Long-term competition assays under regular culture conditions confirmed a large burden for NH4+ release, exclusion by thewt strain, phenotypic instability, and loss of the ability to release NH4+. In contrast, co-inoculation in mild autoclaved soilshowed a much longer co-existence with the wt strain and a stable NH4+ release phenotype. All genetically modified strainsincreased the N content and changed its chemical speciation in the soil. This study contributes one step forward towardsbridging a knowledge gap between molecular biology laboratory research and the incorporation of N from the air into thesoil in a molecular species suitable for plant nutrition, a crucial requirement for developing improved bacterial inoculantsfor economic and environmentally sustainable agriculture.