Use of nitrogen fixing bacteria inoculants as a substitute for nitrogen fertiliser for dryland graminaceous crops: Progress made, mechanisms of action and future potential

A range of free-living and endophytic N2 fixing bacteria have been isolated and used as inoculants on non-legume plants in attempts to maintain or increase yield while reducing the need for fertiliser N. Here, the literature on inoculation of dryland graminaceous crops with N2 fixing bacteria in temperate and tropical agricultural systems is reviewed and the progress made, mechanisms of action of the bacteria and future potential of this approach, assessed. Firstly, we consider the use of Azotobacter spp. in Russia in the 1940s and 1950s and Azospirillum spp. worldwide in the 1970s and 1980s. In both cases, effects on yield were inconsistent. Secondly, factors affecting yield responses to inoculation with Azotobacter and Azospirillum are discussed. A major weakness of using these bacteria as a substitute for N fertiliser is related to their mechanism of action. Evidence indicates that any additional N taken up by plants inoculated with Azotobacter and Azospirillum was primarily derived from the soil and not N2 fixation. It is stressed that the general N deficiency of soil cannot be countered consistently by a procedure that does not add substantial N to the system. Thirdly, we focus on recent projects which have sought N2 fixing bacterial inoculants for graminaceous crops which do add substantial N to the system. Effects of Agrobacterium radiobacter on wheat and barley, Azorhizobium caulinodans on wheat and Gluconacetobacter diazotrophicus, Herbaspirillum spp. and mixed endophytes on sugarcane and maize are considered. It is concluded that currently, no N2 fixing bacterial inoculant is available which can match N fertiliser in consistency to counter soil N deficiency. Endophytic bacteria may have potential as inoculants but substantial experimentation is required before this can be adequately assessed.