The current and potential contribution of asymbiotic nitrogen fixation to nitrogen requirements on farms: a review

Abstract

Significant levels of biological nitrogen fixation from sources other than nodulated legumes have become a tantalizing prospect for decades. Since the benefit to agriculture of nitrogen fixation from nodulated legumes was established, there have been widespread efforts to promote the use of various asymbiotic diazotrophic bacteria to fix extra nitrogen in soil. Despite much optimism by scientists and farmers, this prospect remains to be realised. Recently, the prospect has been pursued with renewed enthusiasm and several commercialised products have appeared. What are the reasons for this fresh enthusiasm? Are the new products based on realistic assessments of their biological potential? Why has it taken so long to advance to a stage where there is still only limited evidence that verifies hope becoming reality? This review assesses the current contribution from asymbiotic nitrogen fixation and re-assesses the prospects for greater contributions from this source. Among the many aspects of this multi-faceted subject that will be considered are: (i) the range of free-living microbial strains currently contributing to signficant asymbiotic nitrogen fixation; (ii) the significance of nitrogen-fixing microbes naturally associated with plants; (iii) the significance of endophytic systems and their role in sugarcane and other Gramineae; (iv) the possibility of extending this range by introducing new strains or discovering new systems capable of contributing additional nitrogen fixation. The case will be made that conditions providing a sustainable contribution for more than a short time are usually missing in such systems so that spontaneous biological nitrogen fixation is usually transient. It will be argued further that if all the positive factors controlling spontaneity at the biothermodynamic level are exploited, significant biological nitrogen fixation may soon be achieved in some of these systems on farms.