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

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. Additional keywords: diazotrophs, saprophytes, endophytes, biofertilisers, cereals, sugarcane. Introduction Since biological nitrogen fixation (BNF) was discovered in the late 19th century, there have been positive reports of the potential benefits from asymbiotic nitrogen fixation (ANF). Because of the greater ease of management, far more attention has been devoted to the symbiotic systems, based on the Rhizobium– legume interaction. In natural ecosystems, Frankia -based systems between plants and microbes are also recognised as reliable and important in habitat colonisation (Dawson 1990; Diem and Dommergues 1990). The highly organised nature of these symbiotic systems as controlled genotype × environment interactions is in strong contrast to the disorganised nature of the asymbiotic systems. The significance of these interactions in achieving the full potential of ANF is a major theme of this review. In an agricultural context the ‘asymbiotic’ systems are generally insignificant. This is partly because there can be no doubt of the much lower magnitude of the BNF involved in contrast to symbiotic nitrogen fixation (SNF), but also because the lack of obvious association with the plant means that the utilisation of ANF is less direct. The affinity with which fixed nitrogen is accumulated by plants from many sources ensures that tracing their use is very difficult. For the purposes of this review, the meaning of the term ‘asymbiotic’ must be clearly understood, particularly as it might now be considered a term less used than formerly and perhaps dated. In this article, asymbiotic will be used to include all BNF not occurring in a genetically defined system where there is a specific interaction between the diazotroph and the host plant resulting in recognisable morphological structures of nodules in the host plant, typical of SNF. Thus, this definition of asymbiotic includes any nitrogen fixation by microbial cells growing independently (free-living) in soil and in terrestrial and aquatic wetland environments as saprophytes, BNF occurring in loose or close association with the plant rhizosphere, and endophytically, but not requiring morphologically defined nodules. Although ANF in marine mangrove ecosystems is beyond the topic of