The greener nitrogen revolution: cereal and other non-legume crop symbiotic nitrogen fixation

Keywords: environmental pollution, Gluconacetobacter diazotrophicus, intracellular colonisation, non-legume crops, seed inoculum technology (N-Fix®), synthetic nitrogen fertilisers, yield improvement.

Despite nitrogen being the key to crop yields, the on-going pollution of the environment from the use of synthetic nitrogen fertilisers produced by the Haber-Bosch process is currently highlighting the need to minimise the use of nitrogen in agriculture: more than half of applied nitrogen fertilisers has been estimated to have been lost to the environment from cereal crops during the past forty years. This paper traces, in detail, the sporadic attempts made from early in the last century to try to enable cereals and other non-legume crops to imitate the ability of rhizobia in legume nodules to fix nitrogen, thereby minimising their need for synthetic nitrogen fertilisers. Initially attempts were made to nodulate cereals by their interaction with rhizobia isolated from legume nodules. This was later extended to the attempted nodulation of non-legume crops by a wide range of nitrogen-fixing bacteria, but all without success.

The commercial success of the Haber-Bosch process for nitrogen fixation delayed the development of any biological nitrogen fixation alternative. It was only with the discovery of the non-rhizobial nitrogen-fixing bacterium Gluconacetobacter diazotrophicus, ‘this most extraordinary diazotroph’ isolated from Brazilian sugarcane juice in the later part of the last century, that the opportunity for the development of a realistic biological nitrogen fixation alternative appeared. The discovery that this bacterium could intracellularly colonise systemically the roots and shoots of a very wide range of crop species, including cereals, without any nodulation led to major commercial interest. Seed inoculum technology based on Gluconacetobacter diazotrophicus (N-Fix®) is now being successfully used commercially in extensive international field trials to produce systemically colonised maize and wheat, and other non-legume crops. N-Fix is able to significantly improve yields of maize and wheat in both the presence and absence of synthetic nitrogen fertiliser. Evidence suggests that these benefits are accruing through a possible combination of intracellular symbiotic nitrogen fixation, enhanced rates of photosynthesis and the presence of additional plant growth factors, pointing the way forward to a greener nitrogen revolution.

Edward C. Cocking, Centre for Crop Nitrogen Fixation, University of Nottingham, Nottingham, United Kingdom.

22 pages, 6 figures, 1 table, 63 references