The prospect of N2-fixing crops galore!

temperature and pressure by reducing N≡N to reactive ammonia. All plant and animal eukaryotes lack the nitrogenase enzyme and are therefore unable to carry out this environmentally benign, biological nitrogen fixation. However, legume crops are able to biologically fix nitrogen by forming nodules in the cells in which they symbiotically fix nitrogen via diazotrophic rhizobia bacteria. Nitrogen is crucial to crop yields but cereals and other non-legume crops do not symbiotically fix nitrogen with rhizobia. This inability leaves agriculture increasingly reliant on the use of synthetic ammonia-based nitrogen fertilizers. These are produced chemically from nitrogen and hydrogen gas at high temperature and pressure in the catalytic Haber–Bosch process, the industrial equivalent of bacterial nitrogen fixation. Chemical fixation of nitrogen can also result naturally from the action of lightning on oxygen and nitrogen gases in the atmosphere yielding nitric acid. There is a thought-provoking science-fiction novel, The Nitrogen Fix by Hal Clement (1980), which is based on the proposition that the production of polluting nitric acid has used up all of the oxygen in the air, so that humans have to live in artificial environments protected from the all-encompassing acid (Figure 1). Although not as extreme, the overuse of synthetic nitrogen fertilizers has resulted in the pollution of aquatic systems by soluble nitrates with associated eutrophication and health hazards. In addition, the atmosphere has been polluted by ammonia and nitrous oxide, the latter being a potent greenhouse gas associated with photochemical smog, fine particulate pollution, ecosystem acidification and climate change. The environmental, energy and economic costs of producing and using ammonia-based fertilizers globally are no longer considered sustainable; if all crops were nitrogen fixing, then nitrogen sustainability of agriculture would be assured.