a CSIRO Agriculture, Private Bag No. 5, Wembley WA 6913 Australia
b CSIRO Agriculture, PMB No. 2, Glen Osmond SA 5064 Australia
Much of the demand for nitrogen (N) in cereal cropping systems is met by using N fertilisers, but the cost of production is increasing and there are also environmental concerns. This has led to a growing interest in exploring other sources of N such as biological N2 fixation. Non-symbiotic N2 fixation (by free-living bacteria in soils or associated with the rhizosphere) has the potential to meet some of this need especially in the lower input cropping systems worldwide. There has been considerable research on non-symbiotic N2 fixation, but still there is much argument about the amount of N that can potentially be fixed by this process largely due to shortcomings of indirect measurements, however isotope-based direct methods indicate agronomically significant amounts of N2 fixation both in annual crop and perennial grass systems. New molecular technologies offer opportunities to increase our understanding of N2-fixing microbial communities (many of them non-culturable) and the molecular mechanisms of non-symbiotic N2 fixation. This knowledge should assist the development of new plant-diazotrophic combinations for specific environments and more sustainable exploitation of N2-fixing bacteria as inoculants for agriculture. Whilst the ultimate goal might be to introduce nitrogenase genes into significant non-leguminous crop plants, it may be more realistic in the shorter-term to better synchronise plant-microbe interactions to enhance N2 fixation when the N needs of the plant are greatest. The review explores possibilities to maximise potential N inputs from non-symbiotic N2 fixation through improved management practices, identification of better performing microbial strains and their successful inoculation in the field, and plant based solutions.
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