Abstract
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 N2fixation. Non-symbiotic N2fixation (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 N2fixation, 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 N2fixation 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 N2fixation. 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 N2fixation when the N needs of the plant are greatest. The review explores possibilities to maximise potential N inputs from non-symbiotic N2fixation through improved management practices, identification of better performing microbial strains and their successful inoculation in the field, and plant based solutions.
Highlights
Non-symbiotic (NS) N2 fixation includes N2 fixation by free-living soil bacteria that are not in a direct symbiosis with plants, and associative N2-fixation
This review summarizes the current knowledge on NS N2 fixation, including measurement techniques, factors that control the function, ecology of N2-fixing bacteria and identifies opportunities to harness this biological process for production and environmental benefits
Further examination of such oligotrophic systems may yield diazotrophic communities that could be adapted to agricultural systems where they might increase the contribution from associative N2 fixation in agricultural crops
Summary
Non-symbiotic (NS) N2 fixation includes N2 fixation by free-living soil bacteria (autotrophic and heterotrophic) that are not in a direct symbiosis with plants, and associative N2-fixation (e.g. associated with the rhizospheres of grasses and cereals). The demand for N fertilisers is expected to exceed 112 million tonnes in 2015 [1] and much of this is produced by the Haber-Bosch process [2], a process which uses large amounts of fossil fuel [3]. This, together with the increasing demand for organically grown agricultural and horticultural products, and the need to address economic and environmental concerns, has rekindled interest in promoting biological N2 fixation in non-leguminous crops. Cleveland et al [7] estimated that the potential global biological N2 fixation (symbiotic and NS) in natural ecosystems is between 100 and 290 million tonnes N year-1. This review summarizes the current knowledge on NS N2 fixation, including measurement techniques, factors that control the function, ecology of N2-fixing bacteria and identifies opportunities to harness this biological process for production and environmental benefits
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