Enhancing Non-symbiotic N2 Fixation in Agriculture

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 N 2 fixation. Non-symbiotic N 2 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 nonsymbiotic N 2 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 N 2 fixation both in annual crop and perennial grass systems. New molecular technologies offer opportunities to increase our understanding of N 2 -fixing microbial communities (many of them non-culturable) and the molecular mechanisms of non-symbiotic N 2 fixation. This knowledge should assist the development of new plant-diazotrophic combinations for specific environments and more sustainable exploitation of N 2 -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 N 2 fixation when the N needs of the plant are greatest. The review explores possibilities to maximise potential N inputs from non-symbiotic N 2 fixation through improved management practices, identification of better performing microbial strains and their successful inoculation in the field, and plant based solutions.

Plant growth-promoting rhizobacteria able to improve NPK availability: selection, identification and effects on tomato growth

“…In cereals, the estimates of biological N2 fixation by free-living bacteria generally range between 5 and 50 kg N ha -1 yr -1 in enriched soil environments and plant residues (Roper and Gupta, 2016). The process of N2 fixation varies between different bacterial genera.…”

Vermicompost and cow dung admixture increases rhizosphere bacterial population and promotes rapid physiological maturity in Maize (Zea mays L.)

“…The study also revealed that the ANFix group of bacteria was closely associated with OrgC. Roper and Gupta [60] inferred that the non-symbiotic nitrogen fixing bacteria are largely dependent upon the carbon stock of the soil, as the organic carbon resources are essential for their enzyme activity. The present findings were in agreement with the above statement.…”

A preview of perennial grain agriculture: knowledge gain from biotic interactions in natural and agricultural ecosystems

“…Diazotrophic nitrogen fixation.-While atmospheric N 2 -fixing bacteria including Rhizobium and Bradyrhizobium form symbiotic relationships with legumes, many grasses and gramineous crops have developed non-symbiotic associations with diazotrophic, free-living bacteria capable of fixing atmospheric N 2 (e.g., Azospirillum sp., Azoarcus sp., Azotobacter sp., Burkholderia sp.). Diazotrophs have been identified in high abundance and diversity in association in the rhizosphere of gramineous C3 and C4 crops such as wheat, maize, rice, sugarcane as well as in many perennial grass species (Roper and Gupta 2016). Diazotrophic community size and composition was found to vary among plant species and plant varieties, including maize (Rodriguez-Blanco et al 2015), sorghum (Coelho et al 2009), switchgrass (Rodrigues et al 2017), and rice (Engelhard et al 2000, Knauth et al 2005.…”