The current and potential contribution of asymbiotic nitrogen fixation to nitrogen requirements on farms: a review

Abstract

Significant levels of biological nitrogen fixation from sources other than nodulated legumes have become a tantalizing prospect for decades. Since the benefit to agriculture of nitrogen fixation from nodulated legumes was established, there have been widespread efforts to promote the use of various asymbiotic diazotrophic bacteria to fix extra nitrogen in soil. Despite much optimism by scientists and farmers, this prospect remains to be realised. Recently, the prospect has been pursued with renewed enthusiasm and several commercialised products have appeared. What are the reasons for this fresh enthusiasm? Are the new products based on realistic assessments of their biological potential? Why has it taken so long to advance to a stage where there is still only limited evidence that verifies hope becoming reality?This review assesses the current contribution from asymbiotic nitrogen fixation and re-assesses the prospects for greater contributions from this source. Among the many aspects of this multi-faceted subject that will be considered are: (i) the range of free-living microbial strains currently contributing to signficant asymbiotic nitrogen fixation; (ii) the significance of nitrogen-fixing microbes naturally associated with plants; (iii) the significance of endophytic systems and their role in sugarcane and other Gramineae; (iv) the possibility of extending this range by introducing new strains or discovering new systems capable of contributing additional nitrogen fixation.The case will be made that conditions providing a sustainable contribution for more than a short time are usually missing in such systems so that spontaneous biological nitrogen fixation is usually transient. It will be argued further that if all the positive factors controlling spontaneity at the biothermodynamic level are exploited, significant biological nitrogen fixation may soon be achieved in some of these systems on farms.

Soil beneficial bacteria and their role in plant growth promotion: a review

“…The %N (N derived from the atmosphere) values were 19.5-38.7, and 38.1-58.2 in basmati and super basmati, respectively. Herbaspirillum seropedicae also acts as an endophytic diazotroph of wheat plants (Kennedy and Islam 2001), colonizing wheat roots internally between the cells. Herbaspirillum seropedicae is also found in roots and stems of sugarcane plant while Herbaspirillum rubrisubalbicans is an obligate endophyte of roots, stems and leaves (Reis et al 2000).…”

Global inputs of biological nitrogen fixation in agricultural systems

“…These values are very speculative but, with current knowledge, it is impossible to offer alternatives. A number of reviews of plant-associated N 2 fixation have clearly highlighted the many methodological problems and inconsistencies in the published studies (Boddey 1987;Chalk 1991;Giller 2001;Kennedy and Islam 2001;Giller and Merckx 2003). One of the key problems is distinguishing between inputs of N by free-living and associative agents and other external sources of N contributing to agricultural soils, e.g.…”

“…For example, Giller (2001) concluded that N 2 fixation by free-living bacteria would rarely exceed 5 kg N/ha/year. However, none of these publications offer a rate figure that could be applied globally, although Kennedy and Islam (2001) were optimistic that perceived problems in manipulating the associative systems to the point that they could be utilised more effectively would be overcome with additional research.…”

Role of Diazotrophic Bacteria in Biological Nitrogen Fixation and Plant Growth Improvement

“…On a global scale asymbiotic nitrogen fixation (ANF) contributes to about 30% to the total biologically fixed N, and can play a significant role in many terrestrial ecosystems (Cleveland et al, 1999). However, in an agricultural context, symbiotic BNF by legumes is considered to be by far the most dominant source of biologically fixed N (Kennedy and Islam, 2001). Other symbiotic systems with agricultural applications include the Azolla-Nostoc (Vaishampayan et al, 2001), and the Frankia-Alder symbioses (Paul and Clark, 1996).…”

“…Other symbiotic systems with agricultural applications include the Azolla-Nostoc (Vaishampayan et al, 2001), and the Frankia-Alder symbioses (Paul and Clark, 1996). All of these symbioses are genetically defined systems that include specific interactions between diazotroph and host-plant (Kennedy and Islam, 2001). …”