Science losing its way: examples from the realm of microbial N2-fixation in cereals and other non-legumes

Abstract

Background

Despite more than 50 years of research, no robust evidence suggests that inoculation of cereals and other non-legumes with free-living and/or endophytic bacteria leads to fixation of agronomically significant quantities of dinitrogen gas (N₂) from the atmosphere. A plethora of new products claims to increase the growth and yields of major cereals and other crops through stimulating N₂-fixation by inoculating with bacteria.

Scope

We review the literature on N₂-fixation by bacteria in the rhizosphere and as endophytes in non-legume plants. We find no unequivocal evidence that these bacteria fix agriculturally significant amounts of N₂ from the atmosphere in non-legumes. Research since the 1930s has followed repeated, overlapping cycles that have concluded that plant-growth-promoting hormones were the primary reason for crop response to microbial inoculants.

Conclusion

We contend that regulations are required to prevent the sale of inoculant products with unsubstantiated and spurious claims. Such regulations should require that unequivocal evidence is provided and independently verified, that: (i) the inoculant bacterium can fix N₂ from the atmosphere (i.e. that it possesses all the genes required to make nitrogenase), (ii) it has a clear mechanism to protect nitrogenase from poisoning by free oxygen, (iii) the bacterium is present in sufficient numbers throughout the growth cycle of the plant, (iv) that enhanced respiration can be detected from the putative N₂-fixing tissues, (v) that inoculation of the non-legume growing in an N-free medium leads to prolific growth and accumulation of nitrogen, and (vi) more than one method is used to confirm quantitatively significant inputs from N₂-fixation in the field.