Microbial products that increase nutrient availability to crops have been of growing interest in recent years. Many products claim to increase the growth and yields of major crops by stimulating nitrogen fixation in soil.
A team of international experts set out to review the available evidence on whether inoculating cereals and other non-legumes with free-living and/or endophytic bacteria leads to the fixation of agronomically significant quantities of dinitrogen gas (N2) from the atmosphere. In this global meta-analysis, the scientists compiled 7859 data pairs from 551 field experiment-based articles published between 1972 and 2022 for a wide range of food crops.
They conclude that there is no clear evidence that these bacteria fix agriculturally significant amounts of N2 from the atmosphere in non-legumes. On the other hand, there is evidence that the observed crop responses to microbial inoculants might be due to plant growth-promoting hormones.
These findings will likely raise questions. Some may also contest them or claim that newly developed products may have overcome past limitations and do fix significant amounts of N2, even in the presence of fertilisers. However, there is a clear need for more rigorous, independent research and field evaluation. Furthermore, the authors define six criteria for regulating inoculant products, which would require unequivocal evidence that:
- the inoculant bacterium can fix N2 from the atmosphere (i.e. that it possesses all the genes required to make nitrogenase)
- it has a clear mechanism to protect nitrogenase from poisoning by free oxygen
- the bacterium is present in sufficient numbers throughout the growth cycle of the plant
- enhanced respiration can be detected from the putative N2-fixing tissues
- inoculation of the non-legume growing in an N-free medium leads to prolific growth and accumulation of nitrogen
- more than one method was used to confirm quantitatively significant inputs from N2-fixation in the field
Developers of microbial products should apply these criteria to their work and similar principles should be followed for other biostimulants.
