Agro-multinationals are eagerly jumping into biological alternatives to fertilisers: bacterial preparations that supposedly fix nitrogen from the air in a form that can be absorbed by plants. Independent experts have serious doubts about the sustainability of those claims and are calling for regulation.
You could call it the holy grail of agronomy. Crops that are not or much less dependent on artificial fertilisers and animal manure, because they manage to extract nitrogen from the air themselves, with or without the help of bacteria. Some plants can do that, leguminous plants such as beans, clover and lupins manage to entice the soil bacterium Rhizobium to ‘move in’ with them in the plant cell of a root nodule. In exchange for providing absorbable nitrogen, they receive energy-rich carbohydrates from their host.
However, major crops such as maize, rice, wheat and potatoes still depend on external sources of nitrogen. That supply is problematic, both because of the high price of fertilisers and because of the greenhouse gases emitted in their production and use. Hence the growing focus of agro-multinationals on biological nitrogen fixation.
In the Netherlands, both Corteva Agriscience and Syngenta have launched a bio stimulant that enables non-leguminous crops to fix nitrogen from the air. Corteva’s product is Methylobacterium symbioticum under the brand name BlueN® and Syngenta works with Azotobacter salinestris, brand name Vixeran. With both products, a grower could supposedly save 30 to 50 kilograms of nitrogen per hectare, equivalent to over 100 kilograms of nitrogen fertiliser.
Excitement
This is not the first time this form of biological nitrogen fixation has set hearts racing. Soil scientist Ken Giller, professor of production ecology in Wageningen, refers to an article he wrote 20 years ago with his Flemish colleague Roel Merckx about the repeating cycles of excitement and disappointment since the early 1960s.
In those days, crops in the former Soviet Union were widely treated with Azotobacterin, a nitrogen-fixing soil bacterium. With less fertiliser, higher yields would be possible. It later turned out that the higher yields were mainly due to the production of indole acetic acid (IAA), a plant growth hormone.
Disappointment all round, but that did nothing to prevent that ten years later there were again high expectations, this time from free-living soil bacteria of the genus Azospirillum spp. Again, however, the observed effects could be attributed to the increased production of indole acetic acid and other growth hormones. In the late 1980s, it was the same story again, this time around Gluconacetobacter diazotrophicus. This bacterium was also found to mainly stimulate the production of indole acetic acid while it also improved plant uptake of zinc and phosphorus.
Fourth wave
So, we are currently experiencing the fourth wave of excitement about biological nitrogen fixation. Once again, it seems to be heading for disappointment. For instance, independent research by a number of cooperating US universities showed that, with a few exceptions, the bacterial varieties on the market there have no effect on growth yields, even with lower rates of fertiliser application.
The researchers conducted a combined 61 years of field trials across 10 US states and with various crops, including, maize, sugar beet and oilseed rape. Application of bacterial preparations had no effect on yields in the vast majority – 59 out of 61 – of the field trials. Application of Utrisha – the US brand name of BlueN – produced conflicting results in one case, which on average amounted to a yield increase of about 12 kilograms per hectare. The rest of the trials, including the one with Syngenta’s US product showed only one effect.
Asked for a comment, Allard Jukema of Corteva Agriscience let it be known that data supporting their claims about BlueN’s yield improvement are based on a two-year study with more than 300 field trials at different sites in US maize producing states. He says trials in Europe on maize, winter wheat and potatoes in different locations also invariably show higher yields and better quality, whether or not combined with lower fertiliser application.
He cannot explain the results of the US trials other than with the assumption that the product was not applied correctly. “We now know through advancing insights that it is not simple to apply correctly” he says.
Positive effect
Although the US study involved a different bacterium than Syngenta is marketing in Europe (see box), we also asked this company for a comment. By email, the spokesperson let us know that field trials with Vixeran conducted in 2022 showed a greater effect than the results described in the US study. In 12 out of 29 trials, conducted in 11 countries, a positive effect was measured compared to a reduced fertilisation schedule. This ranged from just 2% to over 14% more yield.
About the difference with the US study, the company informs us, that the efficacy of live micro-organisms in living plants is influenced by a lot of things, including temperature, soil type, fertilisation, etc. “With such products, it is very important to try to understand why a product did not give the desired result in a particular trial. We see this as a kind of journey, in which we will continue to learn and fine-tune.”
Hormonal effects
Soil scientist Ken Giller cannot help but agree that very many factors play a role when it comes to the efficacy of nitrogen-fixing bacteria. “It is extremely difficult to determine whether nitrogen fixation is taking place,” he says. “In the past, effects, insofar as observable, were repeatedly found to be due to hormonal effects and I suspect this is the case again. Because there are so many confounding factors, a difference in yield of less than 10% on average, as in the case of Vixeran, says nothing about whether or not atmospheric nitrogen has been fixed for the plant.”
Similarly, photos showing bacteria established in the plant’s leaves and roots do not prove nitrogen fixation. “Endophytes colonise the spaces between cells and are not the cytoplasm, where they could have significance for the plant.” The same applies to the biofilm, the ‘slime layer’, which forms on the roots and, according to Syngenta, creates optimal conditions for nitrogen fixation and transport of fixed ammonium into the plant cell.
Genetic inability
In the case of BlueN, the bacteria used cannot fix nitrogen at all. Julie Ardley is a lecturer in molecular microbiology at Murdoch University in Perth, Australia, and one of the world’s leading experts on biological nitrogen fixation. By email, she reveals that claims of biological nitrogen fixation by Methylobacterium symbioticum, the active ingredient in BlueN, cannot be true, because the bacterium does not possess the necessary nif genes. The DNA sequence of the strain in question SB0023/3 was determined back in 2020 and can be found in the NCBI database.
However, in a comment (see our website vork.org), Corteva Agriscience informs that in addition to proven yield improvement, scientists could also verify that Methylobacterium symbioticum SB23 is capable of nitrogen fixation. They are probably referring to a previously sent study by the Technical University of Madrid and Spanish company Symborg SL, which would show that nitrogen fixation was demonstrated via the so-called delta-15N method. A somewhat complicated story that boils down to the fact that nitrogen captured from the air contains a smaller proportion of 15N than the nitrogen absorbed by the plant from the soil.
Murray Unkovich, a researcher at the University of Adelaide who specialises in the delta 15N method, is not impressed. By email, he reveals that the dataset from the study is not robust. “The results of the delta 15N analysis do not provide any evidence for nitrogen fixation and the amount of nitrogen captured is well within the margin of error. Apart from that, delta15N cannot be used as evidence for nitrogen fixation.”
Euan James reveals, when asked, that other factors are probably at play in this case too, such as the aforementioned hormonal action of the bacteria. Species of the genus Methylobacterium are known to be able to produce quite a lot of cytokinins, i.e. plant hormones. James works at the Scottish James Hutton Institute in Dundee and is considered an expert on biological nitrogen fixation. He finds the Spanish research lacks a good description of the field trials, such as data on nitrogen present in the soil.
Disappointment
In short, it seems that the fourth wave of excitement about biological nitrogen fixation is also going to end in disappointment. While Syngenta does assure that the bacterium in their product Vixera does have the necessary nif genes, researchers say that is no guarantee that nitrogen is actually fixed from the air.
According to Giller, it is essential that there is that guarantee. “Even if there were an effect on growth and yield, the lack of nitrogen fixation could have negative consequences, especially for small farmers in low-income countries. They are desperately looking for alternatives, thinking that with these bacterial brews they will need less fertiliser. Meanwhile, they mine out their already poor soil. I can get really angry at companies that sell this stuff and make money by misleading people.”
He is not the only one. Australian researcher Unkovich finds it all quite frustrating. “It sounds so attractive, biological nitrogen fixation to save the world, but proof? It looks suspiciously like greenwashing. Like ‘alternative’ and ‘natural’ therapies, they only have to provide hope for people who want/choose to believe in a green Utopia.” The other experts mentioned in the article are also critical of supposed effects. They would like to see not only the supposed nitrogen fixers, but all bio stimulants subjected to an independent assessment before being marketed.
Box:
BlueN from Corteva Agriscience contains the bacterium Methylbacterium symbioticum SB0023/3, a bacterium that can colonise the intercellular spaces of plants and is known to promote plant growth. The product BlueN was developed by Spanish biotech company Symborg, which has since been acquired by Corteva. In the US, it is sold under the name Utrisha.
Syngenta’s Vixeran has the bacterium Azotobacter salinestris CECT 9690 as its active ingredient, a bacterium from the also Spanish company Ceres Biotic. A similar product is sold in the US (Envita) with a different bacterium as the active ingredient Gluconacetobacter diazotrophicus.
Another company that is making quite a splash with press releases about how much CO2 has been saved thanks to their preparations ProveN and ProveN40 is US-based PivotBio. Their ‘pet’ is Klebsiella variicola (ProveN), assisted by Kosakonia sacchari in ProveN40.
De Nederlandse versie van dit artikel is gepubliceerd in Vork nr. 2, juni 2023
Foto: Wikimedia
