Microbial N-Fertilizers Do Not Reduce Emissions
Impact: Likely no impact on emissions
Technology Maturity: Scaled
This is the Article #9 of a series on the climate technologies shown in my ClimateTech Market Map, with a deeper dive into technical maturity and potential to reduce global warming. Here I'll cover microbial nitrogen fertilizers: bacteria that produce crop-usable nitrogen compounds that in theory allow farmers to reduce the amount of synthetic nitrogen fertilizer they use.
TL;DR: Recent research shows that while these bacteria often promote crop growth, they are not primarily doing this by directly supplying nitrogen. As such, while these bacteria may stimulate nutrient uptake through other pathways, like promoting root growth, they don't replace the need for synthetic fertilizer, and hence can't reduce emissions.
Nitrogen = Crop Yield
The relationship between nitrogen fertilizer application and crop yield is an established fact in agronomy. Nitrogen is the limiting nutrient for most crops and regions, and yield response is almost perfectly linear at lower application rates. The graph below shows one example of the response of yield to increased nitrogen fertilizer application.
Recommended N fertilizer application rates vary considerably based on soil type and crop, but max out at around 220kg per hectare on well managed farms. This application rate represents 22 grams (~ 1oz) per square meter. Not all applied nitrogen makes it into the crop. Depending on region, soil and environment, anywhere from 20% to 70% of applied N fertilizer is lost to leaching, volatilization and denitrification, so only 30-80% is incorporated into the crop biomass. As a result, many researchers have been exploring alternatives to the traditional method of periodic applications of synthetic fertilizer.
Bacterial Nitrogen Fixation in Legumes
Today, most nitrogen input for crops is from synthetic fertilizer produced from methane using the high emissions SMR/Haber-Bosch process. However, biological sources are also important. Bacteria that can convert atmospheric nitrogen (N?) to plant usable nitrogen (NH?+) - also called nitrogen fixation - have been found free-living in the soil near root systems, living in symbiotic root structures, on plant leaves and inside plant tissues. Of these four, only nitrogen fixing bacteria that live inside symbiotic root structures produce nitrogen on the same scale as synthetic fertilizer application. Given their importance, it's worth describing exactly how these root bacteria work.
Legume-associated root bacteria have long been known to be significant sources of nitrogen fixation. In 1880, Hellriegel and Wilfarth, two German agricultural chemists, proved that soil bacteria were responsible for causing legume roots to form root nodules, and the resulting nodules were shown to supply high amounts of nitrogen to the plant.
Subsequently, it was also shown that these root nodules are densely colonized by the same bacteria that cause nodulation. These bacteria are all from a single bacterial family: rhizobia. No other crops apart from legumes (clover, alfalfa, beans, peas, soybeans) experience rhizobial nodulation. This nodule structure encapsulates bacteria within root cells and allows high volume transfers of inputs and outputs between bacteria and plant. The nodule wall also protects the bacteria from oxygen exposure, which would otherwise break down the nitrogenase enzyme that is the key actor in nitrogen fixation. (Oxygen for respiration is supplied to the root cells in a specially bound form.)
The nitrogen produced by these rhizobia contributes the equivalent of 15kg to 325kg of nitrogen fertilizer per hectare to legume crops. For example, soybeans in a maize/soybean rotation in South Dakota get about ~160kg per hectare of nitrogen input from rhizobia fixation (1). This is a huge amount of nitrogen, and it's why legumes have been included in crop rotations as far back as the middle ages.
But - and this is important - this nitrogen is not free. Nitrogen fixation requires lots of energy to break the N? bond, regardless of whether it's a biological or thermochemical process. Legumes typically supply their hosted rhizobia with 5-10 grams of carbohydrates and amino acids in order to get 1 gram of Nitrogen in return. This contribution can constitute up to 20-30% of a legume's entire photosynthetic output - just to feed its hungry rhizobia.
(The magnitude of this carbon drain also has implications for engineering nodulation into other crop plants. Modern wheat, for example, manages to store an astounding 40% of its photosynthetic output into grain production. A rhizobially nodulated wheat producing anywhere close to its full nitrogen requirement would almost certainly be a lower yielding strain.)
Biostimulants and Biofertilizers
The last decade has seen a big push to commercialize other bacteria with plant-growth promoting effects that include not just nitrogen fixation but also nutrient mobilization, soil structure improvement, as well as synthesis of growth hormones, amino acids, and other useful compounds.
Loosely speaking, these are divided into "Biostimulants" which promote growth by producing or promoting regulatory substances like hormones; and "Biofertilizers" which supply nutrients (phosphates or nitrogen) by either directly producing them or converting them from plant-unavailable forms. (Microbial biopesticides are also a major thrust, but I won't cover them here.)
However it is worth noting that there is ambiguity about which bacteria belong to each category and whether these are even the right categories. These commercialized bacteria are mostly soil-dwellers, living adjacent to the root system (rhizospheric), although some colonize leaf surfaces (epiphytic) and some infect the plant without causing nodulation (endophytic).
To give two examples of biostimulant effects: some soil bacteria stimulate root growth by producing ACC deaminase and Indole-3-Acetic Acid (IAA). ACC deaminase works by reducing ethylene levels. Since ethylene inhibits root growth, removing it increases root growth.
IAA is a plant hormone that initiates the formation of new roots and promotes the growth of lateral roots (sideways-growing secondary roots that emerge from the main roots). Because both ACC Deaminase and IAA affect signaling molecules, relatively small amounts of them can have big effects on growth, so you don't need a large bacterial biomass producing them.
The two main nutrients provided by commercial bacterial biofertilizers are nitrogen-fixing and/or phosphorus-mobilization. As far as phosphorus-mobilization goes, most agricultural soils contain lots of phosphorus but little of it is in the form of phosphate ions that can be easily assimilated by plants. Phosphorus mobilizing bacteria secrete weak acids and enzymes that release phosphate ions from more complex compounds.
"Your Mileage May Vary"
Meta-analysis of bacterial bio-fertilizer effectiveness in both controlled and field conditions estimate that bacterial treatments of all kinds produce yield increases of between 12-20% (2). However, the range of effect is large, with a meaningful positive response in only 50% to 70% of cases. Biofertilizers are most effective in tropical and dry soils, and most effective for cereals and legumes.
However, many trials of biofertilizers in temperate soils and for root crops and vegetables show no significant response (e.g. above). Microbial N biofertilizers have been thoroughly investigated for effects on maize in the US Midwest and have been found to have no effect on yield, at any level of synthetic N fertilizer application (3)(4). Like our wheat example earlier, maize has a strongly linear response to nitrogen addition at low application rates, so this is good evidence that at least in these soils, these biofertilizers do not produce meaningful amounts of nitrogen.
The Problem With Non-Nodulating Bacteria
The reasons why these biofertilizers can't produce meaningful amounts of nitrogen are laid out in a recent review by Giller at al. (2024), which leads off with:
"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."
Giller summarizes the issues with commercial microbial N fertilizers as follows:
Giller critically evaluates the evidence for significant nitrogen fixation in multiple bacterial families and finds them all lacking on one or more of these criteria.
Relevant Startups
There are many producers manufacturing a wide range of biostimulants and biofertilizers, from small regionals to multinational divisions. In addition, many startups are producing biostimulants of all kinds including seaweed extracts, humic acids, symbiotic fungi, protein hydrosylates and more.
The following are the startups I have come across, focused on nitrogen fixing biofertilizers:
Pivot Bio has raised $618M for its nitrogen fixing commercial biofertilizer. Its original bacterial strain was a soil-dwelling species, gene-edited to remove nitrogen-inhibition (the mechanism that switches off nitrogen-fixation when soil nitrogen levels are too high). The lead product, "ProveN40", is marketed as a replacement for 40lbs of synthetic nitrogen application. The company has documented solid rates of yield gain among its customer base, but several controlled tests (see Nafziger above) on US maize have demonstrated no effect.
Switch Bioworks and Net Zero Nitrogen are also science-based startups developing nitrogen fixing biofertilizers.
Conclusions
Treating crops with microbial inoculations does increase yield in many crop varieties under many conditions, but results are not guaranteed. In addition, we can conclude that some crops and regions (like mid western maize) simply don't respond at all.
What also seems to be true, somewhat controversially, is that these yield responses are not due to nitrogen from bacterial nitrogen fixation (with the notable exception of rhizobia in legumes). And if these biofertilizers are not replacing nitrogen then their ability to displace synthetic nitrogen fertilizer (with its associated emissions from production and application) is called into question.
While it may be a little early to state that microbial N biofertilizers can't reduce emissions, the weight of the evidence and the science is on that side of the scale.
* Conversion rate used for maize bushels per acre to metric tons per hectare was 0.064.
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Citations
(1) Karki, D. 2020. "Application of Nitrogen Fertilizer in Soybeans" South Dakota University Extension. Available online.
(2) Schütz, L., Gattinger, A., Meier, M., Müller, A., Boller, T., M?der, P. and Mathimaran, N., 2018. Improving crop yield and nutrient use efficiency via biofertilization—A global meta-analysis. Frontiers in plant science, 8, p.2204.
(3) Franzen D. et al. 2023. Performance of Selected Commercially Available Asymbiotic N-fixing Products in the North Central Region. North Dakota State University Extension. SF2080 April 2023. Available online.
(4) Nafziger, E. 2023. What a Year: Crops & Soil Fertility in 2022 (for 2023). 2023 IFCA Conference, January 17, Peoria. Available online
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