Fixing the climate impact of nitrogen from global farming
Anyone considering the contribution of farming to global warming could be forgiven for believing that the only concern is the greenhouse gas methane released into the atmosphere from extensive livestock production systems, particularly those of cattle. This issue of ruminant digestion which generates methane emissions gains most attention in the media and in approaches to policy. There is no doubt that methane emissions from livestock is a major problem; however, in terms of global warming potential (GWP) the gas is relatively short-lived - around 12 years with a GWP value of 25. More of a concern for its GWP is the greenhouse gas nitrous oxide (N2O), which stays in the atmosphere for an average 114 years and has 300 times more GWP than CO2. Globally, about 40% of the total N2O emissions come from human activities, and agricultural soil management was the largest source of N2O emissions in the USA in 2019, accounting for 75% of total N2O emissions
The key issue in soil management in terms of nitrous oxide emissions is the use of synthetic nitrogen fertilizers. At first glance nitrogen fertilizers appear absolutely crucial to agricultural production; ensuring the production of the basic cereal staples (rice, wheat, maize, sorghum) on which the world depends, and ensuring our food security in a time of burgeoning global population. The prospect of removing nitrogen fertilizers or modifying their use leading to yield reductions, would seem to have potentially devastating consequences for feeding mankind and would be unacceptable. Hence, perhaps there is a reticence by policy makers to regulate nitrogen fertilizer use in any way that will potentially impact on crop yields, without there being alternative more sustainable technologies available.
The good news is, there is an alternative provided by biological nitrogen fixation (BNF), and a revolutionary approach developed at the University of Nottingham is transforming the growing industry in this sector. BNF is the means by which certain species of bacteria can convert nitrogen from the air and fix it in a form that can be taken up and used by plants. Natural populations of BNF bacteria that live generally in soil, or those that live around the root zone (the rhizosphere) can be boosted by application of additional natural inoculants that are applied direct to the soil. In the same way- or through application to seed- some BNF bacteria (notably rhizobia) have evolved to form a close, symbiotic association with the roots of a few specific crop species, the legumes (peas and beans). Companies are successfully developing and selling products that boost such BNF, as natural sources of nitrogen around (or in case of rhizobia, within root nodules) the roots can to some degree substitute for synthetic nitrogen fertilizers.
"The good news is, there is an alternative provided by biological nitrogen fixation (BNF), and a revolutionary approach developed at the University of Nottingham is transforming the growing industry in this sector."
However, what is required to transform the sector is a specific technology developed at the University of Nottingham, based on a BNF bacterium which can fix nitrogen where it’s most needed by the plant. Through this technology, a BNF bacterium can be applied to seed to potentially colonize any crop through the emerging roots, move through the plant and fix nitrogen in the leaves. There, sitting alongside the chloroplasts which use sunlight energy and CO2 to produce sugars, the BNF bacteria could form what is known as a diazoplast, fixing nitrogen from the air and converting it to ammonia, a form which the plant can use for growth and grain production.
The University of Nottingham’s technology utilizes the special characteristics of a naturally occurring (non-GMO, organic) BNF bacterium called Gluconacetobacter diazotrophicus - or Gd for short. Isolates of this remarkable bacterium first discovered in sugarcane can colonize a wide range of crops including wheat, rice, maize, potato, soybean, tomato and to significantly impact on crop yields with reduced amounts of nitrogen fertilizers. Extensive international field trials have demonstrated that nitrogen fertilizers can be reduced by between 50- 85% and still maintain crop yields. This delivers a more sustainable technology capable of reducing synthetic nitrogen fertilizer use without a yield penalty for the crop, the farmer or the consumer. In maize and in wheat, a nutritional premium has also been demonstrated. This technology has now been on sale in North America for maize (corn) and soybean growers and has recently been registered in Canada for use in rapeseed oil (canola).
To significantly reduce our reliance on synthetic nitrogen fertilizers, and mitigate the harm caused by agricultural nitrous oxide greenhouse gas emissions, policies which incentivize farmers to take up sustainable BNF technologies are necessary. Only by combining existing methods with new BNF technologies will it be possible to roll out BNF technologies which can begin to reduce our reliance on synthetic nitrogen fertilizers and mitigate against the harm caused by agricultural nitrous oxide greenhouse gas emissions.
Ted Cocking is Emeritus Professor of Botany and Director of the Centre for Crop Nitrogen Fixation in the School of Biosciences.