Fixing the Problem of Nitrogen Limitation in Agriculture

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The Microbiology Society is undertaking a project entitled A Sustainable Future as part of our 75th Anniversary, which aims to highlight the Sustainable Development Goals (SDGs) to our members and empower them to use their research to evidence and impact the goals. Earlier this year, we put a call out to our members to submit case studies in the following three areas: antimicrobial resistance, soil health and the circular economy.

This case study is written by Carolin Schulte, who is a Doctoral Student at the University of Oxford, UK, and a member of the Microbiology Society. It focuses on Soil Health; maintaining the health of our soils has gained increasing prominence in recent years. Soils are essential for the global food system and regulate water, carbon and nitrogen cycles but are put under pressure from population growth and climate change.

What are the challenges that this research addresses?

With the world population predicted to reach 11 billion by 21001 and the area of farmable land decreasing due to the consequences of global warming, there is an urgent need for agricultural practices that generate high yields in a sustainable manner. Despite major advances in agricultural productivity, 13% of the population in developing countries are suffering from malnutrition2, dramatically highlighting that we must take active measures for achieving global food security.

My doctoral research focuses on harnessing microbiology to sustainably increase the availability of nitrogen, which, besides phosphorus, is commonly the main limiting factor for plant growth. Although nitrogen constitutes 78% of the atmosphere, this inert N2 gas is not bioavailable and has to be “fixed” into ammonia, NH3. Leguminous plants, such as peas and beans, have evolved the ability to form intricate symbiotic relationships with certain soil bacteria called rhizobia. In this complex interaction, rhizobia induce the formation of nodule structures on plant roots. Inside these nodules, rhizobia differentiate into nitrogen-fixing bacteroids, which convert inert N2 into NH3 that can be assimilated by the plant host.

What findings and solutions were provided by this research?

Because it is very difficult to study biological nitrogen fixation experimentally, my research combines computational and experimental approaches to understand bacterial metabolism in this extraordinary example of trans-kingdom mutualism. By combining metabolic modelling techniques with various genome-scale omics datasets, I am creating mathematical representations of rhizobial metabolism in the symbiosis. I am using these models to simulate the response to gene deletions or varying nutrient availability, thereby defining mechanisms through which the plant host controls the metabolism of its symbiont. These insights lay the foundation for targeted engineering of rhizobial strains that are more efficient at fixing nitrogen. Understanding fundamental properties of the symbiosis is further important for attempts to transfer symbiotic nitrogen fixation to non-legume crops, such as wheat and corn.

Since establishing these non-natural symbioses is hampered by the requirement to engineer both the plant and the bacterial partner, major research efforts are also aimed at harnessing bacteria that fix nitrogen outside of a nodule environment. As part of my PhD programme, I had the opportunity to explore this area during an internship at Sound Agriculture, a start-up based in Emeryville, California. Sound is developing bio-inspired chemistry to enhance plant-microbe interactions, notably the interaction of free-living nitrogen fixers and corn. For my internship, I joined the research and development team who, amongst other projects, are working on the targeted synthesis of novel molecules and their high-throughput screening for desired properties. This discovery platform has facilitated the identification of compounds stimulating microbial nitrogen fixation and phosphate solubilization, with significant benefits for crop yield observed in field trials.

How can this research support the transition to a more sustainable future?

Current practices of using synthetic fertilisers to prevent nutrient limitation have multiple environmental issues: in addition to the high energy demand of chemical nitrogen fixation, a significant fraction of fertilisers leaches into the environment, causing water pollution and algal blooms. Major changes in agricultural practices are therefore essential to achieving Goal 2 of the Sustainable Development Goals: Zero Hunger, while preserving our ecosystems. Nitrogen-fixing microbes will play a crucial part in this transition, and advances in our understanding of biological nitrogen fixation promise to provide solutions for a sustainable high-yield agriculture.

References

1 United Nations. World Population Prospects: The 2019 Revision, UN Department of Economic and Social Affairs; 2019. https://population.un.org/wpp/ [accessed 13 May 2020]

2 United Nations. Sustainable Development Goals, Goal 2: Zero Hunger; 2020. https://www.un.org/sustainabledevelopment/hunger/ [accessed 13 May 2020]


About the Author

Carolin Schulte is a Doctoral Student at the University of Oxford, UK, and a member of the Microbiology Society. More information on her work is available here.