Legumes: Giving Nitrogen Fixation A Leg Up

Featured Undergraduate post by Ciara O’Flynn.

At first glance, plants seem impressively independent. Unlike us, they can make their own food, through a process called photosynthesis which uses energy from sunlight. This is a pretty neat ability but, plants aren’t entirely self-sufficient. In order to grow and develop fully, they must get a sufficient supply of minerals from the soil – including a particularly important mineral: Nitrogen.

You might be deceived into thinking this is an easy task. Nitrogen does make up 78% of our atmosphere after all. However, plants can only assimilate nitrogen in the form of nitrate or ammonium, which means the nitrogen in our atmosphere first needs to be converted into one of these forms. This can be achieved naturally by bacteria and blue-green algae, but this doesn’t always meet the enormous nitrate demands for plants growing in dense quarters – say for example in a cropping system. This is where legumes come in! Legumes are a type of plant that has a symbiotic association – or a working relationship – with a class of bacteria called rhizobia. These rhizobia congregate in specialised areas called nodules in the legume’s roots. In these nodules, rhizobia love nothing better than to sit around all day and convert inert atmospheric nitrogen to nitrate which legumes and even neighbouring plants can readily absorb.

Rhizobia nodules on legume plant’s roots

Despite the natural advantage that legumes have, they fell from grace of the farmers when inorganic nitrogen fertilizer started being commercially produced. Farmland in Europe cropped with legumes fell from 11.3 million ha in 1961 to a measly 3.4 million ha in 2005. This was due to the higher market value of non-legume crops like barley, wheat and maize having a higher market value than legumes. Naturally, farmers wanted to invest more in these crops than in lower value legumes, and with the discovery of inorganic nitrogen-based fertilizers, they could!

However, recent studies have brought nitrogen-based fertilizers under scrutiny. Surplus nitrogen fertilizer contributes to high levels of the greenhouse gas nitrous oxide and can expedite eutrophication in surrounding water bodies, causing dramatic changes to aquatic ecosystems. Plus, it’s gotten really expensive for farmers to keep buying and applying fertilizer to their fields. So maybe it’s time to learn from the past; from our days of using legume-based cropping systems – which could be a more ecologically and economically beneficial choice.

The effects of eutrophication – fish kills and algae bloom

Dr. Pete Ianetta, a plant biologist based at the James Hutton Institute, has had this very thought. He wanted to see whether a legume based cropping system is superior to a non-legume system which is dependent on inorganic fertilizers. So he and his team set up an experiment which ran for 3 years, in which they included both legumes and non-legumes to test which cropping system was more productive. Different cropping practices including “monocultures, mixtures, intercrops, undersown crops, catch crops, green manures” were set up across arable, pasture and mixed systems to maximize the generality of their findings. Across 8 studies, they used 29 main crops and 59 sub crops which included the likes of maize, faba bean, pea, barley, oat, rye and grass. Then, to help answer their question, Lannetta and colleagues gathered data on the nitrogen balance of the soil, it’s efficiency, surplus and overall annual output, as well as the biomass of the crop and its productivity output (kg/ha). Additionally, they also took into account factors such as potential nitrogen loss when grazed by livestock. They measured these factors for each crop throughout the experiment and then compared the cropping practices to see which was better.

Ianetta and colleagues found that in a crop-year, the nitrogen fertilizer and biomass of a crop weren’t strongly linked to one another. In fact, large quantities of crops were produced without any fertilizer present – such as the legume-based systems across the different cropping practices. This was suggested to be because of the biological nitrogen fixation occurring in the root nodules of the legumes, providing fixed nitrogen to the surrounding crops.

Intercropping peas and barley

I enjoyed hearing about this work as it has the potential to restructure agricultural practices here in Ireland by showing that the sector doesn’t need to be as reliant on inorganic nitrogen fertilizers. This could reduce the greenhouse gas emissions from the agriculture sector here in Ireland. Additionally, the study also debunks the idea that the environment constrains the development of economy; in fact, when managed correctly, it can help it grow! This mentality could promote more ecological awareness and conservation with sectors even spanning outside of agriculture and potentially solve other issues involving the fragility of our wavering natural resources that are occurring today.

To find out more, read the Frontiers in Plant Science article ‘A Comparative Nitrogen Balance and Productivity Analysis of Legume and Non-legume Supported Cropping Systems: The Potential Role of Biological Nitrogen Fixation’.

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About the Author

Ciara O’Flynn is a final year Environmental Science student in the School of Natural Sciences at Trinity College Dublin and Mediator at Science Gallery Dublin. Find out more about her work here:

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