Legumes can improve soil nitrogen content and contribute to more soil carbon capture. Legumes can thus help agriculture to reduce its nutrient and greenhouse gas emissions to the environment. 

There is, however, a slight challenge; when nitrogen is converted and made more accessible to plants, soil carbon becomes more unstable. A better understanding of the processes in the legumes’ root zones can help solve this problem. This is precisely what researchers from the Department of Agroecology at Aarhus University are working on in a new project that has been granted 6.2 million kroner from the Independent Research Fund Denmark. 

One of the benefits of legumes is that they add nitrogen to the soil in an environmentally friendly way via their nitrogen fixation. At the same time, legumes increase the soil’s ability to bind carbon, which reduces the atmospheres’ CO₂ content. Legumes can actually store more carbon in the soil’s organic material than well-fertilised non-legume crops. More nitrogen and carbon in the soil sounds like a win-win situation. 

Delving into the soil’s root zone

Organic material must be decomposed in order for the nitrogen to become available to the plants. With the organic material turnover, carbon becomes more unstable and its storage in the soil more uncertain. More knowledge about what actually goes on in the legumes’ root zone is necessary to overcome this problem. 

- We need to gain a deeper understanding of how carbon and nitrogen behave in the soil in order to enjoy the full benefits of legumes, says the leader of the new project, Senior Researcher Jim Rasmussen from the Department of Agroecology at Aarhus University. 

The researchers have good knowledge about what happens with nitrogen inside the legume plant and its roots, but lack knowledge about the processes and mechanisms that control the soil’s build-up of nitrogen under the legumes.    

The world of small things

Some of the answers can probably be found in the soil’s microbial community – the so-called microbiome. The microbiome contributes significantly to the build-up of soil organic matter but knowledge about some of the processes is lacking.    

- Until now, the common line of thinking has been that plant root turnover is the dominant driver of carbon and nitrogen build-up in the soil, but now it looks like the rapid transport of organic nitrogen from legume roots is even more important for the microbiological build-up of carbon and nitrogen underneath the legumes – but this is where we lack knowledge, says Jim Rasmussen. 

With the aid of new technology, including microdialysis, which is normally used in human medicine for continuous monitoring of transport of compounds in tissues, and advanced use of isotopes, the researchers will survey carbon and nitrogen “traffic” in the root zone. 

- With a better understanding of these mechanisms and activities in legume root zones, legumes can be used more efficiently to promote a sustainable intensification of agriculture, with a reduced need for fertilisation, less emission of nitrogen to the atmosphere and aquatic environment, and increased soil carbon storage, which can help reduce emissions of CO₂, says Jim Rasmussen.  

For more information please contact

Senior Researcher Jim Rasmussen, Department of Agroecology, email: jim.rasmussen@agro.au.dk, telephone: +45 8715 7418