Loss of nitrogen (N) from agriculture to the environment has major consequences for aquatic ecosystems, biodiversity and climate change. For decades, there has been a strong focus on reducing input and surplus of N in agricultural systems in order to limit N losses. However, we live in a world with an ever-increasing population, and food scarcity is a growing global concern.

“Food production largely depends on the input of mineral fertilisers, but we need to consider other ways to increase food production and soil fertility while still reducing the need for industrial N inputs. One step towards that could be the use of cover crops. They have received more and more attention as a tool to retain N in the system thanks to their ability to reduce nitrate leaching,” says Post Doc Chiara De Notaris from the Department of Agroecology at Aarhus University.

In a recently published article, she and colleagues from the Department of Agroecology investigated how cover crop mixtures that include legumes can self-regulate to optimise their biological N₂ fixation while reducing nitrate leaching. Biological N₂ fixation is a process in which molecular nitrogen (N₂) in the air is converted into ammonia (NH₃) or other N -based compounds that can be utilized by plants.

Long-term organic experiment

“We already know that the inclusion of legumes in cover crop mixtures can increase the N inputs thanks to the biological N₂ fixation, and that this further reduces the need for industrial N fertilizer. However, the ability of legumes to reduce N losses is still debated, and some argue that legumes will increase N surplus and therefore cannot be used when the main purpose of cover crops is to reduce N losses. The aim of our study was to investigate how the growth of legume-based cover crop mixtures is affected by different cropping systems and fertility management in a long-term organic crop experiment, and we wanted to see how this affects the biological N₂ fixation and equally important the nitrate leaching,” says Research Assistant Esben Øster Mortensen from the Department of Agroecology at Aarhus University. He worked on the project during his education in Agro-Environmental Management.

The research was based on data collected from 2015-2017 in a long-term field experiment, which includes two organic crop rotation systems with four-year cycles:

• One with one year of green manure in the crop sequence (OGM)
• One with a one-year grain-legume in the crop sequence (OGL)

In the experiment, the researchers used a legume-based cover crop mixture composed of perennial ryegrass, chicory, white clover, and red clover.

The researchers also investigated the effect of manure (+ M) and no manure (-M) with a focus on finding out whether the growth of the cover crop would be affected by the previous main crop, the use of livestock manure as well as its place in rotation order.

Hypothesised correctly

“We believed that the rotation system with the highest N input (OGM + M) would result in the lowest percentage of biological N₂ fixation, while the system with the lowest N input (OGL-M) would result in the highest fixation, and our hypothesis turned out to be pretty close to what actually happened. We also hypothesised that nitrate leaching would be independent from legume biomass as opposed to the opinion of cover crops mixtures with legumes resulting in higher leaching, and that nitrate leaching will be reduced by well-established cover crops,” says Chiara De Notaris.

During the three consecutive growing seasons, the researchers found that non-legumes benefit from more fertile systems, but have a disadvantage when the availability of soil N is low. In the latter case, clover will thrive and the clover biomass accumulate. And results from the plot experiment showed that the higher clover biomass resulted in a higher and more stable biological N₂ fixation. In other words, when the available N in the soil is low, the cover crops regulate themselves in such a way that they can fix more N₂ from the air. 

“We found that the biological N₂ fixation was stable and around an average of 91% of N uptake in the legumes, when the aboveground biomass of the clover plants exceeded approx. 0.4 tonnes per hectare, while a clover biomass below the threshold resulted in a variable biological N₂ fixation, with an average of 73%,” explains Chiara De Notaris.

Self-regulation 

The researchers found that the input of N from biological N₂ fixation is mainly determined by the amount of clover biomass, and that the fixation of N₂ from the air was lowest when the cover crops were either suppressed by the main crop or when clover was outcompeted by non-legume species in the cover crop.

“Our results indicate that legume-based cover crop mixtures can self-regulate in terms of reducing nitrate leaching and N input via N₂ fixation. We saw that the higher the N availability in the soil, the higher was the dominance of non-legumes in the cover crop biomass, which corresponded to a low and variable N₂ fixation. This happened in the treatments where the availability of soil N was expected to be high, just as the risk of nitrate leaching was expected to be high. On the other hand, we found that when clover biomass and N₂fixation were high, the risk of nitrate leaching was also low, ”says Chiara De Notaris.

When the availability of N from the soil is low, this is compensated by the cover crop and the higher amount of clover, which can fixate the much need N₂ from the air to the soil. However, in soils with a high availability of N, there is no need for a high biological N₂ fixation rate, which is why the non-legume species grow well here.  Overall, the results show that legume-based cover crop mixtures are self-regulating in terms of N input via biological N₂ fixation as well as in terms of reducing nitrate leaching.