Aarhus University Seal

The right cover crop mix is the right choice for the environment

Optimising the combination of cover crop species can help reduce agriculture’s impact on the environment by improving nitrogen use efficiency.

[Translate to English:] Afgrøder som majs og byg kan få gavn af den "arv", som efterafgrøder efterlader i jorden. Foto: Janne Hansen

Cover crops play an important role in mopping up nitrogen in the periods between cash crops – and it seems that using the right combinations of cover crop can improve nitrogen use efficiency even further. Researchers from the Department of Agroecology at Aarhus University are setting out to find optimal combinations in a new project to which the Independent Research Fund Denmark has granted 2.9 million kroner.     

Agricultural fields are dominated by monocultures of plant species receiving massive soil inputs of mineral nitrogen fertiliser to boost plant growth. This is not efficient, as the crops take up only about half of the applied nitrogen. The other half is lost from the agroecosystem, which leads to situations that are detrimental to the climate and environment. In addition, utilising only half of the applied nitrogen is inefficient and represents a loss in potential productivity. 

- It is therefore paramount to identify ways to increase the efficiency of nitrogen use by relying more on internal soil-plant nitrogen recycling and less on external nitrogen fertiliser, says the leader of the new project, researcher Diego Abalos from the Department of Agroecology at Aarhus University. 

Making the most of cover crops

Diego Abalos has been involved in recent research in cover crop species diversity at Aarhus University. The results showed that in an intensive system, species richness in itself does not increase nitrogen use efficiency. However, specific plant combinations with high trait dissimilarity are particularly effective at reducing nitrogen losses. The effect of individual species on these losses can be linked to traits related to nutrient acquisition strategies. 

- These results strongly suggest that to increase nitrogen use efficiency a targeted selection of species combinations with specific traits may be more important than increasing species richness per se, says Diego Abalos. The question is: Which combinations are optimal – or rather, which combinations of traits are optimal? 

Cover crops offer a unique opportunity to implement a targeted plant community based on specific traits to improve nitrogen use efficiency. The legacy they leave in the soil can influence nitrogen losses and nitrogen use efficiency in the following cash crop. Some cover crops can deliver on some of the beneficial effects while other cover crops can deliver on other effects. 

In addition, the relative importance of specific traits for soil nitrogen cycling is strongly governed by the level of nitrogen fertilisation during the cash crop phase. The studies aim to find optimum combinations of cover crop plant traits and nitrogen fertilisation levels during the cash crop phase that maximise nitrogen use efficiency. 

Zooming in on cover crop mechanisms

Traits that are expected to be critical for nitrogen use efficiency are those related to high yield potential and include architectural, morphological and physiological traits (e.g. leaf area, root length density and symbiotic relationships with e.g. certain beneficial fungi, such as arbuscular mycorrhizal fungi). 

The researchers aim to reveal the mechanisms through which cover crop plant traits directly modulate nitrogen losses during the cover crop phase. They will assess the traits that regulate nitrogen losses and nitrogen use efficiency via legacy effects in the subsequent cash crop phase in response to different fertilisation levels. Finally, they will propose an optimum traits-based plant combination of cover crop species that increases nitrogen use efficiency in cover-cropping systems at low nitrogen levels.  

The researchers will carry out their experiments in greenhouses and fields. The cash crops in the rotation will be maize and spring barley, while the cover crop species will be various grasses, legumes and brassica. These cover crop species cover a wide range of traits related to nutrient acquisition strategies, biological nitrogen fixation, root exudation and symbiosis with arbuscular mycorrhizal fungi. 

For more information please contact

Researcher Diego Abalos, Department of Agroecology, email: d.abalos@agro.au.dk, mobile: +45 2085 4336