Legume-based cover crops benefit crop yield and reduce nitrate leaching
The use of cover crops in arable farming is more efficient in nitrate leaching reduction than decreasing the field nitrogen surplus. However, this requires well-developed cover crops.
Nitrate leaching, from Danish agricultural fields, has been greatly reduced over the last 30 years, but the environmental target for quality of marine ecosystems set in the context of the EU Water Framework Directive has yet to be reached. Therefore, additional reductions in nitrate leaching are needed in Denmark. This requires updated information on the efficiency of measures to reduce nitrate leaching, also in organic farming.
Nitrogen surplus is the difference between the nitrogen (N) that was applied (e.g. with fertilizers or biological N2 fixation) and the N that was removed (e.g. in grain yield) from the field, and indicates the N that potentially can be lost to the environment. Hence, the higher the N surplus, the higher the risk of leaching. However, the N surplus at country scale has already been halved, and it is questionable whether further reducing N surplus will efficiently reduce leaching.
Cover crops can help to lower nitrate leaching. Under Northern European climatic conditions, most of the leaching occurs during autumn and winter, when it is crucial to keep the soil covered with vegetation, able to “catch” the nitrate and keep it in the system for the following crops. Legumes (e.g., clover, lucerne), due to their ability to fix N2 from the atmosphere, are not considered as efficient as non-legumes (e.g., ryegrass, chicory) when it comes to N removal from the soil. However, in the study “Nitrogen leaching: A crop rotation perspective on the effect of N surplus, field management and use of catch crops” mixtures including both legumes and non-legumes were shown to reduce nitrate leaching by approximately 60%, corresponding to 23 kg N ha-1 y-1, on a loamy sand soil in Denmark. In addition, the study shows how cover crops have a higher impact on nitrate leaching reduction than N surplus (Figure 1).
Figure 1: Nitrate leaching in relation to (a) N input and (b) surplus (kg N ha-1 y-1) in treatments with cover crops (+CC, dots) and without (–CC, triangles).
In spring, when the cover crop is incorporated in the soil, it is important that its biomass is decomposed quickly to release nutrients for the following crops. The inclusion of legumes in the cover crop mixture can make this process more efficient, due to greater net N mineralization of legume residues. In this way, legume-based cover crops can not only reduce nitrate leaching, but also increase the yield of the main crops. In particular, the above-mentioned study reports how N yield in a system with legume-based cover crops can be comparable to N yield in a system with animal manure (applied at a rate of 70 kg total N ha-1 yr-1 ) and no cover crops, but with a significantly lower nitrate leaching (Table 1).
Table 1: Average annual N input, yield, surplus and leaching during the period 2011-2014 in an organic crop rotation. Numbers in brackets are standard errors (n=32). Within each row, values followed by different letters in a row are significantly different. CC = Cover Crop and M = animal Manure (70 kg total kg N ha-1 y-1).
Focus on cover crop biomass
Be it legumes or non-legumes, it is not enough to seed a cover crop to obtain reliable nitrate leaching reductions. As it emerges from the study mentioned above, it is necessary to reach specific thresholds in cover crop biomass to effectively reduce nitrate leaching. This means that cover crops should be big enough to be able to take up the surplus soil mineral N.
How to facilitate the growth of the cover crop? One possibility is to seed it while the main crop is still in the field (undersowing), allowing more time for its establishment and growth. In this case, competition with the main crop can occur, which could be solved by increasing the space between the main crop rows. This is one of the focus points of the project “Row cropping in organic arable farming for increased productivity and sustainability” (RowCrop); results show promising effects of an increased row space (going from 12 to 24 cm) on cover crop growth and weed control.
The project ’Row cropping in organic arable farming for increased productivity and sustainability’ (RowCrop) is a part of the Organic RDD 2- programme, which is coordinated by ICROFS (International Centre for Research in Organic Food Systems). It has received grants from ’Grønt Udviklings- og Demonstrationsprogram (GUDP)’ under the Ministry of Environment and Food of Denmark.
Reference to the cited article:
De Notaris C, Rasmussen J, Sørensen P, Olesen JE (2018) Nitrogen leaching: A crop rotation perspective on the effect of N surplus, field management and use of catch crops. Agriculture, Ecosystems & Environment 255:1-11 doi:https://doi.org/10.1016/j.agee.2017.12.009