Modern agriculture relies heavily on nitrogen fertilisers to get better crops and higher yields, but at the same time, it poses a major challenge for the industry. It is difficult to create a sustainable recycling of nitrogen for the crops without risk of leaching. Globally, agriculture is often based on simple cultivation systems, with the crops having a relatively short period of time to absorb nitrogen. When the crop only absorbs nitrogen for the short period, there is a great risk of loss of nitrogen in the form of nitrous oxide (N2O) emissions or nitrogen leaching. According to researchers from the Department of Agroecology, strategies for maintaining and recycling the excess nitrogen in the soil are needed.

 “We have examined the turnover of different types of cover crops and how much nitrogen is released from them, but we did not stop there. We have also examined the different types in more detail, in order to look at the turnover of root and of the top of each type. We have done this to gain more knowledge about, how much of the nitrogen becomes available for the following crop and how much will be released at a later,” says Senior Researcher Peter Sørensen from the Department of Agroecology. "We also examined the root and top separately, as it may be interesting to harvest the top in some cases. 

Leaching of nitrogen

Cover crops are primarily grown to reduce leaching of nitrogen during the autumn and winter periods, when crops are not grown. Today's statutory cover crops cannot contain legumes that can fixate nitrogen from the atmosphere. However, research shows that if you choose to mix the cover crop with legumes, additional nitrogen may be added to the soil by ploughing the cover crops into the soil which at the same time reduce nitrogen leaching.

“If you mix legumes in the cover crop, they are effective in reducing the leaching as they grow, but they also bind some of the nitrogen and it is subsequently released when the cover crop is ploughed into the soil. And that nitrogen can add extra nitrogen to the following crop, but it may also cause increased leaching in the following years. In other words, the crops do not have as great an effect as one might wish for in terms of leaching, unless the cultivation strategies are adjusted accordingly. It requires knowledge of how the cover crop behaves when it is returned to the soil as well as when and how much nitrogen it releases. That is what we have investigated,” says Peter Sørensen. 

Nitrogen release by different types of cover crops

The researchers examined four different types of cover crops:

• Red clover
• Winter vetch
• Ryegrass
• Fodder radish

For each species, the researchers have examined the turnover of the top and root separately, to create a better picture of what happens if you just leave the roots in the soil or if you plow the whole plant into the soil before establishing a new crop.

“We found that nitrogen is released faster from the top than from root material, so that nitrogen becomes faster available for the new crop. We have figured out how much nitrogen there is in the cover crops, as well as how much and how fast it will be released in the first months,” says Peter Sørensen.

The experiment itself was conducted under spring-like conditions at 10 degrees in the laboratory. And they found that some of the cover crops release a lot of nitrogen within the first three weeks. 

“Winter vetch and red clover release their nitrogen very quickly, while ryegrass and fodder radish instead bind nitrogen initially, after which they release small amounts much more slowly and over a longer period. Where winter vetch and red clover after 100 days have released 30-40% of their nitrogen, then ryegrass and fodder radish have only released approx. 17% from the top,” explains Peter Sørensen.

100 days

The researchers used a period of 100 days to measure the release of nitrogen by the different cover crops. The 100 days represent roughly the period, from the cover crop was ploughed into the soil, and until a new subsequent crop stops the uptake of nitrogen.

“We want nitrogen to be available for the next crop as soon as possible, so it can be absorbed faster. We see in the results that ryegrass and fodder radish only release 10-20% during that period. This means that there is 80-90% left in the soil after harvesting. That nitrogen will be released over a longer period and can contribute to increased nitrogen leaching in the longer term,” says Peter Sørensen. 

“Eventhough cover crops may leave nitrogen in the soil to be leached later, they still play a very important role in reducing nitrate leaching. We just have to take into account their fertiliser effect and try to make the best use of it. By incorporating nitrogen-fixing species, there is a possibility of greater fertiliser effect for the following crop,” explains Peter Sørensen.

Behind the research