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Researchers measure nitrate leaching using vegetation indices

Researchers from the Department of Agroecology at Aarhus University have found a method to assess the effect of cover crops on nitrate leaching using vegetation indices, which can be obtained by satellite or drone images.

[Translate to English:] Foto: AU Foto

Nitrogen is the most important nutrient in crop production. The large inputs and turnover of nitrogen in agricultural systems enhances risks of nitrate leaching to the aquatic environment. The leaching raises major concerns worldwide due to pollution of both groundwater and surface water. Especially in northern Europe, where large amounts of rain during autumn and winter leads to large amounts of nitrate leaching.

“Nitrate leaching is a very serious problem in Denmark. During the winter there are often no crops in the field, and the low evapotranspiration and large rainfall means that there is a precipitation surplus that easily leads to nitrate leaching to aquatic systems,” says Postdoc Jin Zhao from Department of Agroecology.

Cover crops can help

According to the researchers, nitrate leaching depends on the vegetation cover in the fields and how it is managed in autumn and winter. Cover crops can help absorb and retain the excess nitrate in the soil, which would otherwise be leached from the root zone. In addition, cover crops have the advantage that the nitrate they absorb and retain will be released into the soil when they decompose, acting as fertilizer for the following crop. But according to the researchers, it takes a little more than just planting cover crops in the field.

“We have found in previous studies that you cannot be sure that the crops absorb a lot of nitrate, just because they are planted. If they do not have good growing conditions and thus do not grow well enough, they will not be able to reduce nitrate leaching very much. The better the crop is, the more nitrate it will be able to absorb and the less will be leached to the aquatic environment,” explains postdoc Chiara De Notaris from the Department of Agroecology.

It is not just about planting a as many cover crops as possible; it is important to nurture them so that they grow well.

“If conditions are not good enough, we still experience problems with nitrate leaching. When the cover crop reaches some threshold coverage of the field, one can be sure that the leaching is reduced  to a fairly stable level. So it is not just about planting even more cover crops, but about growing them well,” says Chiara De Notaris. Therefore, the researchers believe that it is important to find a way in which the effect of the cover crops can be measured more easily than by physical observations in the field. This may help more farmers grow cover crops and ensure that they grow well, the researchers believe.

Satellites and drones as a measuring tool

Because nitrate leaching can be a major problem in Denmark, several regulations have been set up, where cover crops are included as a measure to stop leaching. The rules, among other things, regulate where to establish cover crops, how much and when.

“In Denmark, rules are set for the use of cover crops to mitigate nitrate leaching, but it is very time-consuming for the authorities to monitor because they have to physically visit each field. That is why we have investigated the possibility of using remote sensing by satellite or drones to measure the plant cover in the field and to determine if the cover crops are sufficiently effective in reducing nitrate leaching,” Jin Zhao explains.

The researchers have investigated three different vegetation indices to find the one that can best be used to measure nitrate leaching and the effect of the cover crops.

“We have examined the Ratio Vegetation Index (RVI), Normalized Difference Vegetation Index (NDVI) and Ratio Red Edge (RRE). The difference between them is the way they are measured and calculated. For example, RRE can be calculated from satellite information, while RVI cannot. So by figuring out which of the three indices works best for measuring nitrogen leaching, we could also conclude whether or not we will actually be able to use satellite measurements,” Jin Zhao says.

The researchers' measurements showed that RRE was the index among the three selected ones that worked best and was most stable throughout the experimental period, even with a large vegetation cover in the fields.

“Ratio Red Edge was stable and worked best in all year of our trial. And RRE can be calculated from satellite information, which means we can use satellites and drones to measure the degree of nitrogen leaching, and that's good news, "Jin Zhao says.

Before this can be implemented for farms in Denmark, the method needs to be further developed.

“We need more studies on this. We based our assessment on three years of observations at one site, so we need expand this to other soils and cropping systems. Hopefully within a few years farmers and authorities will be able to use this method for assessing cover crops;  it will save them a lot of time and labour,” Jin Zhao concludes.


The publication ”Autumn-based vegetation indices for estimating nitrate leaching during autumn and winter in arable cropping systems” will be published in Agriculture, Ecosystems & Environment in March 2020. It is written by Jin Zhao, Chiara de Notaris and Jørgen Eivind Olesen.

For further information

Postdoc Jin Zhao, Department of Agroecology, Aarhus Universitet. Email: jinzhao@agro.au.dk

Postdoc Chiara de Notaris, Department of Agroecology, Aarhus Universitet. E-mail: cdn@agro.au.dk