Cover crops are grown to reduce leaching of nutrients and prevent soil erosion during the wet autumn and winter months in Denmark. The cover crops are most often sown under the main crop or sown immediately after the main crop has been harvested, and it is most often decomposed prior to the following crop so that the nitrogen from the cover crop can be released for the following year's main crop. However, it is also possible to harvest the cover crop as a green manure. Previous research has shown that co-ensiling and storage of green manure can help improve nitrogen release in the following year. For ecologists, green manure can play a major role in bringing nitrogen and nutrients such as sulphur back into the soil. Previous research has shown greater availability of nitrogen when green manure has been through a biogas plant, and researchers from Department of Agroecology have investigated the effect of silage and biogasification of different crops on the availability of nitrogen and sulphur in the green manure.

Three different green manures

Senior Researcher Peter Sørensen from Department of Agroecology has been instrumental in investigating the fertiliser effect under three different conditions.

“We have looked at what the fertiliser effect is if the farmer is harvesting, co-ensiling and using the green manure as mobile green manure in the following year. We have compared that to a green manure that has been through a biogas plant, and last but not least we have also compared a green manure mixed with cattle manure before degassing,” says Peter Sørensen. 

The researchers thus worked with three different kinds of green manure:

1. Co-ensiled cover crops mixed with straw
2. Co-ensiled cover crops mixed with straw and biogas
3. Co-ensiled cover crops mixed with straw and manure and the biogas

It is not abnormal for manure and co-ensiled plant biomass to be mixed in the biogas process, and it was therefore very relevant for the researchers to also look at that mixture in the experiment. In addition, experiments with pure cattle manure were also included. 

“In practice, manure is typically added to the mixture in the biogas plant, and everything indicates that it makes the biogas process itself work better. It simply provides more gas. So, it is interesting to look at whether it is also positive in relation to the availability of the nutrients subsequently,” says Peter Sørensen.

Sulphur availability is especially important for organic farmers

In relation to fertiliser value, the biogas process is positive for nitrogen availability. Compared to sulphur, which is also an important nutrient, the results are not as good.

“A lot of sulphur disappears with the biogas, and we have not known until now whether sulphur would be available after degassing. It turns out that for all the different mixtures of green manure we have worked with, there is actually not much sulphur effect left,” says Peter Sørensen.

According to the researchers, this is bad news, especially for organic farmers. Although they will have a higher fertiliser value compared to nitrogen, through the green manure they will not get sulphur they need.

“Nowadays it is necessary to have sulphur added to the fields. Previously, the fields were given a lot of sulphur as a result of air pollution, but they no longer do so because today the smoke from power plants is so efficiently cleaned,” says Peter Sørensen.

Therefore, it will be necessary for organic as well as conventional farmers to add sulphur in other ways, and here the organic farmers do not have quite as many choices as the conventional ones.

"We are now investigating how the sulphur that is purified from the biogas in biological filters can best be utilised as fertiliser," says Peter Sørensen. "It looks like sulphur from gas filters can be poured into the degassed manure, but it should not be stored in the manure for too long," adds Peter Sørensen. 

Straw can be used more efficiently

"A higher availability of nitrogen during co-ensiling and after the biogas process is not new knowledge, but the combination with straw that we have worked with in these experiments is," says Peter Sørensen.

Straw typically binds nitrogen when it decomposes, and therefore usually helps to make nitrogen less accessible. But researchers have found that by co-ensiling and degassing, the mixture of cover crops and straw can reduce the binding of nitrogen in the straw.

“We can clearly see that by passing the mixture through the biogas plant, the straw does not bind so much nitrogen. This means that you will be able to use straw in the fertiliser and that you can utilise nitrogen as well as the other important nutrients in the straw to a much greater extent than if it is just ploughed into the soil,” says Peter Sørensen.

Instead of leaving the straw in the field and crushing it here, it would be beneficial to use it to provide green energy in the biogas plants and to bring nutrients back to the soil.

A related study

In a similar study, researchers from Department of Agroecology, together with researchers from Department of Engineering, have investigated how to get the most gas out of the biogas process by, for example, co-ensile crops and straw. Here, as in the above study, there is no focus on the nutrients. The results of this study show that there are also benefits to co-ensiling straw and cover crops relative to the amount of gas coming out of it in the biogas plant.

Read more about the study in the article “Co-ensiling of cover crops and barley straw for biogas production” here.

“The conclusion of the two studies is that co-ensiling of crops and straws contributes positively to the availability of nitrogen and the amount of biogas and thus green energy, but that it is not good for the amount of sulphur available. It is therefore important to find out how sulphur that is collected in gas purifiers is best utilised,” says Peter Sørensen.

Behind the research