Sulphur deficiency has been a recurring problem in agriculture in Denmark as well as in the rest of the world. Stricter rules on preventing air pollution from industry have significantly reduced the amount of sulphur particles in the air.

“If you look at Denmark as an example, the atmospheric sulphur deposition has fallen from 20 to 2 kg ha^-1 y^-1between 1970 and 2016. And this is really problematic for crop cultivation, because the mineralization of sulphur from soil organic matter is insufficient to meet the needs of the crops. They need sulphur, and where they used to get it from sulphur deposited with the rainfall, they now have to get it elsewhere. Therefore, the farmer typically fertilizes his/her field with sulphur in the form of sulphate or elemental sulphur, so that the needs of the plants are met,” explains PhD student Doline Fontaine from the Department of Agroecology.

Together with Professor Jørgen Eriksen and Senior Researcher Peter Sørensen, she has investigated how sulphur extracted in the biogas plant's desulphurisation process can be used as a plant-available source of sulphur in the farmer's fields, even after storage. 

Sustainability and bio-based economy

There is a growing focus on sustainability and biobased economics, and with that also comes a focus on nutrient recycling in agricultural systems. Anaerobic digestion produces two main products: digestate and biogas. The digestate corresponds to the material remaining after anaerobic digestion of a biodegradable raw material. Previous research has shown that the anaerobic digestion contributes to a significant increase in the fertilizer value of organic materials, especially when it comes to phosphorus and nitrogen, but not for sulphur. 

“In fact, it happens that some of the sulphur is reduced to hydrogen sulphide during the process in the biogas plant, and this hydrogen sulphide is in a gas form that leaves the system together with the rest of the biogas. However, we can remove hydrogen sulphide from the biogas before the gas is used, for example, to heat houses, to produce electricity or to be injected in the natural gas grid. This is done by purifying the gas with some special filters that can capture the sulphur. In the NutHY project, it is precisely this sulphur filter product we are interested in. We have investigated whether it can be used in crop production, as it contains both sulphate and elemental sulphur,” explains Doline Fontaine.

Stored in manure

When the farmer wants to add nutrients to his field, he must make sure to synchronise applications with the need of his crops. Sulphur produced within the so-called biogas desulphurisation process, as described earlier, is produced continuously and stored until it can be used in the fields.

“The sulphur filter products have generally a low S concentration and can be corrosive; therefore, farmers usually stored them in the existing manure storage facilities. When the sulphur filter product is stored for a longer period of time in a manure tank rather than being added immediately to the fields, it is expected that there will be a turnover of sulphur. Manure is a reductive environment that is very suitable for microbial activity, and it will cause the transformation of nutrients such as sulphur,” explains Doline Fontaine. 

In other words, there is a risk that the sulphur content will be reduced and potentially volatilized when stored in manure. In the project, the researchers conducted a laboratory study of how sulphur can be recovered from the filter products from the biogas desulphurisation process, how sulphur is converted during storage in untreated or digested manure and its subsequent plant availability.

In total, the researchers examined three different sulphur filter products:

1. A biological desulphurisation process (BioF)
2. A chemical absorption process with ash from straw (AshF)
3. A combination of chemical absorption and biological regeneration (Fertipaq)

Results showed that BioF and AshF contained a high proportion of sulphate, while elemental sulphur was the major proportion in Fertipaq.

Less reduction of sulphate in digested manure than in untreated manure

“Our study showed that the reduction of sulphate in untreated manure started after one month of storage and increased significantly after the second month. After four months, more than 50% and after six months, as much as 70% of the initial sulphate content was converted to sulphide. In the case of digested manure, the reduction of sulphate started somewhat later and at a slower rate than untreated manure. The sulphate concentration did not change during the first two months of storage. After six months, 65% of the original sulphate was still there,” explains Doline Fontaine.

In addition, the laboratory study showed that  the filter material made of straw ash (AshF) and mixed to the digested manure had an almost complete conservational effect on sulphate, even over prolonged storage. In sharp contrast,  pure elemental sulphur (Fertipaq) was reduced and oxidized simultaneously and immediately during storage in all types of manure. 

Filter products added to manure significantly increase sulphur uptake

“Our study showed that the fraction of plant available S that is normally around 15-19% of the total S contained in manure increased to 56-90% when the filter materials was added to manure. This shows us that sulphur from filter material is quite available to plants when applied as fertilizers,” says Doline Fontaine.

“The preservation of sulphate and elemental sulphur during storage is important to ensure an optimal efficiency of the sulphur filter products for plant fertilization. There is a minimal risk of reducing sulphate to sulphide when the sulphur filter products are stored in digested manure, especially if the pH value is equal to or higher than 8.2. At a pH of 5.5 to 8.2, it is necessary to keep the storage time as short as possible for all types of manure, even if there is only a small reduction within the first four to six weeks. For the filter product containing mostly elemental sulphur, we recommend a separate storage under conditions that prevent reduction, and eventually to add it to the manure shortly before field application,” concludes Doline Fontaine.