Additive reduces methane during long-term storage of dairy cattle and pig fattening sludge

Livestock farming produces large amounts of greenhouse gases, especially methane, which is particularly harmful to the climate. Among other things, it escapes during the storage of animal excrement, pulp. A study from the University of Bonn now shows that methane emissions can be reduced by 99% with simple and cheap means. The method could make a significant contribution to the fight against climate change. The results have now been published in the journal Waste management.

Greenhouse gases act like a layer of window glass in the atmosphere: They prevent heat from radiating from the Earth’s surface into space. Methane does this 28 times more efficiently than carbon dioxide – it’s (to stick with the image) a kind of invisible double glazing.

Over the past 200 years, the concentration of methane in the atmosphere has more than doubled. This is mainly due to human consumption of meat. First, cows and other ruminants produce methane during digestion. Another important source is animal excrement. “A third of the world’s anthropogenic methane comes from animals,” explains Felix Holtkamp, ​​who is completing his PhD at the INRES Institute of Crop Science and Resource Conservation at the University of Bonn. “It is estimated that up to 50% of it comes from fermentation processes in the pulp.”

Researchers around the world are therefore looking for ways to suppress these processes. Holtkamp, ​​his scientific supervisor Dr. Manfred Trimborn from the Institute of Agricultural Engineering of the University of Bonn and Dr. Joachim Clemens from fertilizer manufacturer SF-Soepenberg GmbH have now presented a promising solution to the problem. “We combined pulp from a farm in the lab with calcium cyanamide, a chemical that has been used as a fertilizer in agriculture for over 100 years,” says Holtkamp. “This brought methane production to an almost complete halt.”

Emissions reduced by 99%

Overall, emissions were reduced by 99%. This effect started just one hour after addition and remained until the end of the experiment half a year later. Long-term effectiveness is important because the pulp is not simply discarded. Instead, it is stored until the beginning of the next growing season and then spread over the fields as a valuable fertilizer. Therefore, months of storage are quite common.

During this period, the pulp is transformed by bacteria and fungi: They break down the undigested organic material into smaller and smaller molecules. Methane is produced at the end of these processes. “Calcium cyanamide breaks this chain of chemical transformations and does it simultaneously at different points, as we were able to see in the chemical analysis of the appropriately treated pulp,” explains Holtkamp. “The substance suppresses the microbial degradation of short-chain fatty acids, an intermediate in the chain, and their conversion to methane. Exactly how this happens is still unknown.”

But the substance has other advantages: It enriches the pulp with nitrogen and thus improves its fertilizing effect. It also prevents the formation of so-called floating layers – these are deposits of organic matter that form a solid crust on the pulp and prevent gas exchange. This crust usually needs to be broken and stirred regularly.

The process also has advantages for the animals themselves: They are often kept on floors called slats. Their droppings fall through openings in the floor into a large container. Microbial conversion allows the stool-urine mixture to foam over time and rise again through the voids. “The animals are then standing in their own excrement,” says Holtkamp. “Calcium cyanamide stops this foaming.” The cost is also manageable – it’s about 0.3 to 0.5 cents per liter of milk for cattle farming.

The “purity law” currently prevents use

It is not yet clear how the method affects the release of ammonia from the pulp. Ammonia is a toxic gas that, although not harmful to the climate itself, can turn into dangerous greenhouse gases. “We have initial indications that the amount of ammonia also decreases in the long term,” says Dr. Manfred Trimborn of the Institute of Agricultural Engineering at the University of Bonn. “We can’t say for sure at this point, though.”

In Germany, an environmental law currently prevents the addition of calcium cyanamide: A strict purity requirement currently applies to conventionally stored pulp.