Globally, agriculture accounts for 32% of man-made methane emissions. As a greenhouse gas, methane is more potent than CO₂, but it decomposes relatively quickly in the atmosphere and has an average lifespan of about 12 years. Methane therefore contributes significantly to global warming, but for a shorter period of time. On the other hand, this means that reductions in methane emissions will be particularly effective in helping to reduce global warming by 2050, which is a climate policy benchmark.

Methane is mainly emitted by cows and other ruminant livestock, but about a third comes from manure stored as slurry on livestock farms. And it is this source that researchers from the Department of Agroecology, together with colleagues from the Department of Biological and Chemical Engineering, Aarhus University and the Department of Food and Resource Economics, University of Copenhagen have studied. 

In a pilot-scale trial, they investigated the potential for using acidification of slurry as a low-cost climate impact tool by reducing methane emissions with a lower dose of sulphuric acid than is normally used to reduce ammonia losses.

Lower costs, same methane reduction

"In a previous study, we had tried to separate the effect of acid addition from the effect of sulphate added with sulphuric acid. It turned out that acid addition and sulphate also separately produced a significant reduction in methane emissions. This suggested that different mechanisms are at play in inhibiting methane production in slurry, and it led to the idea that a lower dose of sulphuric acid might be sufficient to achieve a large climate effect. We decided to test this by dosing different levels of sulphuric acid," explains Professor Søren O. Petersen from the Department of Agroecology at Aarhus University. 

The highest dosage in the experiment with slurry from slaughter pigs corresponded to the level required today for acidification of pig slurry during storage, which is 6 kg sulphuric acid per tonne. In total, five different levels of acidification were tested. And even the lowest level of acidity reduced methane emissions by 50%.

"We have made a preliminary calculation of the total greenhouse gas reduction, taking into account that ammonia is a source of nitrous oxide, and we have looked at the costs of slurry acidification and nitrogen fertiliser purchase. Here we found that the second lowest dose of about 2 kg per tonne gave the best cost-effectiveness," says Chun Ma.

Low-dose acidification has not yet been used in practice, and this gives some uncertainty about the prices used in the calculations

But there is potential for a new climate tool that will not only have a good effect on greenhouse gas emissions, but also on farmers' budgets.

Other methods needed to prevent ammonia losses

"Slurry acidification was developed as a means of reducing ammonia volatilisation, and this requires a lot of acid. In our trials, we have tested the effects of using smaller amounts of acid. Obviously, you then don't get the same reduction in ammonia losses," says Chun Ma.

2.1 kg per tonne was the most effective amount of acid in the experiment in terms of using acidification as a climate agent instead of an ammonia agent. This will then require the use of traditional measures to avoid ammonia losses during storage, i.e. a solid cover such as a tent cover, or maintaining a surface crust on the stored slurry.

Slurry acidification also reduced slurry odour with lower doses of acid. But there was another advantage:

"A positive side-effect of using acidification as a climate tool is that the amount of sulphur released by normal acidification is far too high for the needs of crops. There is therefore a risk that the excess sulphur in the form of sulphate is lost through leaching, like nitrate. What happens when sulphate reaches wetlands and streams is unclear but should be investigated. Either way, it's a risk that can be avoided with low-dose acidification, and that's an attractive benefit," explains Søren O. Petersen.

Full scale this autumn

With support from the Ministry of the Environment, new full-scale trials are set to begin this autumn, allowing researchers to test the climate tool in practice. This will hopefully confirm the effectiveness.

"At full scale, it is important to find out how we can best ensure the acidification effect when fresh, untreated slurry is fed to the storage tank from the barn at regular intervals. Among other things, we will investigate how and how often the slurry tank should be stirred to ensure that the methane production in the entire slurry tank is kept to a minimum," says Søren O. Petersen.