More than 40% of global agricultural land is acidic, which reduces plant growth. However, the well-known practice of liming the field can raise the pH of the soil. This is good for the crop yield, but liming also affects the production and consumption of greenhouse gases in the soil. Now, researchers from, among others, the Department of Agroecology at Aarhus University have investigated how liming affects the fluxes of greenhouse gases in the soil.

“Normally, farmers lime their soil to make the crop grow better and thus get greater yields. We wanted to investigate how liming the soil can affect greenhouse gas emissions and the underlying mechanisms associated with microbial structure and function in an arable soil,” explains tenure-track researcher Diego Abalos from the Department of Agroecology.

Field experiments from 1942

The experiments were carried out on a c. 80-year-old field experiment in Jyndevad in Denmark. The liming treatments were established since 1942.

“Long-term experiments like this in Jyndevad are very valuable to us. Especially when we conduct research in greenhouse gas emissions, we can be more confident in the results when we work with data from such a long period of time. Emissions of greenhouse gases can be strongly affected by, for example, precipitation and temperature, and if you use data from a short timeframe, it may affect the end result,” explains Diego Abalos, who performed the studies in collaboration with Associate professor Lars Elsgaard and postdoc Zhi Liang at the Department of Agroecology. The group worked together with Dr. Peter Dörsch from the Norwegian University of Life Sciences. 

The long-term experiment gave the researchers the opportunity to investigate what happens in the soil when the lime is added and the pH value rises.

Less nitrous oxide and greater uptake of methane

“By using lime, you will not only increase the crop yield, but the nitrous oxide emissions will also be greatly reduced (40-82% reductions). This is due to a pH effect on bacterial enzymes, which we are now starting to understand. In addition, we found that it also helps to increase the soil's uptake of methane,” says postdoc Zhi Liang. 

Although the magnitude of the effect on methane is not very large, arable land can take up methane from the air and oxidize it, and when the pH of the soil increases due to liming, this capacity of the soil seems also to increase. 

“So, you get the combination of higher yields, lower nitrous oxide emissions and higher consumption of methane in the soil. So liming is definitely a very good practice,” says Diego Abalos. 

Slightly higher CO₂ emissions

But there is also a downside. CO₂ emissions increase slightly as the soil pH increases. The increase occurs because the soil microorganisms thrive when the soil is no longer so acidic. And when they thrive, they also emit more CO₂.

“There is carbon in the plants, and part of it is returned to the soil after harvest. Liming increases plant growth and yield, which means that the amount of carbon that is returned to the soil also increases. And while some of the carbon remains in the soil, some is metabolized by the microorganisms and emitted in the form of CO₂,” says Diego Abalos. 

Liming in three rates

To find out how much lime is needed to achieve the best results, the researchers studied an unlimed reference treatment (pH of 3.77) and liming in three rates at four, eight, and twelve tons of lime per hectare (pH of 4.92, 6.39, and 6.84, respectively). The more lime that is added to the field, the higher the pH value. However, the other effects are not quite as straightforward. 

“The exact amount of lime needed to maximize crop yield depends on the crop species and the soil characteristics, such as initial pH, texture and carbon content. In our study, here the crop was spring barley, we found the highest yield by liming moderately – in our experiment, it was eight tons per hectare and not twelve. And when we talk about the emission of nitrous oxide, all you have to do is increase the pH value so that the soil is no longer acidic, you do not reduce more nitrous oxide by increasing the pH-value more than that,” explains Zhi Liang, and adds: "When we looked at the soil's ability to take up methane from the air, we actually found that the lowest rate of 4 tons of lime per hectare was the optimal”.

Challenging to achieve all the benefits at once

Despite the generally positive effects of liming, the study also revealed that it is challenging to achieve an optimal pH value in the soil that can maximize yields while reducing greenhouse gas emissions.

“We saw that the rate of liming that ensured the maximum crop yield also increased CO₂ emissions and reduced the oxidation of methane compared to lower liming rates. However, we also saw that the nitrous oxide emissions were the same at all liming rates, but was greatly decreased compared with the unlimed reference. Therefore, our field experiment suggests that targeting a pH at 6.39 for crop yields seems to be the most appropriate option to balance crop production and mitigation of climate change,” concludes Diego Abalos.