Researchers are now going to monitor carbon in forest and grassland soils

Every summer since 2023, NIBIO's field workers have been recording carbon stock in forest and grassland soils across Norway.

Large amounts of carbon are stored in the roots, trunks, and branches of trees. But forest soil also stores a significant amount of carbon.

“Globally, there's two to three times more carbon stored in the soil than in the atmosphere,” says Shun Hasegawa. He is a research scientist at NIBIO and the leader of the Norwegian Soil Carbon Monitoring Programme.

Hasegawa explains that the boreal forests, which cover much of the Northern Hemisphere, make up about one-third of the world’s forests. These forests store between 20 and 30 per cent of all carbon found on land. Of this, around 60 per cent is stored in the soil. 

Forest soil in Norway

When it comes to forests in Norway, researchers estimate that there is about three times more carbon stored in forest soil than in the trees above ground.

“But today, we still lack knowledge about the size of the carbon stock in forest soils in Norway. Estimates of soil carbon in Norwegian forests vary substantially between studies,” Hasegawa says, adding:

“Moreover, there is no comprehensive record of how soil carbon stocks have changed over time, leaving considerable uncertainty.”

Gathering valuable knowledge about carbon stocks in forests

Now, however, efforts to fill these knowledge gaps concerning soil carbon storage are underway.

In 2022, NIBIO was tasked with starting a nationwide monitoring programme of carbon in forest soils and grazing lands.

“The plan is to visit 3,000 plots in forests and 300 plots in grasslands by 2033,” says Irene Marongiu.

She is a senior engineer at NIBIO and coordinates the fieldwork.

“The goal is to determine how much carbon is stored in forest and grassland soils and how this stock changes over time," she says. 

The newly launched soil carbon monitoring programme will provide valuable new knowledge about forest soil carbon stocks. It will also be useful for Norway’s greenhouse gas accounting.

“The sample plots we're visiting this summer have been systematically laid out along a grid to ensure good statistical representation of the variation found in forests and grassland around Norway,” says Marongiu.

Together with colleagues, Marongiu has already completed two field seasons. Now, equipment is being prepared for the 2025 field season.

Thousands of soil samples

At a sample plot in Frogn municipality, south of Norway's capitol Oslo, Marongiu shows us how soil samples are collected.

She uses soil augers with two different diameters: the wider one for the upper soil layers, and a narrower one for the deeper layers, down to 30 centimetres. Marongiu divides each soil sample into different layers, and the carbon content in each layer is analysed.

Following the field sampling, the soil samples are sent to NIBIO's chemical laboratory. There, thousands of soil samples are analysed every year to determine carbon content.

For each sampling plot, several soil samples are taken. Altogether, the measurements will determine the average carbon content in the soil at that specific location. In addition, soil profiles are taken at some of the sample plots. Soil profiles are a vertical cross-section of the soial from the surface down to the subsoil. 

Marongiu explains that soil consists of weathered minerals – rocks that have been broken down – as well as organic material with varying degrees of decomposition. Organic material is the remains of dead microbes, fungi, plants, and animals.

Several layers of different soil types

Typical soil types in Norwegian forest soils are podzols and brown soils.

“Soil is often divided into layers, and soil profiles provide a lot of information about the properties of the soil,” Marongiu explains.

In forests, the top layer is called the humus layer. It consists of remnants of dead plant material, among other things. The humus layer is rich in carbon. Beneath the humus layer is the mineral soil layer. It has a grainy texture and lower carbon concentrations.

Marongiu explains that the thickness of the various soil layers, how quickly the soil breaks down, and its colour depend on which minerals the soil consists of. But also the climate, the living organisms that inhabit the soil, the forest landscape itself, and how much time has passed since the soil was formed all play a role.

Capturing changes in the carbon stock over time

According to Shun Hasegawa, the soil carbon monitoring programme will serve as an important foundation for further research on soil carbon dynamics in Norway.

“By repeating soil sampling after about 10 years, we can detect changes in the soil’s carbon content over time. This gives us knowledge of how much carbon is being absorbed or lost from the soil in Norwegian forests and grazing lands," he says. 

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Read the Norwegian version of this article on forskning.no

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