Monitoring carbon in forest and grassland soils

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 is two to three times more carbon stored in the soil than in the atmosphere”, says Shun Hasegawa, Research Scientist at NIBIO, and 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. Boreal forests store 20 to 30 percent of all carbon found on land. Of this, around 60 percent is stored in the soil. When it comes to forests in Norway, scientists estimate that there is about three times more carbon stored in forest soils than above ground, in the trees.

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

"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

But efforts to fill these knowledge gaps concerning soil carbon storage is now underway.
In 2022, NIBIO was commissioned to launch a nationwide monitoring program for 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 senior engineer Irene Marongiu at NIBIO, who 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."

The newly launched soil carbon monitoring program will provide valuable new knowledge about forest soil carbon stocks and contribute to Norway’s greenhouse gas accounting.

“The sample plots we are 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”, Marongiu says.

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

And to dust you shall return

At a sample plot in Frogn municipality, south of Norway's capitol Oslo, Marongiu shows us how soil sampling is performed. 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 later the carbon content in each layer is analysed.

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

For each sampling plot, Marongiu and her co-workers take several soil samples and bulk them for each layer. Together, the measurements will determine the average carbon content in the soil at that specific location. In addition, Marongiu obtains and examines soil profiles — a vertical cross-section of the soil from the surface — at some sites to assess soil characteristics.

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.

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— consisting 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, with a grainy texture and lower carbon concentrations.

Marongiu explains that the thickness of the various soil layers, how quickly the soil breaks down, and the colour, all 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 influence these factors.

Capturing changes in the carbon stock over time

According to NIBIO researcher Shun Hasegawa, the soil carbon monitoring program 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”, Hasegawa concludes.