Grasslands in Iceland stored more topsoil carbon with continued grazing, suggesting livestock removal does not always benefit carbon storage.
Researchers have often counted fencing livestock out of a pasture as a low-cost way to store more carbon in the ground. In the grasslands of Iceland, a new study found the opposite. Where sheep and horses kept grazing, the top few inches of soil held more carbon than land fenced off from them for decades.
For this cold northern land, the authors concluded that continued, low-intensity grazing helps soil keep its carbon. The difference was modest: about 8% less carbon in the top 4 inches (10 centimeters) once the animals were gone, and none deeper down. The evidence also comes from one country.
Christian Klopsch led the work as part of doctoral research at the University of Iceland. Anna Gudrun Thorhallsdottir, a professor at the University of Iceland at Holar, supervised it. Co-authors included scientists at the Swedish University of Agricultural Sciences and Lancaster University.
“The grazers are often forgotten actors in the formation and composition of grassland soils,” Thorhallsdottir told Earth.com.
Old fences became a natural experiment
Over the past century, fences went up across Iceland to keep livestock off patches of grazed land. That created side-by-side plots with the same soil, slope and weather, one grazed and one not.
The team found 34 of these pairs in Iceland’s lowlands, with fences that had stood for 20 to 83 years.
In late summer 2022 and 2023, the researchers dug soil pits about 2 feet (60 centimeters) deep on both sides of every fence. They sampled the soil layer by layer and weighed the plants and the roots in the topsoil.
Nearly all the carbon was underground
On average, the top 2 feet of soil held about 58 tons of carbon per acre (130 metric tons per hectare). That was roughly ten times as much carbon as was stored in all the plants, including their roots and dead leaves.
So a small change in the soil can outweigh a large change in what grows on top of it.
Iceland’s soils form on volcanic material, which breaks down quickly into clay minerals that bind carbon and hold it in place.
Fenced land held less in its topsoil
Where grazing had stopped, the top 4 inches (10 centimeters) of soil held 8% less carbon on average than the grazed ground next to it. That thin layer alone carried about a third of all the soil carbon down to 2 feet (60 centimeters).
The roots showed a bigger gap. Ungrazed plots had 29% less carbon in topsoil roots than grazed plots, and 40% less on grassland. Nitrogen, a nutrient plants need to grow, was 9% lower in the topsoil once the animals were gone.
Above ground, the fenced plots did carry more plant material, about 45% more carbon in their shoots and litter. That figure leaves out whatever the animals had eaten, which the team didn’t measure. And the gain stopped growing after 20 to 30 years without grazing.
Not every fenced plot fell behind. Some grassland stayed grassland for more than 50 years without animals, and its topsoil held as much carbon as grazed land. The authors tie that to nitrogen left over from heavy grazing in the past.
Grazed grass may send more carbon down
The authors think grazing changes how grass grows and stores carbon below ground. When animals crop grass, it regrows, and grazed grassland builds dense roots that die back and regrow quickly, releasing carbon into the soil. Dung returns nitrogen, and soil microbes feeding on both leave remains that can stick to soil minerals and stay there.
A study of bison in Yellowstone, too, suggests that grazing can push more of a plant’s carbon below ground.
Left ungrazed, more of the carbon stays above ground in tall shoots and dead litter. The litter likely shades new growth, and as it rots, much of its carbon goes back to the air without passing through the soil. The team didn’t measure these steps directly.
Shrubs and birch replaced the grass
In the far north, grazing animals tend to keep land open as grassland. At nine of the 34 sites, the landscape changed after they left. Grassland gave way to heath, a habitat of low, woody shrubs, at seven sites and to birch woodland at two.
The authors link that shift to nitrogen. Without dung, soil nitrogen ran short, and slower-growing shrubs likely outcompeted grasses that need more of it.
The type of plant cover made a bigger difference to soil carbon than grazing did. Grassland soil held about 43 tons of carbon per acre in its top foot, against about 35 under heath and 25 under mature birch woodland.
That matters because efforts to fight climate change with nature have focused largely on planting trees, and scientists have increasingly questioned that approach in the far north.
“With carbon primarily stored belowground, resilient to disturbances like drought and fire, grazed grassland is a good candidate for a nature-based climate solution,” Thorhallsdottir said.
A missing baseline limits the numbers
The researchers did not plan the fences as an experiment, and they did not measure the soil before the fences went up. So the team couldn’t say how fast carbon builds up each year, only how the two sides differ now.
On grassland, that difference works out to about 130 pounds less carbon per acre (0.15 metric tons per hectare) for every year without grazing.
Across the sites, grazing ranged from constant use to light summer pasture, and the study treated it as one thing. Iceland’s volcanic soils may bind carbon better than soils in other northern regions. And some studies of temperate grassland have found no change in soil carbon after decades without grazers.
“Grazing is not the same as grazing,” Thorhallsdottir said. Local grasses, the kind of animal and moisture will shape the outcome elsewhere, she added.
Whether bringing animals back would restore lost carbon remains open. The authors think it could, but a computer simulation of dry rangelands suggested that shrub-covered land takes far longer to return to grassland than it took to change.
The team has shared its data and calls for future studies to track how often, when, for how long, and how heavily land is grazed.
The full study was published in the journal Global Change Biology Communications.
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