Alpine soils storing up to a third less carbon as summers warm

The top metre of the world’s soils contains three times as much carbon as the entire atmosphere. This means that losing carbon from the soil can quicken the pace of human-caused climate warming.

A new paper, published today in Nature Geoscience, finds this is already happening in the forests of the German Alps. Soils there are losing carbon as summer temperatures rise, the researchers say.

In the last three decades, soil carbon across the German Alps has decreased by an average of 14% – and by as much as 32% for certain types of soils.

The findings might be a sign of how soils could amplify warming in future, other scientists say.

Crucial role

Soils play a crucial role in the global carbon cycle. The figure below, from a News & Views article that accompanies the paper, illustrates how carbon is taken up and released by soils.

Plants absorb CO₂ from the atmosphere through photosynthesis, and transfer carbon into the ground when dead roots and leaves decompose in the soil. Here, carbon is “immobilised” for anything from a week to thousands of years.

Eventually, the carbon is broken down completely, or “mineralized”, releasing CO₂ back into the atmosphere.

Soil carbon cycle. Source: Kirk et al. (2016).

The researchers collected samples of soil carbon from 24 sites in forests and 11 sites in pastures and meadows across the Alps of southern Germany.

“Set 1” of the sample sites (shown as black dots in the map below) is distributed across much of the German Alps – covering an area of around 4,500 square kilometres. “Set 2” (shown as triangles) is concentrated on a 600 square kilometre area of the Berchtesgaden region.

Map of study sites across Germany. Credit: Dr Jörg Prietze.

The scientists chose these sites to match up with samples collected in 1976 and 1987, allowing them to see how carbon levels in the soil has changed over time.

Across the forest sites, they find that levels of soil carbon has decreased by an average of 14% since the first samples were collected.

The size of the decrease is almost identical for the two different locations, the researchers note, with an average decline in carbon of 14.0% for Set 1 and 14.5% for Set 2.

The scientists also find that soils with a higher carbon content to begin with lost more of their carbon over the 30-year study period, averaging 32%.

While the researchers found a decrease in carbon in forest soils, they didn’t find a change in the samples taken from pasture soils.

Carbon appears to be more stable in these soils because of their high mineral content, says Dr Jörg Prietze, lead author of the paper and associate professor of soil science at the Technical University of Munich. The carbon in the soil clings to these minerals and isn’t released into the atmosphere as easily, he explains.

Warming summers

So, what role do rising temperatures play in the decline of soil carbon in the Alps?

Warmer conditions can speed up the turnover of carbon through the soil. Microbes in soils that break down organic carbon work harder in warmer temperatures, releasing more carbon, Prietze explains to Carbon Brief:

Overall, as conditions in the Alps get warmer, more carbon is lost from the soil than is added back in from dead plants, the paper explains.

Average temperatures between May and October across Set 1 of the sample locations (see earlier map) have increased by 0.5C per decade over the last 25 years, the study says. The equivalent temperature change for Set 2 is 0.3C per decade, but this warming isn’t statistically significant. This means the scientists can’t be certain that the warming hasn’t happened by chance.

The results suggest that forest soils in the German Alps have changed from a net sink of carbon to a net source, says Prietze. This means the soils are now releasing more carbon to the atmosphere than they are taking up.

Prof Guy Kirk, professor of soil systems at Cranfield University and author of the News & Views article, writes that the findings of this “exemplary” monitoring study might be a sign of how soils could amplify warming in future, perhaps triggering a self-reinforcing loop. He writes:

This positive feedback would see warming conditions speed up the release of carbon from the world’s soils, which would in turn warm the climate further.

Main image: View from Mt Jenner on to the Königssee. Credit: cc0.photo.