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

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

Posted on 28 June 2016 by Guest Author This is a re-post from Robert McSweeney at Carbon Brief

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.

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

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.

….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.

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:

“[The study’s] evidence that climate change has already started depleting soil carbon in the German Alps raises the possibility that a positive feedback between climate and ecosystems is beginning.” 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.

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