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Climate Hustle

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.

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.

Image showing soil carbon cycle

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

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:

“In many soils under a temperate humid climate, an increase of air temperature results in increased microbial soil organic carbon decomposition rates.”

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:

“[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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Comments

Comments 1 to 4:

  1. The fact that the soils of the world still contain 3 times as much carbon as the atmosphere, despite year of chemical agriculture which has sent soil carbon into the atmosphere at a rapid rate is a cause for hope.  Techniques such as no till agriculture put carbon back into the soil and if widely adopted, would start to suck carbon from the atmosphere.  You will be aware that the atmospheric carbon goes up 8ppm and down 6ppm each year indicating how powerful natural processes are.  If we were truly to recruit this and other natural processes, I bet we could start to reduce atmospheric carbon at a surprisingly fast rate.  (of course this must go along with a cessation of using fossil fuels).  Other measures would include spreading beavers throughout all our catchments, ceasing to catch any whales, (see Monbiot's TED talk), increasing forest plantations, incorporating the wood into long lasting structures, burning the waste wood to the charcoal stage for energy and incorporating the charcoal into our soils and so forth.  Nature would like to be our friend if we would just work with her.  If not, she will reap us.

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  2. It really bothers me when articles like this show up. Not that they are false. It is well known and has been for many decades that forests, especially in warmer climates, are poor sequesterers of carbon in soil, and the carbon on the forest floor is labile carbon, not stable carbon. The vast majority of stable soil carbon is in grassland/savanna biomes as the grasses are what sequester a far larger % of the products of photosynthesis deep in the soil profile. So once again this whole line of inquiry is flawed before it even began, because it is the wrong biome.

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  3. william@1,

    Techniques such as no till agriculture put carbon back into the soil and if widely adopted, would start to suck carbon from the atmosphere

    Although it sounds good, Im ' skeptical that much carbon can be sequestered thay way. It will find its way back to atmosphere in very short time because the two reservoirs are tightly coupled and ever-present microbial action tends to re-ballance any diturbance you create. Think about a model of two pools connected by a shallow, large diameter pipe. How much work do you need to do restrict the equilibrating flow in the pipe while you're trying to move water from one pool to anoher? And if your restriction starts leaking? Your efforts are going to waste.

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  4. You can be skeptical you want Chriskoz, but this study is on the wrong biome and a different carbon pathway. It's meaningless, as is your arguments as to why you are skeptical as well. You are talking about labile carbon, which has a very tiny relative impact on soil carbon anyway. The pathway that has potential to actually be used by agriculture to sequester carbon long term deep in the soil profile is called "the liquid carbon pathway" ie photosynthesis-root exudates-micorrhyzal fungi- glomalin-humic polimers which bind tightly to the soil mineral base and last thousands of years or more if undisturbed. This is a defining property of grassland biomes and is what created the deep rich high carbon mollisols of the world.

     

    http://amazingcarbon.com/PDF/JONES-LiquidCarbonPathway(AFJ-July08).pdf

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