How shrubs are reducing the positive contribution of peatlands to climate

The Swiss Federal Institute for Forest, Snow and Landscape Research (WSL): For the first time, a group of scientists from WSL and EPFL described why on the long run peatlands may not be able to continue fulfilling their role as the most effective carbon stocking ecosystems. They studied the mechanisms behind a phenomenon known as shrub encroachment of peatlands: Complex plant-microbe interactions are at the root of this worldwide vegetation change. The findings have been published online today in Nature Climate Change.

Peatlands (bogs, turf moors) are among the most important ecosystems worldwide for the storage of atmospheric carbon and thus for containing the climate warming process. In the last 30 to 50 years the peat (Sphagnum) mosses, whose decay produces the peat (turf), have come under pressure by vascular plants, mostly small shrubs. A new study by scientists from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and from the Ecole Polytechnique Fédérale de Lausanne describes for the first time what lies behind this change in vegetation and explains why vascular plants are at an advantage over peat (Sphagnum ) mosses in a warmer climate.

The research team closely monitored four peatland sites at altitudes ranging from 600 m to 1900 m over a period of three years. The selected altitudinal gradient reflects the expected changes in climate conditions for the year 2050  in northern Switzerland. They observed that the increase of shrub cover and soil temperature along the altitudinal gradient were responsible for a decrease of almost 50% of the production of new litter by peat mosses, the main contributors to peat accumulation.

The analysis showed that vascular plants can increase the availability of soil nitrogen (a primary nutrient for plant growth) by means of specific compounds contained in their leaves. They exploit the nutrient for their growth through the mediation of specific fungal symbiosis at root level (the mycorrhiza), a process  that becomes more and more frequent when soil temperature increases. At the same time, with higher soil temperature vascular plants release a greater amount of organic matter into the soil through their roots (the so called “root exudates”) and this stimulates the decomposition activity of soil microbes....

A gully with peat moss, shot by Chris Eilbeck, Wikimedia Commons via Geograph UK, under the Creative Commons Attribution-Share Alike 2.0 Generic license

Carbon buried in the soil rises again

University of California-Davis News: A research team that includes a University of California, Davis, plant scientist has identified a source of carbon emissions that could play a role in understanding past and future global change. While earlier studies have found that erosion can bury carbon in the soil, acting as a carbon sink, or storage, the new study published this week in the journal Proceedings of the National Academy of Sciences found that part of that sink is only temporary.

“It’s all part of figuring out the global carbon cycle,” said co-author Johan Six, professor of plant sciences at UC Davis. “Where are the sources, and where are the sinks? Erosion is in some ways a sink, but, as we found out, it can also become a source.” The researchers estimated that roughly half of the carbon buried in soil by erosion will be re-released into the atmosphere within about 500 years, and possibly faster due to climate change. Climate change can speed the rate of decomposition, aiding the release of the buried carbon.

As a case study, the researchers used radiocarbon and optical dating to calculate the amount of carbon emissions captured in soils and released to the atmosphere during the past 6,000 years along the Dijle River in Belgium.

The study’s long time scope — from 4000 B.C. to A.D. 2000 — allowed the researchers to notice the gradual reintroduction of buried carbon to the atmosphere. Significant agricultural land conversion — historically the largest source of global erosion — began primarily in the past 150 years, well under the researchers’ time frame of 500 years. Therefore, most carbon sequestered in the soil during the past 150 years of agricultural history has not been released yet but may become a significant carbon source in the future, with implications for soil management, the study said.

“Our results showed that half of the carbon initially present in the soil and vegetation was lost to the atmosphere as a result of agricultural conversion,” said study co-author Gert Verstraeten, a professor at KU Leaven, Belgium. Six noted that erosion could be minimized by no-till and low-till agricultural methods, as well as by cover cropping, which can ensure that soil is not left bare.... 

Long furrows, shot by Duncan Grey, Wikimedia Commons via Geograph UK, under the Creative Commons Attribution-Share Alike 2.0 Generic license