Carbon cycle accelerates

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Kathiravan Meeran First author Kathiravan Meeran performs measurements in the Ic
Kathiravan Meeran First author Kathiravan Meeran performs measurements in the Icelandic test field near a geothermal fault line. © University of Innsbruck

Soils are the largest natural carbon reservoirs in the world. In the far north, this store is particularly large, but it is also particularly affected by global warming there. A recent study by an international team led by Michael Bahn of the University of Innsbruck investigated how ongoing warming affects the uptake and release of carbon dioxide in subarctic grasslands. The scientists used a geothermally active area in Iceland as a natural "climate chamber.

Subarctic ecosystems store large amounts of carbon. As the climate continues to warm, more and more of it is escaping into the atmosphere. To what extent the uptake of carbon through increased photosynthesis by plants can compensate for the release from the soil was unclear until recently: "After all, if warming increases the decomposition of organic matter in the soil and thus also the supply of nutrients to the plants, the plants should grow better and thereby also take up more carbon dioxide from the atmosphere," says Michael Bahn from the Institute of Ecology at the University of Innsbruck "Surprisingly, this is not the case, as our current study shows."

Natural climate laboratory

Geothermal fault lines in Iceland provide ideal conditions for researchers to explore the longer-term effects of climate warming on ecosystems in the far north. A team led by Michael Bahn used a geothermally active area in Iceland as part of an international project to study the relationship of ongoing warming and nitrogen supply on the carbon cycle. "The research plots were located at different distances from the fault line and warmed to a greater or lesser extent accordingly," Michael Bahn tells us. "By fertilizing part of the plots with nitrogen along this temperature gradient, we were able to study in detail the interactions between warming and nitrogen supply on the carbon cycle."

Warming accelerates the carbon cycle

Warming led to a massive loss of carbon in the soil. "In the area we studied in Iceland, up to 40 percent of the carbon in the topsoil was released to the atmosphere in the first few years after warming," says Michael Bahn. "In subsequent years, there was an adjustment in microbial biomass and the soil carbon balance leveled off again." To understand the path carbon takes in this cycle from the atmosphere through plants and soil and back to the atmosphere, Michael Bahn’s team added the stable carbon isotope 13C during the experiment. Using the isotope, we were able to trace the path of carbon in the ecosystem," explains the ecologist. "This showed that as the temperature rises, the carbon sequestered by photosynthesis in the plants is taken up more by the microbes and released more rapidly by the soil. From the fertilization experiments, it can further be concluded that the productivity of the plants is visibly limited by a nitrogen deficiency as the temperature warms. This led to a decrease in carbon uptake in the ecosystem. As warming accelerated the release of carbon, the ecosystem’s ability to store carbon was thus visibly reduced."

The results have now been published in the journal Global Change Biology. The research was financially supported by the Austrian Science Fund FWF and the European Union, among others.

Publication: Individual and interactive effects of warming and nitrogen supply on CO2 fluxes and carbon allocation in subarctic grassland. Kathiravan Meeran, Niel Verbrigghe, Johannes Ingrisch, Lucia Fuchslueger, Lena Müller, Páll Sigurðsson, Bjarni D. Sigurdsson, Herbert Wachter, Margarete Watzka, Jennifer L. Soong, Sara Vicca, Ivan A. Janssens, Michael Bahn. Global Change Biology, 00, 1-16. DOI: 10.1111/gcb.16851.

Video portrait of Michael Bahn:­eos/szenar­ien-fuer-d­as-klima-d­er-zukunft