Soil and ecosystem carbon dynamics in a warming Subarctic

Nele Meyer1, Juha Mikola2, Tarja Silfver3, Kristiina Myller4, Yi Xu5, Outi-Maaria Sietiö5, Eero Myrsky6, Elina Oksanen4, Matti Rousi7, Kristiina Karhu5
1 University of Bayreuth, Soil Ecology
2 Natural Resources Institute Finland (Luke)
3 University of Helsinki, Faculty of Biological and Environmental Sciences
4 University of Eastern Finland, Department of Environmental and Biological Sciences
5 University of Helsinki, Department of Forest Sciences
6 University of Eastern Finland, Department of Environmental University of Lapland, Arctic CentreBiological Sciences
7 University of Helsinki, Department of Natural Resources Institute Finland (Luke)Sciences

O 1.1 in Soil-vegetation-atmosphere interactions in a changing climate

14.10.2021, 16:00-16:12, H 36

Climate warming in the Subarctic will likely stimulate soil organic carbon (SOC) mineralization but also plant primary production and thereby C input into soil. However, increasing insect herbivory may dampen this positive response. Thus, it remains uncertain whether the Subarctic will become a sink or source for CO2. This study aims at exploring the interactive effect of climate warming, insect herbivory, and plant growth in the Subarctic.

We conducted an open-air warming experiment in North Finland to explore the effects of warming (+3°C) and reduced insect herbivory (using an insecticide) on SOC dynamics, net ecosystem exchange, and plant growth. Further, to understand the long-term effect of plant growth and herbivory, we conducted a comprehensive survey of soil and ecosystem C stocks under living trees, treeless tundra, and under trees that died due to insect outbreaks.

We found a positive effect of warming on plant growth, which went along with larger ecosystem CO2 uptake. As a result of larger C input into soil, SOC contents increased despite the observed stimulation of SOC turnover. The positive effect of plant growth on SOC stocks was also corroborated by the finding that SOC stocks were considerably larger under trees in comparison with treeless tundra. Yet, insect herbivory significantly reduced plant growth and if their numbers increase with warming, they may curtail the positive effect of warming. Surprisingly, we found strong effects of warming or herbivory reduction on soil and microbial variables but most of these effects were not additive. This was in contrast to the additive positive effects of warming and herbivory reduction on plant growth. Our study draws attention to the multiple interacting mechanisms behind warming effects on C dynamics in the Subarctic, creating nonlinear and unexpected responses to changes in temperature. This calls for careful consideration when predicting the fate of Subarctic C sink under climate warming.

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