Breaking the Ice – Consequences of northwards extending agriculture for soil organic matter cycling under a changing climate

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Soils store the largest terrestrial carbon (C) pool and a multiple of the atmospheric C pool. The highest soil organic C (SOC) stocks are found in high northern latitudes, where soils are mostly uncultivated and discontinuously or continuously frozen. At the same time, the strongest temperature increase is also expected in northern latitudes, making this region a global hot spot for climate carbon-cycle feedbacks. Thawing of permafrost will evidently lead to losses in SOC and thereby increase atmospheric CO2. However, global warming will also have another indirect, but strong influence on the carbon cycle of northern circumpolar permafrost soils: human interventions. With increasing area of land that can be cultivated as well as improved living conditions in the North, extensive land use changes will occur and started to occur, which are acknowledged to change a wide range of biotic and abiotic soil properties. The land use changes may thereby amplify soil warming effects. Without quantifying these changes and associated consequences for processes affecting SOC cycling, the impact of human interventions on climate-carbon cycle feedbacks cannot be predicted. To calibrate models that can reliably predict SOC stock changes under a changing climate with associated land use changes, the following aspects will be addressed in the project: Direct biotic and abiotic responses of permafrost soils to cultivation as compared to non-permafrost soils, long-term SOC stock and quality development of agricultural soils (boreal forests converted to cropland and grassland) and potential changes in microbial carbon use efficiency and its temperature sensitivity due to land use change. The latter is critical to understand how cultivated soils will respond to climate change as compared to uncultivated soils with regard to carbon cycling. To quantify carbon use efficiencies, a novel 18O-labeling approach will be applied. The Yukon, the north-westernmost province of Canada, was identified as the perfect research area. Due to the ‘goldrush’, it has an agricultural history of more than a century, even in areas where soils under natural vegetation are still influenced by shallow permafrost. Land use change effects can thus be studied in a chronosequential approach. In relative small distance, permafrost depth varies greatly, enabling to study land-use change effects also along a thermosequence. The fact that agriculture occurs mainly on river sediments constrains confounding factors. Most often, farmers conduct small-scale mixed farming, thus conversion from forest to cropland and grassland can be studied at the same time. The approached farmers have been very open and cooperative, resulting in a large and balanced number of sites that can be sampled, all of which being easily accessible. Results of the project will enable a new view on soil C cycling in the C-rich Northern regions with double pressure: permafrost thawing and deforestation.