SWAMP: A new experiment for simulating permafrost warming and active layer deepening on the Tibetan Plateau

1. Our knowledge on the responses of permafrost ecosystems to climate warming is critical for assessing the direction and magnitude of permafrost carbon-climate feedback. However, most of the previous experiments have only been able to warm the air and surface soil, with limited effects on the permafrost temperature. Consequently, it remains challenging to realistically simulate permafrost thawing in terms of increased active layer (a layer freezing and thawing seasonally above permafrost) thickness under climate warming scenarios. 2. Here, we presented the experimental design and warming performance of a novel experiment, Simulate Warming at Mountain Permafrost (SWAMP), the first one to successfully simulate permafrost warming and the subsequent active layer deepening in a swamp meadow situated on the Tibetan Plateau. Infrared heating was employed as above-ground warming to elevate the temperature of the air and surface soil, and heating rods were inserted vertically in the soil to provide below-ground warming for transmitting heat to the deep active layer and even to permafrost deposits. 3. In 3 m diameter warmed circular plots, the air and the entire soil profile (from surface soil to 120 cm) was effectively heated, with an increase of approximately 2 degrees C in the upper 60 cm, which progressively weakened with soil depth. Warming increased soil moisture across the growing season by inducing an earlier thawing of the soil. Values varied from 1.8 +/- 1.8 to 12.3 +/- 2.3% according to the soil depth. Moreover, during the growing season, the warmed plots had greater thaw depths and a deeper active layer thickness of 12.6 +/- 0.8 cm. In addition, soil thawing duration was prolonged by the warming, ranging from 22.8 +/- 3.3 to 49.3 +/- 4.5 days depending on the soil depth. 4. The establishment of SWAMP provides a more realistic simulation of warming-induced permafrost thaw, which can then be used to explore the effect of climate warming on permafrost ecosystems and the potential permafrost carbon-climate feedback. Notably, our experiment is more advantageous for investigating how deep soil processes respond to climate warming and active layer deepening, compare with experiments which use passive warming techniques such as open top chambers (OTCs).