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).

Effects of permafrost degradation on carbon (C) and nitrogen (N) cycling on the Qinghai-Tibetan Plateau (QTP) have rarely been analyzed. This study used a revised process-based biogeochemical model to quantify the effects in the region during the 21st century. We found that permafrost degradation would expose 0.61 +/- 0.26 (mean +/- SD) and 1.50 +/- 0.15 Pg C of soil organic carbon under the representative concentration pathway (RCP) 4.5 and the RCP 8.5, respectively. Among them, more than 20% will be decomposed, enhancing heterotrophic respiration by 8.62 +/- 4.51 (RCP 4.5) and 33.66 +/- 14.03 (RCP 8.5) Tg C/yr in 2099. Deep soil N supply due to thawed permafrost is not accessible to plants, only stimulating net primary production by 7.15 +/- 4.83 (RCP 4.5) and 24.27 +/- 9.19 (RCP 8.5) Tg C/yr in 2099. As a result, the single effect of permafrost degradation would cumulatively weaken the regional C sink by 209.44 +/- 137.49 (RCP 4.5) and 371.06 +/- 151.70 (RCP 8.5) Tg C during 2020-2099. However, when factors of climate change, CO2 increasing and permafrost degradation are all considered, the permafrost region on the QTP would be a stronger C sink in the 21st century. Permafrost degradation has a greater influence on C balance of alpine meadows than alpine steppes on the QTP. The shallower active layer, higher soil C and N stocks, and wetter environment in alpine meadows are responsible for its stronger response to permafrost degradation. This study highlights that permafrost degradation could continue to release large amounts of C to the atmosphere irrespective of potentially more nitrogen available from deep soils.

Despite the fundamental importance of soil temperature for Earth's carbon and energy budgets, ecosystem functioning, and agricultural production, studies of climate change impacts on soil processes have mainly relied on air temperatures, assuming they are accurate proxies for soil temperatures. We evaluated changes in soil temperature, moisture, and air temperature predicted over the 21st century from 14 Earth system models. The model ensemble predicted a global mean soil warming of 2.3 0.7 and 4.5 1.1 degrees C at 100-cm depth by the end of the 21st century for RCPs 4.5 and 8.5, respectively. Soils at 100 cm warmed at almost exactly the same rate as near-surface (similar to 1 cm) soils. Globally, soil warming was slightly slower than air warming above it, and this difference increased over the 21st century. Regionally, soil warming kept pace with air warming in tropical and arid regions but lagged air warming in colder regions. Thus, air warming is not necessarily a good proxy for soil warming in cold regions where snow and ice impede the direct transfer of sensible heat from the atmosphere to soil. Despite this effect, high-latitude soils were still projected to warm faster than elsewhere, albeit at slower rates than surface air above them. When compared with observations, the models were able to capture soil thermal dynamics in most biomes, but some failed to recreate thermal properties in permafrost regions. Particularly in cold regions, using soil warming rather than air warming projections may improve predictions of temperature-sensitive soil processes.