Warming may alter soil freeze-thaw (F-T) process, such as the start and end dates, frequency and intensity, during winter in cold regions. It’s important to understand the mechanisms of the responses of F-T periods to climate change and their relationship with respiration.
Most previous studies are based on wet, ice-rich arctic soils. However, the Tibetan permafrost has a thin, ice-poor organic layer, which may increase thermal conduction from the air to the deep permafrost, thus the semi-arid alpine permafrost may respond differently.
Based on two-year observation data of the warming (four levels) and precipitation addition platform in Naqu, Prof. WANG Shiping's group from the Institute of Tibetan Plateau Research (ITP) of the Chinese Academy of Sciences investigated how the discontinuous alpine permafrost will response to warming and precipitation addition, and how these changes will affect the annual ecosystem respiration. The underlying surface of the alpine permafrost is semi-arid meadow.
They found that warming delayed the start and end dates of F-T events during the freezing period, and advanced them during the thawing period, thus shortening both the duration of the soil frozen period as well as the total duration of the F-T period from autumn to spring.
Although warming decreased the accumulated CO2 during F-T period, the annual total ecosystem respiration did not change due to the reduced soil water content by warming and the small contribution of Re during the F-T period to the annual result. Although precipitation addition had no effects on F-T period, it increased the Re during F-T period and annually.
This study provides direct in situ evidence that the effects of warming and precipitation addition on F-T events and annual respiration may decouple, because soil temperature (ST) mainly controls F-T events and soil water content (SWC) mainly controls respiration, with no interactive effect.
The contribution of changes in respiration during F-T periods to changes in annual ecosystem release of CO2 could thus be overestimated under warming in semi-arid areas with discontinuous permafrost, such as the Tibetan Plateau.
Better predictions of the influence of climate change on the release of ecosystem C from permafrost will require re-evaluating of the effects of changes in precipitation and their interaction with warming on the cycling of ecosystem C in alpine regions around the world. In particular, the mechanisms of how warming reduces the mean respiration rate without affecting ST or SWC during F-T periods in situ remain unclear.
