Many researches reported that warming alters soil freeze-thaw (F-T) process during winter in cold regions, such as the start and end dates, frequency and intensity. The incubation of soil samples has proved that labile organic C is exposed during soil F-T periods, and carbon-rich permafrost is expected to release more CO2 to the atmosphere through ecosystem respiration (Re) under future climate scenarios, so it’s important to understand the mechanisms of the responses of F-T periods to climate change and their coulping with Re. However, most previous studies are from wet, ice-rich arctic soils. Compared to that, the Tibetan permafrost has a thin, ice-poor organic layer, which may increase thermal conduction from the air to the deep permafrost, and accounting for 75% of the total area of alpine permafrost in the Northern Hemisphere, so we expect a different response in the semi-arid alpine permafrost. In addition, the short-term soil incubation experiment may not represent field conditions, so it’s still unclear how changes in F-T period induced by soil warming affects ecosystem respiration.
Professor Shiping Wang’s group from CETES and Institute of Tibetan Plateau CAS used the two years observation data of the warming (four levels) and precipitation addition platform in Naqu, try to examine how the discontinuous alpine permafrost whose underlying surface is semi-arid meadow will response to warming and precipitation addition, and how these changes will affect the annual ecosystem respiration.
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 and the total duration of the F-T period from autumn to spring (Figure 1). Although warming decreased the accumulated CO2 during F-T period (Figure 2), the annual total ecosystem respiration did’t changed (Figure 3), as 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 provide direct in situ evidence that the effects of warming and precipitation addition on F-T events and annual Re may decouple, because ST mainly controls F-T events and SWC mainly controls Re, with no interactive effect. The contribution of changes in Re 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 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 Re rate without affecting ST or SWC during F-T periods in situ remain unclear.
This work was supported by projects from the National Natural Science Foundation of China (41731175 and 41988101), Strategic Priority Research Program A of the Chinese Academy of Sciences (XDA20050101 and XDA2005010405), National Key Research and Development Program of China (2016YFC0501802) and National Natural Science Foundation of China (31672470).
Article information：Wang, Q., Lv, W., Li, B., Zhou, Y., … Shiping Wang*., 2019. Annual ecosystem respiration is resistant to changes in freeze‐thaw periods in semi‐arid permafrost. Global Change Biology. https://doi.org/10.1111/gcb.14979
Figure1 Warming shifts in the soil freeze-thaw (F-T) periods
Figure2 Mean total ecosystem respiration (Re) during the different freeze-thaw periods
Figure3 Mean total ecosystem respiration (Re) during the thawed period and annually