Why focus on the response of soil carbon emissions to whole-soil warming in alpine ecosystems?

As the largest carbon pool in terrestrial ecosystems, the soil organic carbon pool plays a key role in the global carbon cycling, and its slight change would have a great effect on the carbon-climate feedback. The global surface air temperature has risen by ~1 °C since the Industrial Revolution, and it will continue to increase by up to 4 °C by the end of this century. According to the soil temperature predictions from IPCC models, both surface and deep soils will be warmed at roughly the same rate. However, previous experimental evidence about the responses of soil carbon emissions to whole-soil warming only appears in a small number of forest and peatland ecosystems. The Tibetan Plateau, as the world's third pole, stores a large amount of soil organic carbon (SOC, 7.4 Pg C in the top 1 m depth) and has a double warming rate compared with the global average warming rate. Alpine grassland is the dominant ecosystem in this plateau (nearly 60% of the total area), and stores about 10% of the total SOC in China. However, soil carbon emissions from alpine grassland to whole-soil warming are still not known. Therefore, understanding and quantifying the response of soil carbon dynamics to whole-soil warming is urgently needed in alpine ecosystems to accurately predict the carbon-climate feedback under realistic climate warming in the future.

 How to set up the manipulated field experiment?

This study took a long time. In July 2016, Ying Chen (then an undergrad student) met Dr. Biao Zhu (then an Assistant Professor at Peking University) to discuss potential PhD positions. After the discussion, Ying decided to join Biao’s research group and enter Peking University as a bachelor-straight-to-doctorate student a year later (2017). When Biao was a postdoc at Lawrence Berkeley National Laboratory (2012-2014), he and his colleagues (e.g., Dr. Caitlin Hicks Pries) in the group of Dr. Margaret Torn established a whole-soil warming field experiment in a temperate forest to investigate the soil carbon cycling processes. Biao had started developing a similar whole-soil warming experiment in the alpine grasslands of the Tibetan Plateau since 2015. With the great help of Prof. Jin-Sheng He from Peking University and the directors of the Haibei station (Dr. Guangmin Cao and Dr. Zhenhua Zhang), Biao's group started developing this whole-soil warming experiment at the Haibei National Field Research Station of Alpine Grassland Ecosystems (the Haibei station) with complete experimental facilities, convenient transportation, and adequate logistics. In August 2016, together with a postdoctoral fellow, Dr. Yanhui Hou, we began to delineate the experimental area, divide the plot, bury the soil respiration collars, and investigate the background basic properties of plants and soil. In July 2017, we assembled a pilot pair of warming and ambient treatments and then the system was able to work normally. In June 2018, we set up four pairs of warming and ambient treatments. Soil CO₂ efflux (respiration) was measured during the growing season, every two weeks from June 2018 until September 2020 and approximately monthly from May 2021 until September 2021 (and still continuing). This unique dataset was also part of Ying Chen’s doctoral dissertation (June 2022). This article is the result of the joint efforts of multiple collaborators, including Changchun Guo, Rui Li, Wenkuan Qin, Yunfeng Han, Hongyang Zhao, Qiufang Zhang, Xudong Wang, and others. We also appreciate Prof. Jin-Sheng He and Prof. Margaret S. Torn for their constructive suggestions and comments on the earlier versions of this paper. This article would not have appeared without their contribution.

What is the result?

For the first time, our research provides a perspective to assess the response intensity of soil carbon emission from the alpine grassland ecosystem on the Tibetan Plateau to whole-soil warming. Over 4 years of treatment (2018-2021), we found that whole-soil warming markedly increased total soil CO₂ efflux by 26%, which was much greater than the average effect size (7-8%) by experimental warming in the meta-analysis across global grasslands or alpine grasslands (none were whole-soil warming experiments). Moreover, the SOC-derived heterotrophic respiration increased by 37%, while the root-derived autotrophic respiration increased by 12% (but non-significant) by the whole-soil warming. Over the four years (2018-2021), the warming-induced relative increase in CO₂ effluxes did not change significantly. However, the large SOC pool across the 0-100cm profile had not been significantly altered after ~3.3 years of warming (from June 2018 to August 2021). These results demonstrate that the expected future, which is long-term whole-soil warming, could create a much stronger effect on soil carbon emission than what is estimated by previous warming experiments that do not distinguish SOC- versus root-derived responses or that only warm surface soils. Overall, our findings remind other scientists and policymakers that in order to predict and mitigate the carbon-climate feedback more accurately, the huge response of soil carbon emissions to real warming scenarios (whole-soil warming) must not be ignored, especially in alpine ecosystems. More details can be found in our paper ‘Whole-soil warming leads to substantial soil carbon emission in an alpine grassland’ published in Nature Communications.

 What about the future?

Together, whole-soil warming will likely continue to stimulate the degradation of soil carbon and cause more CO₂ emissions in the alpine grassland. However, substrate limitation and adaptation of soil microbial communities have been reported to occur after long-term warming, therefore longer-term measurements are required to test this. This field experiment has been in operation for 6 years (since June 2018), and we have been collecting data on soil respiration (and othe related variables) for 6 years so far. We are also analyzing this longer dataset and expect to find more interesting results, especially whether longer-term whole-soil warming will significantly alter the size of soil carbon pool. Another knowledge gap that we are more concerned about is whether the alpine ecosystem becomes a carbon source or a carbon sink after the whole-soil warming. A manuscript on this scientific issue is being written and is expected to be published as soon as possible. It is expected that our research on the whole-soil warming will help reveal the response of the carbon cycle in alpine ecosystems to realistic climate warming and the underlying mechanisms.