Soil Organic Carbon Mineralization And Its Temperature Sensitivity In The Treeline Ecotones

Soil is the largest carbon pool in terrestrial ecosystems and its carbon stock depends on the input and output of carbon.Soil organic carbon mineralization is one of the important processes in the ecosystem carbon cycle,and is also the main form of soil carbon release to the atmosphere.As one of the important ecological ecotones in alpine regions,the treeline ecotone connects alpine forest and alpine shrub,and is a sensitive area in response to climate change.Soil carbon stock is abundant within the treeline ecotone and its carbon turnover plays an important role in the global carbon cycle.There are many studies on soil organic carbon mineralization in different ecosystem types around the world,but no consistent conclusion on the changes of soil organic carbon stability and the factors influencing it in the treeline area.Therefore,we performed an incubation experiment at different temperatures（15℃and 20℃）for 300days based on soils from the treeline ecotone in the Gongga and Taibai Mountain near 30°N latitude,soil organic carbon mineralization rates were measured,and the basic physico-chemical properties of soils、organic carbon components、microbial community structural characteristics of the soils were analyzed,the relevant factors affecting the cumulative mineralization per unit of soil organic carbon and temperature sensitivity were also analyzed to investigate the soil organic carbon mineralization pattern and potential in the treeline ecotone,and to provide data support for further revealing the response of soil carbon in the treeline ecotone to future global warming and for predicting the trend of global climate change.The main research findings are as follows:（1）The soil organic carbon（SOC）content significantly decreased from forest to shrub in the treeline ecotone of Gongga Mountain,while SOC content significantly increased from forest to shrub in the treeline ecotone of Taibai Mountain.The content of particulate and mineral-bound organic carbon decreased significantly from forest to shrub in the treeline ecotone of Gongga Mountain,and showed a significant increase in the Taibai Mountain.The diversity and abundance of soil fungi and bacteria varied significantly between treeline ecotones,at the same time,soil fungi and bacteria in the treeline ecotone were similar in composition and structure.The soil fungal communities in the treeline ecotone of Gongga and Taibai Mountain were the same at the phylum taxonomic level,while the dominant soil bacterial phylum differed greatly,showed the largest relative abundance of soil bacteria in the treeline ecotone of Gongga Mountain was Acidobacteriota,while the Proteobacteria in the treeline ecotone of Taibai Mountain.（2）SOC mineralization patterns differed between treeline ecotones,with the highest SOC mineralization rates and cumulative mineralization of forest and shrub soils in the Gongga and Taibai mountains,respectively.The results of Spearman’s correlation analysis showed that the stability of SOC in treeline ecotones was mainly influenced by the mineral protection of SOC,the stronger the mineral protection of SOC,the more stable of the SOC.Meanwhile,the abundance and diversity of soil microorganisms were also important reasons for the difference of SOC stability in the treeline ecotones,and SOC would be more stable as the diversity of soil fungi and bacteria increased.（3）The Q_10-Cminin the treeline ecotone of Gongga Mountain was2.48±0.10,while the Q_10-Cminin the treeline ecotone of Taibai Mountain was as high as 3.81±0.18.The treeline ecotones exhibited high temperature sensitivity,therefore they are more vulnerable to climate change under the background of global warming.The temperature sensitivity of SOC mineralization did not differ significantly between different sampling sites in the treeline ecotones.The significant differences in soil temperature sensitivity between the two treeline ecotones were mainly influenced by soil p H,which mainly affected Q₁₀values by regulating the structure of soil microbial communities.It was also influenced by the abundance and diversity of bacterial communities,soil bacteria in the treeline ecotone of Taibai Mountain showed higher temperature sensitivity due to the higher abundance of the Proteobacteria.In summary,SOC mineralization varied significantly from forest to shrub within the treeline ecotones,but no significant difference in temperature sensitivity.However,the differences in SOC content and composition under different vegetation types,as well as differences in soil p H and microbial community structure,caused significant differences in SOC mineralization and temperature sensitivity among different treeline ecotones.These results are useful for predicting the changes of soil carbon stocks under climate warming.In the future,as the climate continues to warm,the treeline will continue to move upward,causing changes in SOC content in the treeline ecotones,which will affect changes in soil carbon mineralization.