Carbon Sequestration Mechanism And Assessment Of Carbon Sink Capacity In The Taklimakan Desert

"Carbon neutrality"is one of the ultimate human efforts to cope with the climate crisis.Accurately assessing the carbon balance of each ecosystem and fully exploiting its carbon sequestration potential has become one of the most important research directions in relation to the balance between climate change and economic and social development.It is also the most economical,safe,and effective means to sequester carbon and increase sinks.At present,most studies on the global carbon cycle focus on highly productive systems such as forests,grasslands,wetlands and agricultural lands.Studies have shown that vast desert ecosystems,which have been neglected for a long time,can sequester large amounts of CO₂ through non-photosynthetic processes and play a positive role in promoting carbon neutrality,reducing carbon loss,and mitigating climate warming.However,the contradiction between CO₂ flux data obtained from different observation methods and the lack of clarity on the internal driving mechanisms of desert carbon sinks have made desert carbon sink still suspected by researchers after years of research.At the same time,this have prevented the accurate determination of the contribution of deserts to the global carbon cycle,and limiting the exploitation of desert carbon sequestration potential.In view of this,this study conducted a comparative observation experiment on CO₂fluxes in the Taklimakan Desert based on different observation methods,and analysed the reasons for the contradiction between different observation methods.At the same time,the contribution of CO₂ flux of each components in the shifting sand was split,and the internal processes and driving mechanisms of shifting sand carbon sinks were fully understood in combination with the interaction of water and heat factors.On this basis,an empirical estimation scheme of CO₂ flux of shifting sand was established considering the interaction of hydrothermal interaction and soil properties.In addition,the carbon sink capacity,source-sink conversion,and response to climate change of the shifting sand area were assessed in the Taklimakan Desert.Finally,the impact of the establishment of desert artificial protective forest on the carbon sequestration in the desert during the desertification control process and the response of the protective forest to precipitation events were analysed.The main findings of this study are as follows.（1）A comparative analysis of the differences in simultaneous observations of open-path eddy covariance（IRGASON）,closed-path eddy covariance（EC155）,and soil carbon flux meters（LI-COR8100A）in the non-vegetated shifting sand of the Taklimakan Desert found that the observations of EC155 and LI-COR8100A are highly consistent,and they accurately reflect the real CO₂ exchange in the desert.The anomalous negative CO₂ fluxes observe by IRGASON during the daytime would overestimate the carbon sequestration capacity of the Taklimakan Desert if uncorrected.On this basis,we found that incomplete correction for self-heating and spectral effects due to the extreme arid environment of the desert was the main cause of the apparent discrepancy between the high-frequency CO₂ fluxes of IRGASON and the CO₂ fluxes observed from EC155.The corrected IRGASON observation results show that the shifting sand releases CO₂ during the day,peaking at midday;the shifting sand absorbs CO₂ at night and shows carbon sequestration.The daily CO₂exchange of the shifting sand has a unimodal distribution throughout the year,with April-September as a carbon source and the rest of the year as a carbon sink.In the dynamic balance of release and absorption throughout the year,the absorption effect is relatively strong and the shifting sand in the Taklimakan Desert has a significant carbon sequestration effect.（2）The process and exact magnitude of the CO₂ flux contribution of each component（sand,moisture,salt/alkali,microbes）of shifting sand are accurately described by disassembly experiment and temperature-controlled experiment.We found that the contribution of soil moisture and soil microbes to CO₂ fluxes of shifting sand are very limited due to prolonged extreme drought in desert areas.In contrast,the contribution of expansion/contraction of soil air containing CO₂ caused by heat fluctuations to total CO₂ exchange is surprisingly strong.This long-term cryptic process combined with chemical carbon sequestration by soil salts/alkalis dominates the release and uptake of CO₂ from dry shifting sand,respectively.They are tightly regulated by the soil temperature difference,which represents the flow of soil heat,and the rate of change of soil temperature,which represents the absorption/release of heat by the soil.On this basis,a parametric scheme for estimating CO₂ fluxes of dry shifting sand was developed.（3）Based on hydrothermal interaction experiments in the Taklimakan Desert hinterland,it was found that high soil moisture can significantly stimulate the rapid release of CO₂ from sand in the short term by rapidly increasing microbial activity and organic matter diffusion when the soil moisture at 10 cm is above the threshold of0.05 m³ m^-3,with a maximum rate of 3.68μmol m^-2 s^-1.At this point,the CO₂ fluxe of shifting sand is synergistically influenced by both soil temperature and soil moisture.In contrast,the stimulating effect of low soil moisture is not as pronounced,and it is only tightly controlled by surface temperature.In addition,as the decreases of organic carbon matrix and the increase of soil alkalinity,the carbon sequestration of shifting sand is gradually increases and is strengthened at low temperature.Conversely,the carbon sequestration of shifting sand gradually diminishes.On this basis,we developed and calibrated an empirical estimation scheme for CO₂ flux of shifting sand that takes into account both hydrothermal interactions and soil properties.This improves the understanding of desert carbon sequestration mechanisms and provides a possible scheme for assessing total amount of desert carbon sequestration and its contribution to the global carbon cycle.（4）The establishment of desert artificial protective forest adds plant photosynthesis to the original inorganic carbon sequestration process,increasing the carbon sequestration capacity of desert ecosystems by a factor of about 150,with an annual CO₂ uptake of about 1000 g m^-2.This not only improves the regional ecological environment,but also transforms desertified land into an important terrestrial carbon sink.In addition,the carbon sequestration capacity of planted forests shows different responses to different amounts of precipitation in the desert.Precipitation of less than 2 mm has no effect on the carbon sequestration capacity of planted forests.The precipitation of 2 mm to 6 mm causes a brief decrease in the saturation water vapour pressure differential（VPD）,which promotes photosynthesis and enhances the carbon sequestration capacity of planted forests.However,the duration of this process is short.While precipitation greater than 8 mm reduced VPD and promoted photosynthesis,the increase in soil moisture stimulated the decomposition and transport of organic carbon by soil microorganisms,which in turn stimulated enhanced soil respiration.This offset some of the enhanced photosynthesis and cause the protective forest to show an initial inhibition of carbon sequestration after precipitation.Thereafter,as soil respiration decreases,carbon sequestration capacity shows a brief increase.（5）The shifting sand in the Taklimakan Desert can sequester about 1.05×10⁶ tons of CO₂ per year.The area with carbon sequestration occupies two-thirds of the total shifting sand in the Taklimakan Desert.The shifting sand in Kashgar and Aksu regions,located on the western and northern margins of the Taklimakan Desert,has become the main area of CO₂ emission in the desert due to relatively high precipitation and soil organic carbon content.However,with global warming and the increase in extreme precipitation events in deserts,it will stimulate the release of more CO₂ into the atmosphere by shifting sand.This not only gradually weakens the carbon sink capacity of the shifting sand in the Taklimakan Desert,but also gradually reduces the carbon sink stability.Therefore,from the perspective of desert carbon sequestration,desertification prevention and control not only directly improves the regional ecological environment,but also is endowed with the important significance of consolidating and improving the carbon sink stability and carbon sequestration capacity of ecosystems in arid regions.However,if desert precipitation further increases,it will effectively benefit the growth of desert vegetation in the long run.The process of desert carbon sequestration may gradually increase with the help of plant photosynthesis.In summary,in view of the key scientific problems existing in the current research on desert carbon sequestration,this paper systematically carried out the research on the driving mechanism of carbon sink of shifting sand in Taklimakan Desert along the framework of"feature analysis-mechanism exploration-impact assessment".At the same time,the changes in carbon sequestration capacity and its risks of the shifting sand in Taklimakan Desert were evaluated by using the established empirical estimation scheme of CO₂ flux.The research results have improved the understanding of the carbon sink mechanism of desert ecosystems,and laid a foundation for accurately assessing the total carbon sequestration of global deserts and their contribution to the carbon cycle.In addition,the research results further deepen the important significance of desertification control in consolidating and improving the carbon sink stability and carbon sequestration capacity of ecosystems in arid areas.