人类活动影响下天山北麓碱化土壤碳库构成及其固碳潜力

Soil is the largest carbon pool in the terrestrial ecosystems and plays an important role in the global carbon cycle. In recent years, studies of soil organic carbon from wide scopes and deep perpectives have emerged and made a series of research progress. Accounted for 40 percent of the total land area on earth,the arid soil as the main part of the soil carbon pool stores carbon in the inorganic format (secondary carbonate). In particular, for the soil in the northwest arid region in China, the inorganic carbon reservoir is 2～5 times of the organic carbon reservoir. However, the status of soil inorganic carbon in the terrestrial carbon transfer and the carbon sequestration in the Earth's surface is still unknown. There is a large desert area containing alkalinized soil with rich carbonate in the northern foot region of Tianshan mountain in Xinjiang, China, which serves as a great inorganic carbon pool. This project intends to understand the processes of inorganic carbon sequestration, accumulation, and transfer in alkalinized soil as well as the interaction between the processes and the human factors using the methods of field survey, laboratory simulations and element geochemistry. Quantitative analysis of the distribution features of the secondary carbonate and primary carbonate in alkalized soil profile will be performed to investigate the corresponding contributions to soil inorganic carbon, objectively evaluate the carbon sequestration potential of soil with different alkalization degree in regional carbon cycle, and improve the estimation accuracy of the regional-scaled soil carbon pool. This study also discusses the impact on the dynamic changes of soil inorganic carbon from human activities. In summary, this research not only provides great insight into understanding soil inorganic carbon sequestration and the impact from human activities, but also advances the exploration of the "Global Carbon Missing Sink" problem.

土壤是陆地生态系统中的最大碳库，在全球碳循环中起着非常重要的作用。近年来，土壤有机碳的研究广泛而深入并取得了一系列的研究成果。占世界陆地40%的干旱性土壤中，土壤碳库则以无机碳（次生碳酸盐）为主要存在形式。目前，人们对土壤无机碳在陆地碳转移及其在地球表层系统碳截存中的地位还了解不多。新疆天山北麓存在着大面积富含碳酸盐的荒漠碱化土壤，是巨大的无机碳库。本项目尝试通过野外定点观测、室内分析模拟、元素地球化学等方法，对该区域碱化土壤无机碳固定、积累、迁移过程及其与人类活动间的耦合关系进行研究，定量分析碱化土壤原生、次生碳酸盐的剖面分布特征，探讨其对土壤无机碳库的可能贡献，客观评价不同碱化程度土壤的固碳潜力及其动态变化对区域碳循环过程的影响，以提高区域尺度土壤碳库估算精度，为进一步了解土壤无机碳截存，特别是人类活动对无机碳截存的影响提供理论支持，同时也是对全球"碳失汇"问题的探索。

在“粮食安全”和“全球变暖”两个重大问题驱动下，本项目在围绕天山北麓碱化土壤无机碳库特征和人类活动与碱化土壤固碳效应两个核心内容，在对区域绿洲-荒漠交错带的小气候特征，土壤碱化程度、有机质与养分特征进行分析的基础上，针对天山北麓的残余强碱土、活性碱化盐土以及基于碱性荒漠灰钙土背景的不同防渗条件下的平原水库下缘土壤碳库构成、强碱土土壤无机呼吸与影响因子模拟、绿洲-荒漠生态系统碳库迁移、人类活动与区域碳库汇/源效应的耦合作用等进行了一系列相关研究，主要结论如下：1） CO2是碳酸盐的淋溶与重结晶过程中不可缺少的重要组成部分，强碱土与大气交换CO2的过程主要以通过水为作用介质的无机碳的物理化学过程为主；强碱土土壤呼吸中，无机呼吸部分占到总的呼吸的 99.37%；2）天山北麓西端艾比湖滨岸活性碱化盐土的无机碳库最大，其1m厚度土壤中的总碳密度约为17 kg•m-2，无机碳占近78%；3）天山北麓东部以碱性荒漠灰钙土为背景、有防渗处理的新平原水库下缘，土壤总碳密度约为13 kg•m-2，无机碳占比约64%，而未经防渗处理的老旧平原水库下缘土壤总碳密度约为16kg•m-2，土壤无机碳占比降至55%，老旧平原水库在造成土壤次生盐碱化的同时，提高了土壤有机碳汇潜力；4）40年来天山北麓新人工绿洲快速扩张，沼泽消失，水库干涸，灌丛大幅减少，裸盐碱地面积40年里减少了70%，新生草地在裸盐碱地上形成；开垦导致的天然植被碳库向作物碳库迁移是植被碳汇无效化过程，天然植被的减少或消失是碳汇潜力降低过程，裸盐碱地向新生草地迁移是有效碳汇补偿过程，新人工绿洲扩张使研究区植被固碳潜力降低，成为弱碳源；2016年，耕地土壤有机碳储量占比超过50%，土壤碳库重新达到平衡,人类活动是影响研究区土壤与植被碳库迁移及其碳汇/碳源效应的决定性因素；5）裸盐碱地土壤碳库在向耕地、灌丛地、草地土壤碳库的迁移过程中对开垦造成的碳排放产生了极为重要的有效碳汇补偿，这种补偿能在一定程度上为 “碳失汇”做出贡献；6）若维持现有耕地规模不再进行新的垦荒活动，采用节水灌溉技术，40年垦荒造成的土壤碳库损失可以逐渐得到恢复并重新形成碳汇效应。