青藏高原多年冻土“老碳”释放对多梯度实验增温的响应

Climatic warming occurring on the permafrost regions is likely to cause widespread thawing of permafrost, leading to permafrost's old carbon release by microorganism decomposition, and triggering the positive feedbacks of permafrost carbon cycling to climatic changes. Most of current studies focus on the total flux of soil or ecosystem respiration, the relative contribution of the old soil carbon loss, however, has seldom been quantitatively assessed due to the confounding effects of the decomposition of new carbon inputs. This knowledge gap has severely hindered our ability to predict permafrost carbon balance in future climate. Here, we want to propose a project centered on the scientific topic of “Does old soil carbon loss increase in response to climate warming in typical permafrost on the Tibetan Plateau?”. We plan to conduct an in situ field investigation of old soil carbon decomposition at a typical permafrost site, based on a permanent field station that is located in the Naqu Prefecture of the Tibetan Plateau. To quantitatively assess the relative contribution of old carbon loss to the total soil respiration, we plan to combine the carbon isotope techniques (δ13C + △14C) and Markov Chain Monte Carlo (MCMC) method. Furthermore, the sensitivity of old soil carbon loss in response to climatic warming would be studied based on the field station that is facilitated with infrared heating lamps, which enable the multi-gradient experimental warming for the soils. The outcomes of this research project would be of great value to the studies concerning the permafrost carbon cycling and its feedbacks to future climatic warming, but also be of practical significance to the assessment of ecosystem vulnerability of permafrost to future warming scenarios.

气候变暖引起多年冻土融化，可能导致被冻土长期封存的大量土壤有机碳(老碳)分解释放，诱发冻土碳循环与气候变暖之间的正反馈。然而，以往的研究主要关注土壤或生态系统总呼吸通量对气候变化的响应，缺乏对“老碳”和“新碳”等呼吸组分的量化区分，严重限制了我们对未来冻土碳平衡的预测能力。本项目围绕“气候变暖是否会加剧多年冻土老碳分解释放？”这一核心问题，选择青藏高原典型多年冻土作为研究对象，依托西藏那曲市境内的野外多梯度红外增温实验平台，测定土壤总呼吸及不同深度土壤和气体样品的同位素特征(δ13C+△14C)，结合马尔科夫蒙特卡洛(MCMC)方法，量化老碳分解释放对冻土碳释放的相对贡献及其季节动态，并进一步揭示老碳分解释放对多梯度增温的响应规律及其温度敏感性。本项目成果将为深入认识冻土碳循环与气候变化的反馈关系提供关键数据，为不同变暖情景下青藏高原冻土生态系统的脆弱性评估提供依据。

本项目针对多年冻土碳释放及其对气候变暖的响应开展研究。一方面，依托青藏高原典型多年冻土野外多梯度模拟增温实验平台，揭示不同碳释放组分的大小及其对梯度增温的响应规律，验证“气候变暖加剧多年冻土老碳分解”假说。另一方面，比较分析多年冻土分布区与非冻土区土壤碳释放过程及其对气候变暖响应敏感性的差异，验证“多年冻土对气候变暖具有更高敏感性”的假说。. 首先，项目组在西藏安多县和青海祁连县选择了两处典型多年冻土安装了温湿度长期监测设备，建立了同时覆盖活动层和永冻层的典型多年冻土地温长期高频监测站点和持续三年的监测数据库。与此同时，项目组系统分析了青藏高原多年冻土区、高纬度冻土区及非冻土区生态系统碳通量及组分对气候变暖的响应特征，主要发现如下：（1）梯度增温显著改变了多年冻土活动层冻融循环的周期特征。响应于梯度增温，多年冻土活动层的冻结长度平均减少7.9天/℃；（2）梯度增温显著增加多年冻土区生态系统碳释放，导致生态系统由弱碳汇转化成强碳源，土壤呼吸增量大于70%，表层和深层土壤CO2浓度和通量均显著增加，老碳占比也在增加；（3）土壤有机碳释放的温度敏感性呈现随纬度升高的普遍规律，证实多年冻土区土壤碳更高的脆弱性；（4）全球异养呼吸被普遍高估，其后果导致生态系统固碳能力被显著低估，且误差在多年冻土区最为显著。. 上述研究成果以SCI论文形式发表在Global Change Biology, Global Biogeochemical Cycles, Science Bulletin 等国内外知名期刊，完成项目计划书预期目标。项目组主要成员2人次参加了国内学术会议，交流学术进展。项目执行期间共培养研究生2名。