热喀斯特对土壤碳组分的影响及其途径—以热融沟为例

Permafrost carbon cycle is receiving great interest among the global change research community. However, previous studies mainly focused on the effects of increased active-layer thickness on regional carbon cycle, with less involved in the impacts of thermokarst. Moreover, little is known about the changes in soil carbon composition and the pathways in which soil carbon is lost, since previous studies mostly concentrated on carbon cycling processes. In this project, we try to establish paired-sites along three typical thermo-erosion gullies on the Tibetan Plateau, in which surface soil samples are collected from the collapse plot (inner gully) and control plot (out gully). We will then employ organic geochemical techniques, such as biomarker and nuclear magnetic resonance techniques, to explore whether and how the formation of thermo-erosion gully alters soil carbon content and its composition (e.g. carbohydrates, aliphatic lipids and lignin). Meanwhile, to evaluate soil carbon released into the atmosphere, we will determine soil heterotrophic respiration and methane fluxes. To evaluate soil carbon transferred to the nearby river, we will further collect water samples from inner and out gully to examine the changes in composition of the dissolved organic carbon (DOC) and particulate organic carbon (POC) based on ultraviolet-visible (UV-Vis) absorption spectra, three-dimensional fluorescence spectra and nuclear magnetic resonance spectra analyses. The implementation of this project could improve our understanding of permafrost carbon-climate feedback on the Tibetan Plateau.

鉴于冻土碳循环与气候变暖之间潜在的正反馈关系，冻土融化如何影响区域碳循环成为当前全球变化研究中关注的焦点问题。然而，以往的研究主要关注活动层厚度增加造成的影响，对热喀斯特现象的关注较少；并且，以往的研究主要关注碳循环过程的变化，尚不清楚土壤碳组分的变化及其损失途径。本项目拟以青藏高原多年冻土区发育的热融沟为对象，采集沟内斑块和沟外对照区土样，基于生物标志物、核磁共振等方法揭示热融沟的形成对土壤碳组分(简单糖类、脂类物质、木质素等)的影响。进一步地，通过微生物群落和酶活性分析，结合土壤呼吸和甲烷通量测定，进而解析土壤碳的气体损失途径。同时，通过在沟内、外溪流采集水样，结合紫外-可见吸收光谱法、三维荧光光谱法、核磁共振等技术，分析水体中可溶性、颗粒有机碳含量及其组分的变化，进而解析土壤碳的横向转移损失途径。本项目的实施有助于加深学术界对冻土碳循环与气候变暖之间反馈关系的认识。

冻土融化对生态系统碳循环的影响受到全球变化研究群体的广泛关注。然而，以往的研究主要关注活动层厚度增加的影响，对于热喀斯特地貌形成对冻土生态系统碳循环影响的认识相对不足。为此，本项目依托冻土热融塌陷研究平台，结合同位素标记、核磁共振等技术，研究了土壤碳含量及其组分沿塌陷序列的变化，解析了氮可利用性在调控土壤碳激发效应中的关键作用，揭示了热喀斯特形成后DOC生物可降解性对光照和氮添加的响应，阐明了温室气体通量以及土壤总氮转化速率沿塌陷序列的变化机制。结果发现：(1)塌陷早期，沟内土壤碳含量与对照没有差异，塌陷中、后期显著低于对照。并且，塌陷后期土壤活性碳组分显著降低而惰性碳组分显著增加。(2)土壤碳激发效应的强度沿随冻土融化形成的自然氮梯度呈现下降趋势，且激发效应强度沿自然氮梯度的变异主要取决于微生物代谢效率，但不受微生物胞外酶活性调控。(3)无机氮添加对热喀斯特区DOC生物可降解性没有显著影响，而光照可以通过改变DOC组分而显著提高其可降解性。(4)塌陷早期和中期CH4排放通量相对较低，但塌陷后期CH4通量显著增加；CH4通量的变化主要受土壤质地和充水孔隙度的影响。冻土融化初期土壤N2O通量显著增加，而融化中期和后期土壤N2O排放通量逐渐恢复至对照水平。土壤含水率和NO3-浓度等非生物因素以及土壤微生物功能基因(nirS和nosZ基因)共同调控土壤N2O通量的变异。(5)土壤总氮矿化速率在塌陷早期、中期和后期均呈现下降趋势，而土壤总硝化速率仅在塌陷初期增加。土壤水分是导致总氮转化速率沿塌陷序列变化的重要驱动因素：土壤含水率下降会影响土壤微生物量、微生物群落结构和酶活，进而降低塌陷序列上的总N矿化速率；土壤水分下降也会影响土壤通气性和氨氧化细菌及古菌丰度，进而促进塌陷初期的总硝化速率。上述研究丰富了学术界对冻土碳循环与气候变暖之间反馈关系的认识。在该项目的支持下，项目组成员在Nature Geoscience、Nature Communications、Ecology、Global Change Biology等刊物共发表SCI论文22篇，顺利完成了各项研究任务，实现了预期目标。