西南高山树线交错带土壤有机碳稳定性及相关生化特征研究

Alpine treeline ecotone (ATE) in the southwestern China, located between the subalpine coniferous forest and alpine meadow, is one of the most important transition zones of terrestrial ecosystems in China even in the world. ATE is experiencing significant changes in location or plant community structure under the pressure of global environmental change. Ecotone change has been identified as a potential early indicator of climatic change. It has been proved that an upward altitudinal shift in the alpine vegetation belt is occurring at different rate from eastern Switzerland, Ural to western Austria, or increased plant growth and density in the northern arctic latitudes during the past century by climatic change. However, biomonitoring parameters for upward elevation migration of the alpine-nival (uppermost) flora are long-term indicators of decades to centuries..Soils of alpine areas are noted darker and richer in humus than those of lowlands with larger C accumulations, and regarded as an huge cold carbon pool. These soils are also observed increasing fractions of particulate and labile SOM with increasing altitude in natural treeline ecosystems, which means that C pool of the higher elevation soil is characterized with higher sensitivity and lower stabilization than that of lowlands. However, soil labile C fractions, such as particulate organic C and light fractions, are considered as appropriate early indicators for carbon stock changes under soil warming. Studies on temperature sensitivity of soil respiration and soil enzyme activity, Q10, also indicate that decreases with temperature increasing in cold system. Therefore, some chemical, biological and biochemical parameters of soil carbon for evaluating responses of ATE to climatic change may be more sensitive than those biomonitoring parameters of ATE..In this study we planned to carry out soil regular sampling, soil profile temperature and humidity monitoring in situ, ATE homogenized soil column incubation in situ for soil respiration activity monitoring and regular sampling in the specialized altitudinal gradients in the ATE, and laboratorial control experiments of temperature sensitivity of ATE homogenized soil. soil microbial functional groups, thermodynamic parameters of soil respiration, enzyme activity, and labile organic carbon fractions is determined by methods of soil chemical, biological and biochemical analysis including PLFA and enzymology. Our objectives are to elucidated soil temperature dynamics, spatial distribution, stabilization and temperature sensitivity of soil carbon pool, tempo-spatial heterogeneity of temperature sensitivity of soil respiration, microbial functional groups, and related soil enzymes along ATE environmental gradients, which is helpful to find suitable biochemical indicators of short-term effects of ATE soil carbon dynamics in the context of global environmental change.

西南高山树线交错带（ATE）位于亚高山针叶林与高山草甸之间，是我国最重要的陆地生态系统过渡带之一，土壤碳库容量大而稳定性低。在全球环境变化压力下，ATE正在发生位置或群落结构的明显变化。本项目在西南ATE各典型海拔和植被环境梯度上，开展ATE原生特征生境断面的土壤剖面定期采样调查、ATE特征土壤剖面温湿度原位监测、均质化土柱原位培养土壤呼吸活性定期监测、均质化土柱原位培养定期采样及土壤温度敏感性控制实验等途径，利用包括PLFA和土壤酶学在内的土壤化学、土壤生物与土壤生物化学手段，首次对西南ATE环境梯度上土壤温度变化特征、碳库规模及活性有机碳库特征、微生物功能类群、土壤呼吸与酶活性动态与温度敏感性等方面进行研究，揭示全球变化背景下ATE土壤碳库及稳定性、微生物功能类群、相关土壤呼吸与酶活性等温度敏感性与热动力学特征方面的时空演变规律，探索表征ATE土壤碳动态及相关生化过程的短期效应指标。

高山树线交错带（ATE）是重要生态过渡带和环境敏感区，揭示土壤有机碳稳定性及对环境变化生化效应特征，为研究全球变化下青藏高原土壤有机碳库动态提供理论基础。本项目对海拔梯度上（特别是ATE）土壤温度、有机碳稳定性、土壤微生物群落的时空分布特征，相关酶温度敏感特征、凋落物分解特征以及树线位置动态等方面，进行原位研究。结果显示，ATE不同位置在低温季节（从10月至次年5月），土壤10cm温度低于0℃均超过180天；土壤10cm日均温、极端温度ATE中部比上部和下部都低，并非极端低温决定乔木生长（树线位置）。土壤有机碳总量随海拔（3200m~4100m）上升，颗粒态碳（或易氧化碳）占比与海拔呈显著正相关，微生物量碳、水溶性碳以及轻组分碳与海拔无明显相关；ATE区域以粗有机质形式存在的有机碳占土壤有机碳总量90%甚至更高，有机-无机复合有机碳不超过3%。ATE土壤微生物群落结构分析表明，微生物群落结构与功能类群在空间和季节上差异明显，3400m~3800m土壤微生物群落相似较高，与3200m或4000m土壤微生物群落差异显著；0~10cm土层和10~20cm土层间土壤微生物群落也差异明显。丰度季节变化大的微生物类群主要集中于厚壁菌门、变形菌门、酸杆菌门、放线菌门一些属。土壤酶温度敏感性特征表现在低温区间（0~20℃）敏感性高；凋落物原位培养研究，显示氮沉降和升温处理将使其中有机组分周转周期延长。ATE乔木种群（红杉）密度和数量持续增加（过去40年），ATE内乔木密度增加或树线位置上移或二者同时发生，乔木在ATE内多丛状集群分布，但随年龄越小集群度越弱，显示ATE环境逐步向乔木适生方向发展。因此，ATE土壤有机碳以POC等不稳定有机碳形式储存，腐殖化过程及产物不占主导地位，且稳定性随海拔升高下降；另外土壤酶在低温阶段对增温响应比高温段更敏感，土壤升温可能引起相关酶过程急剧变化；然而高山树线上移趋势明显，加上增温和氮沉降等可能导致更多有机碳以不稳定有机碳形式储存下来，因此在青藏高原增温背景下土壤有机碳库及有机碳生态过程变化可能更加复杂。