西藏高寒农牧交错带生态系统土壤有机碳物理保护下的化学稳定机制

The stability of soil organic carbon (SOC) play a very important role in the SOC the response to the ecological conditions, soil management and climate change, which is closely related to key carbon components and molecular characteristics. The SOC with different carbon molecular characteristics vary in their degree of stability. This project selected different degraded grasslands (Normal grassland, slight and severe degraded grassland) and farmland soil in the alpine, arid and farming-pastoral ecotone in Tibet as the research objects to study the SOC pools and its composition under different ecological situations. By combining with the composition of soil particles and the distributing characteristics of the aggregates (>0.25 mm macroaggregates and <0.25 mm microaggregates) to elucidate the physical protection performance differences of soil organic carbon under different ecological conditions. It adopts spectroscopy to study what constitutes the major functional groups of the soil organic carbon. Besides, the changing characteristics of the carbon functional groups during the mineralization of soil organic carbon by simulating the temperature and humidity changes will analyzed. The project is to answer the questions: (1) Are there significant differences among the soil organic carbon contents and their stability under different ecological conditions in the farming-pastoral ecotone in Tibetan alpine zone? 2. Is there consistency between the stability of the soil organic carbon tested by the physical-chemical method of soil and the stability of carbon functional groups tested by spectroscopy? 3. How do the soil organic carbon dynamics of the soil particles under different ecological conditions and the aggregates with different size fractions respond in sensitivity to the changes of temperature and humidity and what are the possible changing characteristics of the corresponding functional groups? Finally, the study will provides a scientific basis for different ecological carbon dynamics and the future long-term possible trend prediction in the alpine farming-pastoral ecotone.

土壤有机碳稳定性是土壤碳系统响应于生态条件、土壤管理及气候变化的核心，与关键碳组分及分子特征密切相关。本项目选择西藏高寒、干旱农牧交错带不同退化程度草地及农田土壤作为对象，研究不同生态条件下土壤有机碳库及构成；结合土壤颗粒组成及不同粒径团聚体分布特征，研究不同生态条件土壤有机碳物理保护性能差异；采用光谱技术，研究土壤有机碳主要官能团构成，并通过室内模拟温度、湿度变化，研究土壤有机碳矿化过程中碳官能团变化特征。旨在回答：1、西藏高寒区农牧交错带不同生态条件土壤有机碳含量及稳定性特征是否具有显著性差异；2、基于土壤物理-化学方法测定土壤有机碳稳定性与基于光谱技术测定碳官能团稳定性之间是否具有一致性；3、不同生态条件土壤颗粒、不同粒级团聚体土壤有机碳动态对温度、湿度变化敏感性响应及相应的官能团可能的变化特征；最终为高寒农牧交错带不同生态系统碳动态及未来长期的可能变化趋势预测提供科学依据。

土壤团聚体对有机碳物理保护及不同稳定态碳官能团构成是有机碳稳定的重要基础，为阐明西藏农牧交错带生态区不同土地利用方式对土壤有机碳的影响，及土壤有机碳在各级团聚体中的分布特征，旨在揭示土壤团聚体在有机碳物理保护中的作用，及土壤有机碳官能团的构成特征。以西藏高寒农牧交错带藏北亚寒带干旱气候区和臧东南温暖湿润气候区作为研究区，主要开展了不同土地利用方式下土壤各级团聚体、团聚体有机碳含量、土壤微生物区系数量及细菌主要功能群、不同海拔高度不同层次土壤有机碳主要官能团分布特征等。主要结果和发现如下：（1）干筛法所得不同粒径团聚体以粒径>2.0 mm的团聚体占比较高，自然生态土壤具有较高的平均重量直径（MWD）和几何平均直径（GME），而农田受人为扰动，结构性变差；（2）气候条件和土壤条件可能共同作用了土地利用方式对土壤有机碳的影响效应，寒冷、干旱的气候条件下，土壤有机碳对土地利用方式改变响应的敏感度要低于温暖湿润区，即温暖湿润气候带土地利用方式改变加剧了土壤有机碳损失的风险；（3）各粒径土壤团聚体有机碳主要以粒径大于0.25 mm而小于2.0 mm范围的团聚体具有高的有机碳浓度，这表明这部分团聚体对土壤有机碳具有良好的容纳和物理保护性能；（4）土壤δ13C值表现为草地高于农田，而δ15N表现为农田高于草地，草地土壤δ13C值随着层次的加深而增加，与土壤有机碳含量的垂直分布呈相反趋势，δ15N在农田和草地上均表现为随土壤层次的加深而增加；（5）土壤有机碳官能团以烷基碳和烷氧碳为主，而脂肪碳和羰基碳含量较低，高海拔区顽固性有机碳官能团占比较高，而低海拔区则以非稳态碳为主；稳定态碳官能团占比随土壤深度的增加而增加，即深层土壤有机碳可能以稳定态的老碳为主，枯落物层及表层土壤有机碳损失的风险高于下层矿质土壤；（6）亚寒带样区草地和农田利用方式下，α-变形菌纲Alphaproteobacteria为优势菌纲，相对丰度达到14.17%-24.06%，退耕土壤中细菌多样性指数最高；不同土地利用方式下，参与新陈代谢Metabolism的细菌相对丰度最高，为51.92%-52.47%。最终结果可以为揭示西藏高原高寒农牧交错带不同土地利用方式下，土壤有机碳数量和质量差异，及未来可能的长期变化预测提供科学依据。