温性草甸草原根系碳储量及碳素转化对放牧强度的响应

More than 80% of the grassland ecosystem's carbon distributes underground. The roots are important carbon sinks and mineral nutrient pools of grassland, and moreover, the main source of soil organic carbon(SOC). However, below-ground carbon process are poorly understood due to the restrictions of measurement methods and there is an urgent need for further research. Grazing is one of the major factors that control the net primary productivity(NPP) and carbon cycle of grassland ecosystem. The objective of this project, with new and more comprehensive methods and dataset, is to draw a better and conclusive understanding of the impact of grazing intensity on underground carbon dynamics of grassland ecosystem. In this study we selected temperate meadow steppe, which has the highest NPP and SOC in all temperate grassland types, to achieve the above specific objective. The research will be conducted in Hulun Buir, where the Huluner Grassland Ecosystem Research Station located. The station had built a grazing experiment platform in 2008, and carried out continuous observation during 2009-2013 to quantify the impact of six grazing gradients on grassland biodiversity, above- and below- ground biomass, carbon and nitrogen cycle, soil features and microbial properties. These studies raised new scientific issues which will be focused in this study. To address these scientific issues, we will adopt stable isotope techniques and in-growth soil core methods to explore the root biomass allocation, root carbon input, death and decomposition under different grazing intensity, to trace the underground carbon flow from root to soil carbon pool, to develop a carbon transfermation model to describe the diminishing of stable isotope C in underground carbon process, and to reveal the potential mechanism for response of grassland ecosystem carbon budget to grazing disturbance . The result will helpful for fully understanding of China's terrestrial ecosystem carbon cycle, and provide comprehensive knowledge for effective management of grazing ecosystems.

草地生态系统碳库绝大部分分布在地下。根系是草地重要的碳汇、也是土壤有机碳的主要来源，由于测量方法限制，草地生态系统地下碳过程知之甚少,迫切需要开展深入研究。放牧是影响草地物质生产和碳循环的关键人为因素，明确放牧活动对草原生态系统碳库的影响，对全面理解我国陆地生态系统碳循环具有重要意义。 温性草甸草原是第一性生产力及土壤碳密度最高的温性草原类型。本项目依托呼伦贝尔草原生态系统国家重点野外实验站，基于大型草地放牧控制实验平台，利用稳定性同位素技术，研究不同放牧强度下草甸草原生物量分配、地下生物碳输入及碳素分解转移特征，探索草甸草原生物量碳与土壤碳之间的转化规律，建立稳定性同位素在地下碳素流动过程中的递减模型（碳素转化模型），揭示草甸草原生态系统地下碳库对放牧干扰的综合响应机理。不仅有助于全面理解放牧干扰下草原植被碳库的源汇机制，同时为草地放牧生态系统的有效管理提供基础依据和理论启示。

不同强度的放牧活动对草地生态系统碳循环造成复杂且多元化的影响，特别是草地生态系统地下碳库对放牧强度的响应机制仍不明确。本研究以此为背景，选择呼伦贝尔温性草甸草原为研究区，基于同位素脉冲标记、植物根系生长芯等技术手段，系统分析了不同放牧强度对草甸草原植物群落各组分及土壤有机碳碳素动态变化的影响，揭示草原生态系统地下生物碳素转移过程对放牧强度的响应模式，结合地上、地下碳素转移过程，进一步探讨并量化放牧强度对草地生态系统碳素分配机制的影响。重要结果如下：.1. 植物群落地上部植物活体碳储量随着放牧强度的增加显著下降，并间接导致立枯和凋落物的碳储量也发生明显下降。植物群落活根碳库随着放牧强度的增加呈现先上升后下降的趋势。放牧主要影响20cm深度以上的表层土壤有机碳，且重度放牧土壤有机碳储量最高，这可能是由于过度放牧增加了土壤容重导致的一种短期效应。.2. 光合碳向根系（活根）和土壤分配比例均随着放牧强度的增加呈先上升后下降的趋势，两者最适宜放牧强度分别为0.18–0.3 Au.ha-1、0.4–0.5 Au.ha-1，说明轻度放牧可能对碳素向根系分配更有利，而中度放牧则更有利于碳素向土壤碳库分配。随着放牧强度的加剧，根系–土壤碳素分配比值逐渐上升，说明植物将更大比例的碳素从根系分配到土壤中，这可能是植物应对极端放牧的一种响应策略，从而获取更多的营养物质。.3. 光合固定总量随放牧强度的增加表现出显著的下降趋势。标记后31天，轻度放牧植物向地下根系-土壤分配比例最高，分别为18.08%、17.08%，碳素总流失比例最低，为43.49%；中度放牧植物地上活体13C比例最高，为22.99%；重度放牧严重降低碳素在植物-土壤系统的分配比例，碳素流失量最高，但从13C的绝对固定量来看，仍是无放牧处理最高。. 本研究成果为全面理解草原生态系统碳循环、研究草地碳库对人类活动的响应模式提供理论基础。