基于陆面过程模型量化研究活性氮沉降增加对青藏高原高寒草地碳汇功能的影响

Reactive nitrogen (Nr) includes all biologically, photochemically, and radiatively active N compounds in Earth’s atmosphere and biosphere. Since the mid-19th century, anthropogenic Nr inputted in terrestrial ecosystems has dramatically increased, leading to changes in ecosystem structure and functioning. The Tibetan Plateau (TP) provide an ideal platform to study the coupling process of multi-sphere (e.g. atmosphere, hydrosphere, biosphere and other spheres) under the background of global change. In the TP, both surface temperature and Nr wet deposition dramatically increased during recent several decades. Evidences from multi-level Nr fertilization experiments suggest that alpine grassland ecosystems in the TP are N-limited, and are therefore sensitive to enhanced Nr deposition. However, the regional scale responses of alpine grasslands carbon (C) cycle to elevated Nr deposition and their trend during past several decades remain unclear. In this project, based on datasets of in situ observations, remote monitoring, and experimental results, we will optimize the latest version of a land surface process model, which considered C-N interactions. Then, we will quantitatively estimate the temporal and spatial patterns of alpine grassland C balance responses to enhanced Nr deposition during past 3 decades. This project will shed light on the mechanisms of alpine ecosystem C cycle responses to enhanced Nr deposition, and further improve the understanding of the C-N interactions on the TP.

活性氮是指大气圈和生物圈中具有生物活性、光化学活性或辐射活性的含氮化合物。自20世纪中叶以来，陆地生态系统人为活性氮输入急剧增加，对生态系统结构与功能造成了深刻影响。青藏高原是全球变化的敏感区和先兆区，是研究全球变化背景下大气圈、水圈、生物圈等多圈层耦合过程的典型区。数十年来，青藏高原持续变暖，且活性氮湿沉降迅猛增加。控制实验揭示，青藏高原高寒草地生态系统受氮限制，对活性氮输入增加非常敏感；但是，区域尺度青藏高原高寒草地对活性氮沉降增加的响应及其变化趋势仍不明确。本项目拟利用已有定位观测、遥感监测、控制实验等研究结果优化耦合了碳氮循环的陆面过程模型，基于模型模拟定量研究近30年活性氮沉降增加对青藏高原高寒草地生态系统碳汇功能影响的时空格局。本项目的实施，有助于揭示青藏高原生态系统碳循环对氮沉降增加的响应机制，丰富青藏高原高碳氮循环研究的知识体系。

青藏高原高寒草地生态系统是全球变化的先兆区和敏感区，近几十年经历了快速气候变暖、大气氮沉降增加和大气CO2浓度升高等。阐明陆地生态系统碳汇功能对全球变化的响应对于理解和预测陆地碳循环-气候系统反馈至关重要。本项目基于观测揭示了青藏高原腹地高寒草原-高寒草甸-高寒沼泽过渡带生态系统碳-氮循环特征，发现空间上随土壤水分增加，土壤NH4+/NO3−增加，这可能是由于土壤湿度增加促进了反硝化速率和降低了土壤pH共同控制。同时，土壤水分增加缓解植物生长水分限制和氮限制，促进生态系统碳吸收。与此不同，围封促进了草地碳吸收，却降低土壤可利用氮，加剧植物生长氮限制，制约了高寒草地碳固定持续增加的潜力。野外氮添加实验结果则表明，氮添加缓解了高寒草原植物生长氮限制，但却改变了土壤可利用氮形式。因此，青藏高原腹地高寒草地植物生长的氮限制特征受土壤水分条件、草原管理方式、大气氮沉降变化等协同控制。进一步地，结合模型模拟和实验观测数据分析发现陆地生态系统碳-氮耦合模型普遍高估了氮对生态系统碳吸收的限制作用。模拟和观测整合分析结果揭示，1960年代以来，相比于大气氮沉降变化和气候变暖，大气CO2升高是陆地生态系统碳汇增加的主要驱动因素。这说明开展模型模拟和实验观测的整合分析有助于更准确认知陆地生态系统碳循环对全球变化的响应机制。本研究为高寒生态系统碳循环和全球变化研究领域提供了数据支撑和科学线索。