增温对青藏高原多年冻土区土壤碳呼吸损失与冻融迁移的影响研究

Permafrost soils in Qinghai-Tibet Plateau store high soil organic carbon (SOC) stocks, especially in the subsurface layers inducing by cryoturbation. Facing the warming climate in the plateau, the changes in SOC of the permafrost are paid increasing attention, and the analysis of the effect of warming on SOC decomposition, especially on old carbon is necessary. Furthermore, the effect of warming on the cryoturbation in these soils and consequently the effects on the redistribution of SOC along the soil profile should been considered. In this study, warming experiments using open-top chambers (OTCs, two treatments with annual average air temperature increase respectively by 1.8℃ and 3.6 ℃ × two treatments with one short-term warming treatment established in 2017 and one longer-term warming treatment established in 2013) are established at Tibetan alpine meadow and swamp meadow, respectively. The ecosystem respiration of the two grasslands under the warming and control treatments is estimated and the effects of warming on the flux and seasonal dynamics of ecosystem respiration are analyzed. Moreover, the ecosystem respiration is separated into autotrophic respiration and soil heterotrophic respiration based on the analysis of δ13C and Δ14C of ecosystem respiration and autotrophic respiration, and the effects of warming on soil heterotrophic respiration are evaluated. In addition, CO2 of ecosystem respiration of the top layer (top 20 cm layer) is also collected and estimated for δ13C and Δ14C values, and then these values are used to calculate the proportions of surface soil and subsurface (under top 20 cm) soil heterotrophic respiration to total soil heterotrophic respiration. Meanwhile, the effects of warming on soil old carbon respiration in top layers and subsurface layers are analyzed. Finally, the distribution of soil particle size distribution and 137Cs activity along the soil profiles are respectively compared between the warming treatments and controls, and based on these comparisons, the effects of warming on cryoturbation and the consequent SOC redistribution due to cryoturbation are analyzed. The results and conclusions of this study can provide some insights in the responses of permafrost SOC to warming.

全球变暖背景下，青藏高原多年冻土土壤碳变化及机制研究具有重要的科学和实践意义，而冻土老碳在增温下的呼吸损失及土壤碳在冻融作用下的剖面迁移变化是评估土壤碳动态的关键。以青藏高原典型多年冻土区高寒草甸和沼泽草甸为研究对象，利用开顶式气室模拟增温(两个增温梯度×两个增温持续时间)，研究两类草地生态系统呼吸对增温的响应，并通过呼吸排放的δ13C与Δ14C分析，区分生态系统呼吸中自养和异养呼吸，量化老碳的排放贡献及对增温的响应；结合表土(20 cm)呼吸排放及δ13C与Δ14C测定，评估增温对表层和下层土壤异养呼吸及其中老碳排放的不同影响。同时通过分析土壤粒径组成和137Cs活度剖面分布在增温下的差异以及粘粉粒吸附碳库的估算，研究冻融作用的变化对土壤碳迁移的影响，并结合以上研究综合评估增温对冻土土壤碳呼吸损失及冻融迁移的双重作用，为气候变化下青藏高原多年冻土区碳源汇的评估提供科学依据。

青藏高原多年冻土是世界上高山冻土集中分布区，由于低温的限制作用，冻土土壤储存了大量的碳，对于区域及全球碳循环具有重要的调控作用。在当前全球变化背景下，青藏高原面临着快速且剧烈的增温过程，强烈影响着冻土土壤碳储存。当前亟需回答的一个关键问题是：在增温背景下青藏高原冻土区是碳源还是碳汇? 为了回答上述科学问题，本项目一方面在典型多年冻土区构建了不同增温幅度和增温持续时间的人工模拟增温平台；另一方面根据大量的前期工作和基础，建立了小流域尺度的观测样地，通过研究不同坡向及坡位的碳库差异性，来表征长期天然增温的影响。人工模拟增温结显示：增温显著促进了冻土区两类典型草地（高寒草地和高寒沼泽）的生态系统呼吸速率，增加了土壤碳的呼吸损失，但同时促进了高寒草地的生产力。通过检测土壤碳库的变化，研究发现不管是低幅度增温方式（2.4℃）还是高幅度增温（4.9℃），短期（3年）增温对高寒草甸土壤有机碳浓度和储量影响不大；长期（6年）增温则显著提高土壤有机碳浓度和储量。同时通过微生物残体分析发现，增温下增加的土壤有机碳主要来自于土壤团聚体包裹和黏粉粒结合的植物源有机碳，而不是微生物残体碳。然而，植物源有机碳的输入同时促进了土壤中老碳的分解，降低了土壤碳的稳定性。上述研究表明，人工模拟增温同时促进了冻土区植被生产力以及土壤碳呼吸分解速率，但生产力的提升抵消了土壤碳的损失，从而增加了土壤碳储存，仍表现为碳汇。此外，通过对比典型小流域不同坡向碳循环及碳库发现，温度较高的阳坡生态系统呼吸较高，但其植被生产力及土壤碳库也显著高于阴坡。进一步说明在长期的天然增温背景下，青藏高原多年冻土区仍然可能表现为碳汇。此外，首次提出并验证了利用137Cs检验和评估冻扰作用的直接方法，研究显示增温增强了冻土的冻扰过程，促进了土壤碳氮元素的深层迁移及物理保护，为认识冻扰作用对土壤碳氮的物理保护作用及对增温的响应提供了科学依据。本项目的开展不仅从不同时空尺度上（短期和长期以及样点和小流域尺度）提供了增温下青藏高原多年冻土区碳源汇变化的证据，而且从土壤碳来源、稳定性及冻扰作用等方面深入理解了冻土碳对增温的响应机制。