冻融作用对青藏高原高寒草甸土壤导水率的影响机制

China is the third permafrost/seasonal frozen ground distribution area of the world. Most of permafrost/seasonal frozen ground are concentrated in the Tibetan Plateau. Although soil is frozen in a long period over the Plateau, the soil surface is suffering frequent daily freezing-thawing alternations. This intensive soil freezing-thawing can changes soil water condition and infiltration process, therefore have obvious influence on hydrologic process of permafrost/frozen ground area. However, impact of freezing-thawing on soil hydraulic conductivity under the particular environment such as the Plateau is unclearly yet. Besides, as the most popular vegetation types in frozen ground area of the Plateau, alpine meadow is undergoing large-scale degradation in recent years. The degradation may have impact on the effect of freezing-thawing action, but little study focus on it. Naqu area in the central of the Qinghai-Tibetan Plateau where alpine meadow and seasonal frozen ground are widespread is selected as the study area. Field observations of soil freezing-thawing process and infiltration experiments will be conducted under different degraded plots. Indoors simulated freezing-thawing experiments and soil infiltration experiments under different freezing temperatures, freezing times, freezing-thawing rates and freezing-thawing cycles will be performed with different degraded alpine meadow soil samples. Besides, basic soil properties and root characters of the soil samples will be analyzed. This study aims to clarify the influence and mechanism of impact of freezing-thawing on alpine meadow soil hydraulic conductivity with considering the degradation degrees of the alpine meadow soil. This study will provide theoretical guidance for hydrologic process modeling studies in the permafrost/seasonal frozen ground area in the Plateau.

我国是世界上第三冻土大国，冻土在我国的青藏高原地区分布最为集中。青藏高原土壤的总体冻结时间较长，表层土壤的日冻融交替频繁，强烈的冻融作用改变了土壤的水分状况和入渗过程，对冻土区水文过程具有显著影响。然而，高原特殊冻融环境下冻融作用对土壤导水率的影响尚不明了。作为高原冻土区主要植被的高寒草甸近年来出现大面积退化，草甸退化引起土壤的相关变化对冻融作用改变土壤导水率可能会有影响，但相关研究鲜有报道。为此，本项目选择青藏高原中部的那曲高寒草甸季节性冻土典型区为研究区，通过不同退化程度样地土壤冻融过程观测和田间入渗试验，结合不同退化程度样品在不同冻结强度、冻结时间、冻融速率、冻融循环次数条件下的室内模拟冻融试验和入渗试验，以及样品土壤基本特性和根系特征的分析结果，在考虑草甸退化程度的条件下，探求冻融作用对高寒草甸土壤导水率的影响特征与作用机制，以期为冻土区水文过程及其模拟研究提供理论指导。

青藏高原地区土壤冻结时间长、土壤冻融交替频繁，强烈的冻融能够改变土壤的水力性质，对冻土区水文过程产生影响。本项目选取青藏高原中部那曲地区选取不同退化的草甸样地为研究区，采样调查了土壤的基本性质；布设土壤温度湿度自动记录仪监测土壤的冻融过程；结合田间冻融土壤采样和入渗试验以及室内控制条件下的冻融模拟试验，研究不同冻融过程对草甸土壤饱和导水率的影响；通过统计方法确定冻融作用对高寒草甸土壤导水率影响的关键要素及其影响机制。结果表明，冻结层分布越浅，地表稳定入渗率越低；冻结温度越低，冻结土体对土壤导水率的影响相对较大；随着冻融循环次数的增加，土壤饱和导水率大体上呈先增加后减少并趋于稳定，在植被条件较好时高含水量下相同冻融循环次数对土壤饱和导水率的影响显著高于低含水量条件，在植被较差时高含水量下相同冻融循环次数对土壤饱和导水率的影响低于低含水量条件。影响冻融作用对土壤饱和导水率的因素包括三个主成分，成分一包括冻结强度、冻融循环次数、土壤初始含水量、孔隙度、砂粒含量和草甸退化度；成分二包括土壤有机质、根系含量和粘粒含量；成分三为冻结深度。土壤孔隙度的变化是冻融作用改变土壤入渗性能变化的主导因素。经过冻融用后，土壤中的大孔隙减少而中孔隙和小孔隙增多，引起土壤水分和导水性的显著变化。对于根系较少的严重退化草甸土壤经过冻融后，土壤水体在冻结时体积膨胀移动土壤颗粒，降低了土壤团聚体的稳定性；当土壤解冻时，土壤中的细小颗粒随融解水体发生运动，堵塞了土壤中的有效孔隙，降低了土壤饱和导水率。然而，草甸土丰富的根系一方面能够降低冻结水体膨胀对土壤结构的破坏，另一方面还会抵御解冻时细颗粒的移动及其对大孔隙的堵塞效应。因此，退化度较低的草甸土经过冻融作用后土壤饱和导水率呈增加的趋势。