冻融是影响寒区土壤形成的主要因素，能够显著影响土壤结构，进而会影响土壤中的水分循环、热量与养分的迁移以及微生物群落的分布。本文以青海湖流域高寒草甸为研究对象，通过采集原状土柱和土壤团聚体样品，基于室内冻融模拟实验、CT扫描和图像解译的方法，研究了冻融循环对土壤孔隙结构的影响；通过磷脂脂肪酸（PLFA）法测定了土壤微生物群落的季节变化，探究了季节冻融过程对土壤微生物群落的影响，最终探讨了冻融循环过程中土壤孔隙结构与土壤微生物群落之间的关系。主要研究结果如下：

（1）在土壤团聚体尺度上，随着冻融循环次数的增加，土壤总孔隙度呈现先减小后增大的趋势，其中3-5 mm直径土壤团聚体的孔隙度受冻融循环的影响最大。冻融循环10次后，2-3 mm和3-5 mm直径团聚体的大孔隙（>80 μm）的孔隙度和孔隙连通度均增大，即冻融循环促使2-3 mm和3-5 mm直径团聚体的水气运输能力增强。

（2）在剖面尺度上，随着冻融循环次数的增加，土壤总孔隙度呈现先增大后减小的趋势，冻融循环3次后土壤的孔隙度达到最大，这种变化趋势主要和土壤较大孔隙（等效直径>1000 μm、表面积>100 mm²和体积>10 mm³）的变化有关。随着冻融循环次数的增加，土壤孔隙数量密度、表面积密度、节点密度均呈现先增大后减小的趋势，而孔隙等效直径、分枝密度、长度密度均呈现逐渐增大的趋势。冻融循环使土壤中一些连通的大孔隙变成非连通的相对较小的孔隙，孔隙连通度降低，土壤水分稳定入渗率降低，不利于水分在土壤中的垂直运移。

（3）不同土层深度的土壤孔隙结构对冻融循环的响应不同。随着冻融循环次数的增加，在0-75 mm土层深度，土壤总孔隙度先增大后减小；在75-150 mm土层深度，土壤总孔隙度主要在冻融循环5次后降低，冻融循环10次后又有所增大；在150-280 mm土层深度，土壤总孔隙度呈现逐渐减小的趋势。

（4）季节冻融过程中土壤微生物群落差异显著。土壤总微生物量在不稳定冻结期＞不稳定融化期＞冻结期＞融化期，其中细菌的微生物量高于其他种群的的微生物量。冻结期土壤O层（枯枝落叶层）的微生物量比季节冻融其他阶段均低，但是土壤A层（淋溶层）、B层（淀积层）和C层（母质层）的微生物量却高于季节冻融其他阶段。不稳定冻结期和冻结期真菌/细菌（F/B）的值高于融化期和不稳定融化期，不稳定冻结期和冻结期革兰氏阴性菌/革兰氏阳性菌（G^-/G⁺）的值低于融化期和不稳定融化期。冻结期土壤微生物群落在土层间的差异比其他季节冻融过程低。高寒草甸不稳定冻结期和冻结期的真菌更占优势，微生物群落的稳定性相对较强，而且土壤养分的有效性较高。

（5）土壤孔隙结构参数和土壤微生物之间有显著相关关系。细菌、革兰氏阴性菌（G^-）、革兰氏阳性菌（G⁺）、真菌、放线菌的微生物量和总PLFA均和土壤孔隙的长度密度、表面积密度、等效直径和水力半径呈显著正相关关系，G^-/G⁺与土壤孔隙的表面积密度、等效直径和水力半径呈显著正相关关系，细菌、G⁺、真菌的微生物量和总PLFA还和土壤孔隙的数量密度呈显著正相关关系。土壤中等效直径为700-1000 μm、表面积为1-100 μm²和体积为0.1-10 μm³的孔隙更有利于土壤微生物的生存。因此，冻融循环可以通过影响土壤孔隙结构特征从而影响土壤微生物分布。

Freeze-thaw is an important physical factor that significantly affects the soil structure in cold alpine regions, which can affect the hydrological circulation, heat and nutrient migration and microbial community distribution in soil. Alpine meadow of Qinghai Lake basin was selected as study object to detect the effect of freeze-thaw cycles (FTCs) on the soil pore structure by excavating intact soil columns and aggregates and the methods of indoor freeze-thaw simulation experiment, computed tomography (CT) scans and images interpretation, and to study the effect of seasonal freeze-thaw process on soil microbial community by determining the seasonal change of soil microbial community using phospholipid fatty acid (PLFA) method. Then the relationship between soil pore structure and microbial community was studied. The main results are as follows:

(1) With increasing FTC frequency, the total soil porosity decreased first and then increased on the scale of soil aggregates and the porosity of 3-5 mm aggregates was significantly affected by FTCs. After 10 FTCs, the porosity of macropores (>80 μm) and pore connectivity increased in the 2-3 mm and 3-5 mm aggregates. Therefore, the water and gas transport capacity of the 2-3 mm and 3-5 mm aggregate increased affected by FTCs.

(2) With increasing FTC frequency, the total soil porosity increased first and then decreased on the scale of soil profile and reached the maximum after 3 FTCs. The effect of FTCs on the total soil porosity of alpine meadows was mainly related to the change of macropores (equivalent diameter >1000 μm, surface >100 mm² and volume >10 mm³). In addition, with increasing FTC frequency, the number density, surface area density and node density of soil pores all increased first and then decreased, and the equivalent diameter, branch density and length density of soil pores all increased. The FTCs decreased the pores connectivity and the stable infiltration rate of soil water, and changed the connected macropores into some unconnected and relatively small pores, which could not facilitate greater water flow to deeper soil layers.

(3) The responses of soil pore structure to the FTCs were different at different soil depths. With increasing FTC frequency, at the soil depth of 0-75 mm, the total soil porosity first increased and then decreased; at the soil depth of 75-150 mm, the total soil porosity decreased significantly after 5 FTCs and increased after 10 FTCs; at the soil depth of 150-280 mm, the total soil porosity decreased gradually. 

(4) There were significant differences in soil microbial community during seasonal freeze-thaw process. The soil microbial biomass decreased in the following order: unstable-freeze period > unstable-thaw period > freeze period > thaw period. The microbial biomass of bacteria was higher than that of other populations. The microbial biomass in soil humus horizon (O) under the freeze period was lower than that of other seasonal freeze-thaw periods, while the microbial biomass in soil horizon of A, B and C was higher than that of other seasonal freeze-thaw periods. The values of fungi/bacteria (F/B) in unstable-freeze period and freeze period were higher than that in thaw period and unstable thaw period, and the values of gram-negative bacteria/gram-positive bacteria (G^-/G⁺) were were opposite. The differences of soil microbial communities at different soil layers in freeze period were lower than that in other periods. The fungi in unstable-freeze period and freeze period were more dominant in alpine meadows, which indicated the stability of microbial community and the availability of soil nutrients were relatively higher than other seasonal freeze-thaw periods.

(5) There were significant correlations between soil pore structure and soil microbial biomass. Bacteria, G^-, G⁺, fungi, actinomycetes and total PLFA were significantly positively correlated with the length density, surface area density, equivalent diameter and hydraulic radius of soil pores. G^-/G⁺ was significantly positively correlated with the surface area density, equivalent diameter and hydraulic radius of soil pores. Bacteria, G⁺, fungi and total PLFA were positively correlated with the number density of soil pores. The pores with equivalent diameters of 700-1000 μm, surface areas of 1-100 mm² and volumes of 0.1-10 mm³ were more conducive to the survival of soil microbe. Therefore, the FTCs could affect the distribution of soil microbe by affecting the characteristics of soil pore structure.