祁连山南麓高寒禾草-矮嵩草草甸土壤水源涵养功能的特征

青藏高原被誉为“中华水塔”,高寒草甸是主要植被类型但其水源涵养功能有待准确量化。以祁连山南麓高寒禾草-矮嵩草草甸为研究对象,通过分析2014—2018年的植被生长季(6—9月)土壤体积含水量的长期观测数据,探讨了土壤有效水源涵养量(土壤现实持水量与最小持水量之差)和水文调节功能(有效水源涵养量的时间变化速率)的变化特征及其环境调控机制。结果表明:高寒草甸0—100 cm年均土壤有效水源涵养量为(44.3±8.7)mm(平均值±标准差,下同),呈现出双峰型的季节趋势,最高峰和次高峰分别为6月下旬的(57.8±14.4)mm和9月中旬的(59.2±15.7)mm。浅层(0—20 cm)、中层(20—60 cm)和深层(60—100 cm)土壤有效水源涵养量占比分别为53.1%,34.9%和12.0%,土壤有效含水源涵养量随土层深度增加表现为对数衰减(R²=0.82,p<0.001)。增强回归树的结果表明土壤有效水源涵养量的季节变化主要受控于土壤温度,尤其是5 cm土壤温度,二者呈现出显著负相关。不同深度的年均土壤有效水源涵养量和土壤黏粒比例显著负相关(R²=0.99,p=0.004)。根系区(0—40 cm)年均土壤吸湿速率和脱湿速率分别为(0.21±0.02)mm/h和(0.22±0.02)mm/h,t检验的结果表明除了0—5 cm之外,根系区土壤脱湿速率显著大于吸湿速率。分析表明土壤温度是土壤吸湿和脱湿速率的显著环境驱动因子。因此,土壤温度是高寒禾草-矮嵩草草甸土壤有效水源涵养量和水文调节功能的主要影响因素,维持土壤的低温是高寒草甸水源涵养功能保育和提升的重要基础。

Soil Water Conservation Function of an Alpine Graminoid-Kobresia Meadow on the Southern Slope of the Qilian Mountains

The Qinghai-Tibetan Plateau is well-known as ‘water towers for China', and alpine meadow is one of the most important vegetation types while its water conservation capacity remains unclear. The continuous volumetric soil water content data of an alpine graminoid-Kobresia meadow were analyzed to quantify the effective soil water storage(the difference of measured soil water storage and minimal soil water storage)and soil hydrological regulation function(the temporal rate of effective soil water storage)and their environmental controls during the growing season(mid-June through end-September)on the southern foot of the Qilian Mountains from 2014 to 2018. The results showed that the annual total(0—100 cm)effective soil water storage averaged(44.3±8.7)mm(Mean±S. D., the same below)and exhibited a bimodal season pattern, with the first peak of(57.8±14.4)mm at the end of June and the second peak of(59.2±15.7)mm in the middle of September; the shallow layer(0—20 cm), mid-layer(20—60 cm)and deep layer(60—100 cm)accounted for 53.1%, 34.9% and 12.0% of total effective soil water storage, respectively; effective soil water storage was negatively related to soil depth logarithmically(R² = 0.82, p< 0.001). The boosted regression trees showed that the seasonal variations of effective soil water storage were mainly regulated by soil temperatures, especially by 5 cm soil temperature, where they negatively correlated. The annual soil water storage along soil depths was closely related to the ratio of soil clay(R²=0.99,p=0.004). The average annual soil moisture absorption rate and dehumidification rate of the root zone(0—40 cm)were(0.21±0.02)mm/h and(0.22±0.02)mm/h, respectively. The t-test showed that, except 0—5 cm soil layer, the soil dehumidification rate was significantly greater than the moisture absorption rate in the root zone. Correlation analysis revealed that soil temperatures more than soil moistures drove the season variations of soil moisture absorption and dehumidification rate. Therefore, soil temperature was the main factor influencing the effective soil water conservation and hydrological regulation function of alpine graminoid-Kobresia meadows. These findings suggested that maintaining low soil temperatures would be an important basis for persevering and improving the water retention function of alpine meadows.