Study On Variation Of Soil Water And Heat In Typical Ecosystems In The Middle Part Of Qilian Mountains

Soil water and heat are important physical properties of soil,and their distribution characteristics and dynamic changes affect the eco-hydrological processes of vegetation ecosystems and soil.In addition,soil water and heat are of great significance to maintain the stability of ecosystems.The Qilian Mountains area is an important ecological barrier in our country,and it is known as the"ice source reservoir"in China.It is an important formation area of surface water resources in the Shulehe river,Heihe river,Shiyanghe river and other inland river basins.Frozen soil is widely distributed in the Qilian Mountains,and the freezing and thawing processes significantly affect the Qilian Mountains'ecological and hydrological processes.Therefore,characteristics of soil water and heat changes in the Qilian Mountains play important roles in stabilizing the alpine ecological environment and ensuring an ecological security in the northern Qinghai-Tibet Plateau.In this study,we selected two typical ecosystems which are communities of the subalpine meadow and Qinghai spruce forests in the Qilian Mountains as the study objects.The interannual variation characteristics of soil water and heat in subalpine meadows during freezing and thawing period,the interannual variation characteristics of soil water and heat in Qinghai spruce forest during freezing and thawing period under low(2750 m),medium(3050 m)and high(3250 m)altitude gradients,and the differences of soil water and heat changes on the daily and annual scales of the two ecosystems were analyzed.Results showed that:(1)The subalpine meadow soil entered the freezing and stable development stage in mid-November,entered the thawing stage in mid-to-late March,and end of the freezing in mid-early April.The frozen depth was 100–140 cm in the study period.During the study period,the earliest freezing time was delayed year by year and the earliest thawing time was advanced year by year.In addition,the freezing duration,freezing rate,and freezing depth of subalpine meadows all decreased with time during the study period.(2)The soil layer with 10–40 cm depth in Qinghai spruce forest at three altitude gradients entered the freezing and stable development stage in mid-early November,entered the thawing stage in mid-to-late April,and end of the freezing and thawing period in early May.As the altitude increased,the earliest thawing time was delayed,and the thawing rate gradually decreased.Compared with the low and high altitudes,the earliest freezing time,the longest freezing time,the fastest freezing speed,and the largest soil temperature change range at the middle altitude.In addition,there was no difference in the earliest freezing time,the earliest thawing time and freezing duration of different years during the study period.(3)The soil temperature in the two typical ecosystems showed a sinusoidal variation trend on both the daily and annual scales.Compared with the soil of Qinghai spruce forest,the response of soil to temperature changes in the subalpine meadow was more lag,the heat release stage of the subalpine meadow soil was longer,and the correlation with atmospheric temperature in the subalpine meadow was smaller.On the daily scale,the daily variation of the unfrozen soil water content(USWC)in the two ecosystems was very weak.Compared with the soil of subalpine meadow,the difference in the vertical profile of USWC in the Qinghai spruce forest was smaller,and the stable period of USWC in the Qinghai spruce forest was longer.(4)The changing trend of USWC in the two ecosystems displayed an"U"shape during the freezing and thawing period.There was a good power function correlation between the absolute value of the negative soil temperature and USWC(y=ax^b),and the correlation decreased with the increase of depth.The USWC of subalpine meadow in different years was different under the same temperature and depth,and USWC increases yearly.In summary,there was certain differences in soil water and heat changes between the two typical ecosystems in the central Qilian Mountains.The soil temperature and USWC of subalpine meadow were higher,and the freezing time was shorter.The subalpine meadow ecosystem showed a warming trend year by year in the six freezing and thawing periods.The freezing and thawing characteristics of soil in picea crassifolia forest ecosystem were different in altitude.In addition,the shallow soil temperature of the two ecosystems was significantly affected by temperature,and the correlation between soil temperature and air temperature of Qinghai spruce forests was higher.