Characteristics Of Surface Dew Under Different Types Of Sand-fixing Shrubs In The Eastern Margin Of Hobq Desert

As the main water source in arid ecosystem except rainfall,dew plays an important role in the stable maintenance of ecosystem structure and function.Although there have been many studies on the formation process and source characteristics of surface dew,the mechanism of the formation process and source characteristics of surface dew under the sand-fixing shrubs in arid sandy areas are still not clear enough.Therefore,different types of sand-fixing shrubs(Salix psammophila,Artemisia ordosica,Caragana korshinskii)were selected in the eastern margin of Hobq Desert,and the bare land was taken as the control in this study.Observation points were set at the root of shrub,the one half of the crown,and the crown width edge,respectively.The amount of dew was observed with the microlysimeter,while the source of dew was observed with the top-sealed and bottom-sealed micro-lysimeter,and its source of different depths of soil layers were observed with the micro-lysimeter as a drawer type.The main results are as follows:(1)Compared with the control,the existence of sand-fixing shrub reduced the surface dew,the surface dew of Salix psammophila,Artemisia ordosica and Caragana korshinskii decreased about 16%,15% and 12%,respectively.Under the sand-fixing shrubs,the surface dew in different months was in the order of July > August > September > June,and the surface dew in different positions was in the form of shrub root < one half of the crown width < crown width edge.Different locations and different months have significant effects on surface dew,while sand-fixing shrub types have no significant effects on surface dew.(2)The formation and evaporation of surface dew presented a "single peak" curve,and its peak value gradually moved backward over time(June–September).Surface dew started to form at 19:00 and reached its maximum value between 7:00-9:00 the next day,and it continued to form about 1 hour after sunrise.When the dew reached its maximum value,it began to evaporate,and the evaporation curve decreased rapidly first and then slowly.Compared with the control,the presence of sand-fixing shrubs is not conducive to the formation and evaporation of surface dew.The surface dew had a significant negative correlation with air temperature and soil temperature,a significant positive correlation with atmospheric relative humidity,and no significant correlation with wind speed.The surface temperature under the sand-fixing shrubs increased from root to crown edge in daytime,but it was opposite at night.Sand-fixing shrubs have an interception effect on near surface airflow.Sand-fixing shrubs buffer surface temperature changes by intercepting solar radiation to slow water condensation and evaporation.(3)The dew at the depth of 0-10 cm mainly came from soil,and the water vapor ratio from soil and air was about 6:4.From June to September,the contribution rates of soil to surface dew were 54%,61%,57% and 74%,respectively.Due to the influence of sand-fixing shrubs,the sources of surface dew at different locations also showed certain differences.The dew increased gradually from root to crown margin in Salix psammophila,the contribution from soil under Artemisia ordosica was relatively stable,and the contribution from soil under Caragana korshinskii decreased.(4)In the process of dew condensation,the main source of dew in 0-3cm soil layer was air and its contribution gradually decreased with the process of condensation.The proportion of dew in 3-6 cm soil layer was similar to that in air and soil,and the dew in 6-10 cm soil layer was mainly from air vapor at the initial stage of condensation.,and was mainly from soil at the end of condensation,and the maximum can reach 67%.(5)The surface dew mainly formed in 0-3 cm soil layer,followed by 6-10 cm soil layer and 3-6 cm soil layer.Compared with the control,the contribution of dew in 0-3 cm soil layer decreased,and that in 3-6 cm and6-10 cm soil layer increased.The water vapor from the air can only provide water vapor in 0-3cm and 3-6 cm soil layers because of its weak transport capacity,while the water vapor in the soil had a strong transport capacity and can provide water vapor in 0-3cm soil layers.(6)At the initial stage of water vapor condensation,the contribution of0-3 cm soil layer to the dew was larger,and the contribution of 3-6 cm and6-10 cm soil layer was smaller.At the end of condensation,the contribution of 3-6 cm and 6-10 cm soil layer increased,and the contribution of different soil layers from large to small was 6-10 cm,0-3 cm and 3-6 cm.This may be related to the high precipitation during the experimental observation period(August).The high precipitation led to the increase of soil water content and water vapor in the 6-10 cm soil layer,making the water vapor in 6-10 cm contribute more to the condensation process in August.In addition,the contribution of different soil layers to surface dew was closely related to the variation of surface temperature gradient.