| Vegetation restoration on the Loess Plateau has obtained remarkable achievement,controlling soil and water loss and increasing carbon sequestration.However,the negative feedback of soil moisture caused by vegetation restoration has led to the widespread appearance of soil desiccation on the Loess Plateau and restricted the sustainable and healthy development of vegetation.The key factor limiting the vegetation restoration and plant growth is still soil moisture in the loess region of Northern Shaanxi.Therefore,it is very crucial to study the vertical distribution of soil moisture in 0-1000 cm and its influence mechanism for revealing the characteristics of soil water supply and consumption in forestland,the construction of forest and grass vegetation,and the improvement of the stability and quality of plantation forest ecosystem.In this paper,different vegetation types in jinfoping watershed and different slope microtopographies in Hegou watershed of Wuqi County,Northern Shaanxi Province,were taken as the research objects.We observed soil moisture in the 1000 cm soil layer,studied the vertical distribution characteristics of soil moisture in different forestlands,and analyzed the impacts of soil physical and chemical,slope microtopography and vegetation restoration on soil moisture.The main conclusions are as follows:(1)In terms of the overall characteristics of soil moisture in jinfoping watershed in loess region of northern Shaanxi,soil moisture is subject to normal distribution in all layers after normal test.In the depth range of 0-1000 cm,as the depth of soil layer increased,the soil moisture decreased firstly and then increased,and the coefficient of variation of soil moisture waved.In the depth range of 0-1000 cm,there are significant differernces among different vegetation types,the grassland is significantly higher than other vegetation types,and the Populus simonii is the lowest(P<0.05).In terms of temporal change(2017-2019),the shallow soil moisture is more dramatic,while the deep soil moisture is relatively stable.(2)In 0-100 cm soil layer,as the soil depth increases,soil bulk density and soil water content increased,while soil porosity,saturated water content and soil saturated hydraulic conductivity decreased.In 0-1000 cm soil layer,the soil organic carbon content is significantly higher in the 0-100 cm soil layer than in the 100-1000 cm soil layer,indicating that there is surface aggregation of soil organic carbon.The correlation between soil bulk density and soil porosity and most other soil properties reached significant or extremely significant levels(P<0.05 or P<0.01),indicating soil bulk density and soil porosity could represent soil properties,while soil bulk density and soil porosity have extremely significant correlation.In order to facilitate monitoring,soil bulk density can be selected as the representative index of soil quantitative properties in this area.The results of grey correlation analysis showed that the effects of silt content,clay content,soil non-capillary porosity,soil saturated water content,soil total porosity,soil capillary porosity,soil bulk density,saturated hydraulic conductivity,sand content and soil organic carbon on soil moisture decreased in turn.(3)The soil moisture of each soil layer obeys normal distribution in 0-1000 cm soil layer for five microtopographies such as gully,ephemeral gully,sink hole,platform and scarp and undisturbed slope.The minimum and maximum values of average soil moisture were found in the 580-600 cm soil layer and 0-10 cm soil layer with value of 9.18% and14.17%,respectively.As the depth of the soil layer increased,the average soil moisture firstly decreased and then increased,while the coefficient of variation showed fluctuating changes.There were significant differences in different microtopographies(P < 0.05),especially in the 100-600 cm soil layer,the soil moisture in gully and sink hole significantly lower than in other microtopographies and in undisturbed slope.According to the multiple linear regression model analysis between soil moisture and soil particle and soil organic carbon in undisturbed slope,the depth of influence of microtopographies on vertical distribution of soil moisture exceeded 1000 cm for a gully,740 cm for a sink hole,360 cm for an ephemeral gully,480 cm for a scarp and only 220 cm for a platform.The gully and sink hole are significantly deeper,probably because the shallow soil moisture of gully and sink hole is better,and during the natural recovery of the vegetation,the vegetation invade these microtopographies preferentially.However,under the influence of climate warming and drying,vegetation can only use the deep soil moisture to maintain its life activities,which leads to the obviously decrease of soil moisture in sink hole and gully.(4)Differernt artificial forestlands showed different degree of soil water storage deficit(CSWSD),especially the deep soil water storage deficit is more serious,which is mainly affected by vegetation types.The water consumption depth of Armeniaca sibirica,Hippophae rhamnoides,Robinia pseudoacacia and Populus simonii is more than 1000 cm,while the water consumption depth of Pinus tabulaeformis is 760 cm.The water cosumption depth of Pinus tabulaeformis is relatively small,probably because Pinus tabulaeformis is a young tree.The soil desiccation index(SDI)is the highest in Populus simonii(18a)(80.76%)and Robinia pseudoacacia(19a)(76.28%),and the lowest in Pinus tabulaeformis(5a)(36.45%)and Armeniaca sibirica(8 a)(33.99%),which is mainly the result of tree species,tree age and vegetation characteristics.The soil desiccation degree of Hippophae rhamnoides(18a)shrubland is significantly lower than that of two kinds of adult arbor forests.It is suggested that Hippophae rhamnoides is the main type of artificial forest and grass vegetation construction,especially in semi-arid areas with annual average rainfall less than 500 mm. |
