Water Use Efficiency Of Soil And Water Conservation Forests Vegetation And Its Influencing Mechanism In Hilly And Gully Region Of Northern Shaanxi

The loess hilly and gully area is a key area for ecological restoration in our country.After a long-term comprehensive improvement of soil and water conservation,soil erosion has been effectively controlled.However,the contradiction between insufficient water resources and water consumption for vegetation growth has become an important limiting factor affecting the continuous restoration of soil and water conservation forests.Therefore,in order to reveal the water use efficiency of soil and water conservation forests and its impact mechanism,this study was conducted in the Wuliwan watershed in the hilly and gully area of northern Shaanxi.We selected the Armeniaca sibirica,Hippophae rhamnoides,Pinus tabulaeformis,Populus simonii,Robinia pseudoacacia,Robinia pseudoacacia and Amygdalus davidiana mixed forest that were planted in 2000,and selected the 20～45 years of restoration of Caragana korshinskii forests and the restoration of 14～45 years of Robinia pseudoacacia forests as the research objects.Based on the stable carbon isotope method and gas exchange method,the water use efficiency of different soil and water conservation forests and the change characteristics and differences of vegetation and soil factors in the same period were analyzed.Using statistical methods such as correlation analysis and redundancy analysis to explore the key influencing factors that affect the water use efficiency of soil and water conservation forests,for providing a scientific basis for the efficient use of water resources and sustainable and stable restoration of soil and water conservation forests.The main research results obtained are as follows:（1）There are significant differences in water use efficiency of different types and different restoration years of soil and water conservation forests measured by stable carbon isotope method.The water use efficiency of different types of soil and water conservation forest vegetation ranges from 73.93 to 113.29μmol·mol^-1.Among them,the water use efficiency of Pinus tabulaeformis,Robinia pseudoacacia in mixedforest and Hippophae rhamnoides is the highest,and the Populus simonii has the lowest water use efficiency.In different years of soil and water conservation forests,the water use efficiency of Caragana korshinskii after 45 years of restoration was reduced by 19.9%compared with 20a,while the water use efficiency of Robinia pseudoacacia first decreased and then increased.It was the lowest at 30 years of restoration,which was 70.60μmol·mol^-1,and at 45 years was the highest,which was 104.22μmol·mol^-1.The instantent water use efficiency of different soil and water conservation forests was calculated by the gas exchange method.The range of each type of forest land was 2.26～6.39μmol·mmol^-1.The instantent water use efficiency showed that the Pinus tabulaeformis and Robinia pseudoacacia in mixed forest was higher,and in Populus simonii and Armeniaca sibirica were lower.And the tallest Pinus tabulaeformis is 2.8 times that of the lowest Armeniaca sibirica.The instantent water use efficiency of Caragana korshinskii and Robinia pseudoacacia soil and water conservation forests ranged from 2.13 to 4.91μmol·mmol^-1.Caragana korshinskii recovered at 45 years was reduced by 43.5%compared with 20a,while Robinia pseudoacacia showed14a>20a>45a>30a.The water use efficiency obtained by the two methods has a very significant correlation.The stable carbon isotope method can express the water use efficiency of the vegetation over a period of time.The gas exchange method is to test the condition of the leaves at the time,which has certain limitations.（2）The photosynthetic and physiological characteristics of different types and different restoration years of soil and water conservation forest vegetation are significantly different.Among different types of soil and water conservation forests,Hippophae rhamnoides and Populus simonii have higher photosynthetic rate,transpiration rate,stomatal conductance and intercellular CO2 concentration.The difference in leaf water content of various types of vegetation is small.The leaf chlorophyll SPAD value of Hippophae rhamnoides is the highest,which is 60.4.Armeniaca sibirica is the lowest at 24.0.In different years of soil and water conservation forests,the photosynthetic rate,transpiration rate and stomatal conductance of Caragana korshinskii decreased with the increase of years,the intercellular CO2 concentration first decreased and then increased,and there was no significant difference in the leaf water content.Significant,but the transpiration rate,stomatal conductance,and intercellular CO2 concentration of Robinia pseudoacacia 30a were significantly higher than those of other plots.In general,the leaf moisture content of Robinia pseudoacacia was higher than that of Caragana korshinskii,while the chlorophyll SPAD value was reversed.（3）Planting of soil and water conservation forests can improve soil moisture status and permeability.The soil water storage in the 0～100cm soil layer of different types of plots is the largest in the Pinus tabulaeformis forest,reaching 188.4 mm,and the Hippophae rhamnoides is the smallest at 169.6 mm.Among the plots of different ages,Caragana korshinskii 30a and Robinia pseudoacacia 14a are the largest,181.1 mm and 191.3 mm,respectively.Different soil and water conservation forests have significant differences in soil saturated hydraulic conductivity.The saturated hydraulic conductivity of Pinus tabulaeformis and mixed forests reaches 0.76 and 0.67 mm/min,respectively,at 0～10 cm,which is significantly higher than that of other types of forest land.In plots of different ages,the soil saturated hydraulic conductivity of 45a and 30a Caragana korshinskii forest increased by 3.1 times and 1.4 times,respectively,compared with 20a,and Robinia pseudoacacia 45a increased by 45.7%compared with 14a.As the depth of the soil layer deepens,the difference in soil moisture content and saturated hydraulic conductivity between different soil and water conservation forests decreases.（4）The continuous restoration of soil and water conservation forests has significantly improved the soil structure and nutrient content.Different types of plots have different effects on accumulating soil carbon and nitrogen components,increasing total phosphorus content and adjusting p H.In general,Robinia pseudoacacia and Robinia pseudoacacia and Amygdalus davidiana mixed forestcan improve soil carbon and nitrogen content.The mixed forest has the highest soil total phosphorus content of 0.62 g/kg.The soil p H of Populus simonii and mixed forest is the highest,with an average of 8.33.And the Armeniaca sibirica is the lowest with 7.80.With the extension of the restoration period,the content of soil accumulated carbon and nitrogen nutrients showed an increasing trend.After 45 years of restoration,the total phosphorus content of the soil,Caragana korshinskii and Robinia pseudoacacia forests increased by 8.7%compared with that of 20 years,and there was no obvious regularity in soil p H changes.The soil bulk density of Hippophae rhamnoides and Populus simonii in different types of soil and water conservation forests is lower at 0～20 cm,1.10 and 1.11g/cm³,respectively.And the soil porosity is 58.4%and 58.0%,respectively;the average soil bulk density of Caragana korshinskii forests in different years is 1.14 g/cm³,the maximum at 30a is 1.22 g/cm³,the soil porosity is between 54.0%～58.7%,the soil bulk density of Robinia pseudoacacia forests of different years is between 1.14 g/cm³～1.29 g/cm³,and the maximum is at 20a.The average soil porosity is 54.5%.（5）Through correlation analysis and redundancy analysis of various vegetation and soil indicators,it is concluded that there are significant correlations among multiple indicators,indicating that various vegetation and soil factors affect and correlate each other during the growth process of plants using water;The influencing factors are sorted and screened.In this study,leaf photosynthetic rate,stomatal conductance,soil p H,organic matter,ammonium nitrogen content,saturated hydraulic conductivity,and bulk density are the vegetation and soil factors that affect plant water use efficiency.In summary,the Pinus tabulaeformis and Hippophae rhamnoides soil and water conservation forests have higher water use efficiency and are suitable for planting in a drier environment.Mixed afforestation will increase the water use efficiency of Robinia pseudoacacia.The photosynthetic physiological characteristics of vegetation,soil nutrient and water properties,and different forest ages will affect water use efficiency.It is necessary to evaluate and protect the water use and health recovery of soil and water conservation forests from various aspects.