Study On The Mechanism Of Soil Water And Salt Transport In The Patch Pattern Of Desert Vegetation In The Southern Margin Of Junggar Basin

In recent years,the shortage of water resources in the arid and semi-arid areas of Northwest China has become more and more serious.The shortage of water resources has led to a very fragile regional ecological environment,which is mainly manifested in the early dry water shortage,serious water shortage leading to the degradation and desertification of patchy vegetation,and the area of desertification is increasing year by year.In this paper,Haloxylon ammodendron vegetation with typical patch pattern in the oasis desert ecotone in the southern margin of Junggar basin is taken as the research object to reveal the hydrophysical characteristics of soil and the water transport law of natural vegetation,and to answer the scientific questions about the water use source and water transport law of desert vegetation and the ecological hydrological process of vegetation patch pattern.The main conclusions are as follows:(1)The root system of Haloxylon ammodendron absorbs water for plant growth,which makes the soil moisture content of vegetation patch significantly higher than that of bare land patch.Taking Haloxylon ammodendron as the highest point of humidity,it gradually reduces to the periphery,forming a "wet island" with Haloxylon ammodendron as the center.Vegetation coverage will weaken solar radiation and light,so that the vegetation patch becomes a low-temperature center and bare land patch due to the effect of environmental factors.The block became the center of high temperature and gradually increased the temperature along the vegetation patch to the bare land patch,forming a "cold island effect" centered on the vegetation patch.As a desert vegetation,Haloxylon ammodendron has the function of salting out,which makes the salt content of the soil where Haloxylon ammodendron is located lower than that of the surrounding bare land.The lowest point of Haloxylon ammodendron is gradually increasing towards the surrounding area,forming a "low salt area" centered on Haloxylon ammodendron.The above phenomenon of hydrologic Island fully shows that Haloxylon ammodendron has strong environmental adaptability to desert environment.(2)According to the analysis of water use of Haloxylon ammodendron,it can be seen that desert plants have formed a certain self-regulation mechanism under long-term drought stress.In the dry and rainless season,due to the low water content in the soil,it is not easy to be absorbed by plants,so plants mainly absorb and utilize the deeper soil water through the deep main root system;however,when the rainwater infiltrates into the soil,the lateral root system of desert plants will be activated,and their water use strategies will be changed,and the water in the soil will be used preferentially.According to the research,under the rainfall of 200 ml,the soil is shallow the contribution of layer water to the growth of Haloxylon ammodendron increased by 12%.(3)The adaptability of Haloxylon ammodendron to the environment is not only reflected in its absorption of soil water,but also in its consumption of water,that is,the liquid flow rate of Haloxylon ammodendron.Specifically,the liquid flow rate of Haloxylon ammodendron appears a short dormancy state when the solar radiation is the highest in the middle of the day,which is called "noon break",because the solar radiation is too strong in the middle of the day,the vegetation closes the stomata in order to reduce evaporation;The dry liquid density of Haloxylon ammodendron decreased with the decrease of soil moisture content,and the difference between day and night increased with the decrease of soil salt content;the liquid density of Haloxylon ammodendron was positively correlated with solar radiation,and the fitting result was: Y=0.04853X2-0.24635X+0.9865(Y is flow stem of Haloxylon ammodendron,g * m^-2 * h^-1;X is solar radiation,w * m^-2);R²=0.9167,P=0.0052.There was a significant positive correlation with the atmospheric temperature,the fitting results were: Y=0.0059X2-0.0095X+ 0.01964(Y is flow stem of Haloxylon ammodendron,g * m^-2 * h^-1;X is the atmospheric temperature,?);R²=0.8416,P=0.0069.However,it has a significant negative correlation with the space air humidity.The fitting results are: Y=0.1376X2-0.482X+ 0.0198(Y is flow stem of Haloxylon ammodendron,g * m^-2 * h^-1;X is the saturated vapor pressure VPD,hpa);R²=0.87846,P=0.0065.(4)The stability of Haloxylon patch pattern in the oasis desert ecotone in the southern margin of Junggar basin is the result of the interaction between Haloxylon vegetation and arid environment.The general hydrochemical types of patches in bare land are as follows: the surface soil is mainly SO₄^2--Ca²⁺,in which the Mg equivalent of SO₄^2-is 80-100%,and the Mg equivalent of Ca²⁺ is 60-90%;with the increase of soil depth,it gradually becomes SO₄^2--Na+·Ca²⁺,in which the Mg equivalent of Na+ is 60-90%;finally,in deep soil,they mostly exist in the form of SO₄^2-·Cl-Na+.The milligram equivalent of chlorine is 80%-90%.With the increase of soil depth,the general hydrochemical types of vegetation patches gradually change from SO₄^2--Ca²⁺to SO₄^2--Na+·Ca²⁺,in which the Mg equivalent of SO₄^2-is 80%-100%,the Mg equivalent of Ca²⁺ is 60%-90%,and the Mg equivalent of Na+is 30%-60%.In the deep soil,the content of SO₄^2--Na+·Ca²⁺is the highest,and the Mg equivalent of Na+ is 60%-90%.Haloxylon ammodendron can affect the soil environment to some extent,and reduce the secondary salinization of soil.(5)The diurnal and diurnal changes of soil water content indicated that the root system of Haloxylon ammodendron had a reverse hydraulic lifting effect;after precipitation,the utilization ratio of Haloxylon ammodendron to the surface and deep groundwater of the soil increased while the utilization ratio of Haloxylon ammodendron to the middle soil decreased,and the "photosynthetic noon break" of the leaves all strongly indicated that the root system of Haloxylon ammodendron had a hydraulic downward effect.The redistribution of water in Haloxylon ammodendron root system can maintain the balanced distribution of soil water,prolong the decrease time of soil water storage,reduce the stress of arid climate on Haloxylon vegetation,and make Haloxylon vegetation better survive in arid environment,which is also the basic reason for the formation and stability of Haloxylon vegetation patch pattern in oasis desert ecotone.