Mechanisms Of Self-Healing After Drawing Damaged Of Soil And Water Conservation Shrub Roots In Semi-Arid Areas

The complex and changeable climatic conditions and geomorphic types in semi-arid areas have formed extremely sensitive and fragile ecological zones.The research and practice of soil erosion control in these regions have not been interrupted for many years.One of the hot research topic is that restoration of damaged ecosystems through plant practices.Plant roots interpenetrate and entwined in the soil to form the "reinforcing" and"anchoring" effects on the soil,to improve the ability of soil to resist all kinds of erosion,and this mechanism has become the main embodiment of soil consolidation by plants.In this study,four kinds of typical shrubs in arid and semi-arid areas:Caragana microphylla,Hippophae rhamnoides,Artemisia ordosica and Salix psammophila were selected as subjects,by means of material mechanics and plant physicochemical analysis,through field in situ simulation test and laboratory control experiments,the basic ultimate tensile strength properties of straight roots and lateral-root branches of four plants were studied in parallel,and researched the influencing factors of the variation of root tensile properties at the same time.The changes of growth index self-repairing capability and ultimate tensile strength properties self-repairing capability of shrubs roots after instantaneous drawing damage and continuous drawing damage were respectively studied.The effect of the main chemical components of four kinds of shrubs roots on the ultimate tensile properties was analyzed from microcosmic perspective.On the basis of these,the influence factors such as limiting force,repair rate and chemical composition of root system were synthesized,the self-healing soil-reinforcement indexes of different shrubs roots were determined by establishing evaluation index system.This study can lay a foundation for the establishment of mechanical model of soil-reinforcement under root failure load.It has important theoretical and practical significance for expanding the theoretical system of soil erosion resistance of plant root system,and it has guiding significance for building accurate vegetation allocation model in fragile ecological areas.The main conclusions in this paper are as follows:(1)No matter the basic ultimate tensile force and tensile strength of undamaged roots,or the ultimate tensile force and tensile strength of drawing damaged roots after self-healing,there were significant differences among the four kinds of shrubs,which were all manifested as:Caragana microphylla>Salix psammophila>Hippophae rhamnoides>Artemisia ordosica.The ultimate tensile force and tensile strength of four kinds of shrubs in the early growth stage were significantly different from the period of vigorous growth.The vigorous growth period of Caragana microphylna was larger than that of the early growth stage,and the other three species were vice versa.The ultimate tensile force of four kinds of shrubs increased with the increasing of root diameter as a power function,and the ultimate tensile strength decreased with the increasing of root diameter as a power function or an exponential function.The ultimate tensile force and tensile strength of lateral-root branches were weaker than that of the straight roots of four kinds of shrubs,indicated that lateral-root branches was the vulnerable part of the root network.The ultimate tensile properties repair rates after drawing damaged of four kinds of shrubs were manifested as:Salix psammophila(65.42%)>Hippophae rhamnoides(44.76%)>Caragana microphylla(37.83%)>Artemisia ordosica(30.32%).The growth index repair rates after drawing damaged of four kinds of shrubs were manifested as:Salix psammophila(75.36%)>Caragana microphylla(67.94%)>Hippophae rhamnoides(61.97%)Artemisia ordosica(61.03%).The self-healing soil-reinforcement indexes of four kinds of shrubs roots were determined as follows:Caragana microphylla(5.03)>Salix psammophila(2.89)>Hippophae rhamnoides(-1.97)>Artemisia ordosica(-5.96).The results showed that the root system of Caragana microphylla had the strongest ability of sustainable soil-reinforcement,followed by Salix psammophila,Artemisia ordosica had the worst ability.(2)Continuous pull-out to the growth and ultimate mechanical properties of shrubs roots was significantly greater than that of instantaneous pull-out,the self-healing ability after continuous drawing damaged was weaker.After continuous pull-out damaged,the root growth index and ultimate mechanical repair rates of Caragana microphylla were 78.06%and 56.42%,respectively,of the instantaneous pull-out damaged,the two kinds of repair rates of Salix psammophila were 80.58%and 73.06%,respectively,of the instantaneous pull-out damaged,the two kinds of repair rates of Hippophae rhamnoides were 69.66%and 67.55%respectively,the two kinds of repair rates of Artemisia ordosica were 75.74%and 58.59%respectively.(3)The degree of damage significantly affected the self-healing ability of root growth index and ultimate mechanical properties of shrubs roots.The inhibitory effect of severe damage on the activity,growth rate,ultimate tensile force and tensile strength of shrubs roots was significantly greater than that of mild damage,and severe damage was highly likely to result in root death.After severe damaged,the repair rates of root growth index and ultimate mechanical properties of Caragana microphylla were respectively 80.55%and 86.24%of mild damaged,the two kinds of repair rates of Salix psammophila were 82.44%and 86.24%,respectively,of the mild damaged,the two kinds of repair rates of Hippophae rhamnoides were 82.50%and 72.45%respectively,the two kinds of repair rates of Artemisia ordosica were 84.74%and 72.82%respectively.(4)The duration of repair could significantly affect the self-healing ability of growth index and ultimate mechanical properties of shrubs roots after drawing damaged.The longer the repair time after damaged,the better the roots system could eliminate the negative feedback that generated by external damaged,and the stronger the sustainable soil-reinforcement ability.After self-healing for 12 months,the root growth index repair rate and the ultimate mechanical properties repair rate of Caragana microphylla were 1.48 and 3.32 times respectively of self-healing for 3 months,the two kinds of repair rates of Salix psammophila were 1.38 and 5.29 times respectively of self-healing for 3 months,the two kinds of repair rates of Hippophae rhamnoides were 1.75 and 3.81 times respectively,the two kinds of repair rates of Artemisia ordosica were 1.18 and 8.16 times respectively.(5)The influence of root type on the self-healing ability of growth index and ultimate mechanical properties of shrubs roots after drawing damaged varied from plant to plant.The root growth index repair rate of Caragana microphylla lateral-root branches was 3.77%slightly higher than that of the straight root,and the ultimate mechanical properties repair rate of straight root was 20.80%significantly higher than that of the lateral-root branches.The root growth index repair rate of Salix psammophila straight root was 1.74%slightly higher than that of the lateral-root branches,and the ultimate mechanical properties repair rate of lateral-root branches was 6.35%higher than that of the straight root.The two kinds of repair rates of Hippophae rhamnoides lateral-root branches were 2.42%and 2.20%respectively higher than that of the straight root.The two kinds of repair rates of Artemisia ordosica lateral-root branches were 5.19%and 5.82%respectively higher than that of the straight root.(6)The contents of five kinds of chemical components in shrub roots were significantly different among plant species.The content of cellulose,hemicellulose and holocellulose of Caragana microphylla root was the highest among four kinds of shrubs,and the content of lignin and wood fiber ratio of Hippophae rhamnoides root was the highest.Both root type and root diameter could significantly affect the distribution of chemical components in root system.The content of root chemical composition had different effects on the ultimate tensile force and tensile strength of four shrubs,significant correlations were generally found between components variables and mechanics variables.The results showed that the chemical composition indirectly affected the sustainable soil-reinforcement of shrubs roots by changing the ultimate mechanical properties.The average contribution rate of root chemical composition to the ultimate tensile force of the four shrubs were:cellulose(80.67%)>holocellulose(72.49%)>hemicellulose(64.00%)>wood-fiber ratio(60.24%)>lignin(52.76%),the results showed that cellulose was the most important chemical component affecting the tensile mechanical properties of shrubs roots.