| In vulnerable ecosystems,there exists a intimate interaction between soil and forests.Studying the soil nutrient and biological property variation characteristics in different sand-fixation forests is meaningful for understanding the effects of forests' soil amelioration and desertification prevention.In this paper,we mainly do research on how to protect the sand-fixation forest and improve the soil effect of the transition zone between the desert and oasis in Minqin.This study are carried out mainly through the sampling survey in the field,the standard set,the spot sampling,the sample analysis indoor,data processing and mathematical statistics.The key factors,such as the physical and chemical properties,the microbes,the enzyme activities and the anti-erosion effect are investigated.Then,the internal mechanism of the soil conservation for sand fixation is exploited,and a theoretical basis is provided for the construct and management of sand fixation.The main contributions are as follows:1.Conservation soil effect of sand-fixation forests depends on the type of tree species and age,which has significant differences in the windward and leeward side.Haloxylon forest was the best choice to be promoted all around the ecotone for it's not only significantly improve the sand-fixing effects,but also improve the soil charateristics and microbe activities of the moving dunes.(1)When the distance between the windward side and the plant is more than 1m,the influence of the sand-fixation forest on the stream of sand flow in the solid surface and wind velocity is small,where the structure eigenvalues of the sand flow are greater than 1,and there appears the wind erosion.The structure eigenvalues reduce and approach 1 when closing the plant and the depth of erosion becomes smaller.In contrast,when the distance is less than 1m,the stream of sand flow and the velocity decrease significantly,and the reduction of the stream of sand flow on the leeward side and the velocity is greater than the extent of the windward side;when the distance between leeward side and the sand-fixation forest is 0.5m,the structure eigenvalues of the sand flow are less than 1,and there appears the wind erosion;at the distance of 1m between leeward side and the plant,the structure eigenvalue reduces to the minimize value.When the distance between leeward side and the plant is more than 1m,the stream of sand flow and the velocity will increase;with the increasing of the distance between leeward side and the plant,the structure eigenvalues increases and is close to 1,and the Aeolian thickness becomes small and small.(2)The result of stream of the sand flow,the structure eigenvalues of the sand flow,and the depth of the wind erosion in sand-fixation forest with different species: Bare sandy land> Nitraria> Haloxylon forest with 20-year-old > Haloxylon forest with 40-year-old > Willows Tamarix> Haloxylon with 30-year-old.The roughness range in the leeward side of the sand-fixation forest is 0.57 cm ~ 4.18 cm,where the largest one is the Haloxylon forest with 30-year-old,the minimum is the white thorn;and the farther of the leeward side away from the plant,the smaller of the surface roughness.With the growth of the forest age,the stream of sand flow,the structure eigenvalues of the sand flow and the depth of erosion are reducing;the thickness of aeolian,the reduction of the wind speed and the roughness of the surface are increasing with different age spindle forest.The death and degradation of Haloxylon forest with 40-year-old,the stream of sand flow,the structure eigenvalues of the sand flow and the depth of erosion are bigger than that of Haloxylon forest with 30-year-old.(3)The wind speed near the sand-fixation forest increases with the height increasing at the same observation point.The height of the plant has significant impact on the effect of wind.When the wind passes through the sand-fixation forest,the closer to the top of the forest,the smaller the impact of sand-fixation forest on wind speed;the closer to the central of the sand-fixation forest,the greater the influence of wind speed.2.The effect of improvement soil physical and chemical properties the physicochemical properties of soil in sand fixation forest depends on soil depth,tree species and age.Within a certain time,the physicochemical properties of the soil have been improved with the establishment of bare sand vegetation,Soil amelioration were greatest in 30 s aged.(1)In the soil layer with the depth of 0-60 cm,with the increase of the soil depth,the moisture content of the soil,bulk density,p H value increase;but the content of the soil silt and clay,the porosity,the organic matters,the total of nitrogen,the total of phosphorus,the total of potassium,the nitrogen,the phosphorus and the potassium decrease.(2)In the soil layer with the depth of 0-60 cm in the sand-fixation forest with different tree species,the moisture content of the soil,bulk density,p H value are smaller than that of the bare sandy;the soil porosity,the silt and clay content,the organic matter,the total of nitrogen,the total of phosphorus,the total of potassium,the alkali solution nitrogen,the phosphorus and the content of rapidly-available potassium are greater than that of the bare sandy;with the depth of 0-20 cm,the moisture contents of the sand-fixation forests are more than that of the bare sandy,and with the depth of 20-60 cm,the moisture contents of the sand-fixation forests are less than that of the bare sandy.(3)The proportion of sand,silt and clay are different in different the sand-fixation forests.The coarse sand and fine sand are demonstrated with the highest content,followed by silt,and the content of clay is the lowest;in addition,and the proportion of silt and clay is far less than the proportion of coarse sand and fine sand.(4)For the Haloxylon forest with different age,the soil bulk density,the content of coarse sand and the soil p H value increase with the growth of forest age;they decrease to the minimum about 30 years old,and later increase gradually with the increasing of age.The soil moisture,the soil porosity,the soil clay and the content of silt,the soil organic matter,the total of nitrogen,the total of phosphorus,nitrogen,phosphorus and the content of potassium increase with the growth of the forest age,and they increase to the peak about 30 years old,and later decreases gradually with the growth of age.With the growth of age,the total of soil potassium content in Haloxylon Forest shows irregular variation.(5)The cumulative effects of different sand-fixation forests on the total of nutrients and the effective nutrient in the solid surface soil are different;for the various sand-fixation forests,the organic matter and nitrogen,the content of phosphorus and the content of potassium in the Haloxylon forest are the largest.The soil nutrients of the sand-fixation forests have the characteristics of low-nitrogen,low-phosphorus,high-potassium,and there is no significant difference in the content of the total of potassium.3.The different sand-fixation forests types and forest age,the different effect of improvement of soil biological properties.This study concluded that along with the restoration of Minqin Oasis-desert ecotone,all sand-restoration forests were becoming more and more adaptive to the desert environment.Populations of soil microbes and activities of enzymes were growing which means the biological characteristics were better than before.(1)The soil bacteria,actinomycetes and fungi,catalase,urease,invertase and phosphatase activity of the sand-fixation forests are significantly greater than that of the bare sand.It is shown that for the microbial components of the sand-fixation forests,the proportion of bacteria is shown the largest,followed by actinomycetes,and the minimum is fungi.(2)The soil microorganism distribution and enzyme activities were significantly different with depth.The population of soil actinomycetes,fungus and the activity of sucrase in Haloxylon forest were negatively correlated with the depth.(3)In the soil layer with the depth of 0-60 cm,the contents of the soil bacteria,actinomycetes,fungi,catalase,urease and phosphatase activities in the Haloxylon forest are the biggest,and 2.29 times,2.36 times,10.17 times,2.63 times,1.65 times and 1.71 times of the bare sand,respectively.(4)For the Haloxylon forest with different age,with the increasing of the age,the number of microorganisms,catalase,urease,phosphatase activity in the topsoil reached the maximum for the Haloxylon forest at 30-year-old;then the Haloxylon forest degraded with the growth of the forest age,and the various indicators decline for the Haloxylon forest with 40-year-old.4.soil conservation effect factor,Soil mechanical composition,soil nutrients,microbial number and soil enzyme activities in different sand-fixation forests were well positive correlated,which significantly improved the growth of plants and the quality of soil.(1)For the different sand-fixation forest,there is a positive correlation between the soil organic matter and the coarse sand significantly,and a negative correlation is shown among the clay,the silt and the soil organic matter.In addition,the negative correlation are reflected among the soil organic matter,the soil microbial and the activity,and among soil nitrogen,potassium and the number of bacteria,fungi,catalase,urease and phosphatase activity;and also among the number of bacteria,the soil phosphorus and four activity;and the correction among the content of the total of potassium,catalase,urease and phosphatase activity.Therefore,improving mechanical composition of the degraded soil,and has the important significance for improving soil nutrients and biological properties of sand-fixation forests.(2)For the different sand-fixation forest,there are significant positive correlation among fungi,actinomycetes,soil catalase,urease and phosphatase;and also between soil fungi and invertase.It reaches a significant level of the correlation among three kinds of microbes of different sand-fixation forest;for the four kinds of soil enzyme activities,the correlation among the enzyme are highly significant except for the catalase and invertase.The factors affecting soil microbial quantity impact on soil enzyme activity,we should pay attention to the adverse effects of external factors in sand-fixation forests management and protection process.(3)For the different sand-fixation forest,there are significantly negative correlation among the thickness of Aeolian,the reduction of wind speed,and the roughness of the surface with the content of coarse sand;and significantly positive correlation with the content of silt and clay;and a positive correlation between the sand flow and the coarse sand;and finally,a negative correlation among the sand flow with the content of silt and clay.There is no significant correlation among the thickness of Aeolian,the sand flow with the total of potassium and actinomycetes;and also no significant correlation between the roughness of the surface and the total of potassium,and among the reduction of wind speed with the content of nutrient,actinomycetes and catalase activity;the four factors have significant correlation with the content of the other soil nutrient,the number of microbes,the soil enzyme activity.Conservation soil effect and soil improvement effect of sand-fixation forests have good correlation,therefore,windbreak and sand-fixation are prerequisite for improving soil effect. |
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