Vegetation Succession And Soil Physical And Chemical Properties During The Decline Of Shrub Dunes Of Nitraria Tangutorum In Mu Us Sandy Land

Vegetation is the most sensitive and intuitive symbol of desertification development in each stage,and the ecological process of its growth,development and death is the feedback of the growth,development and decline of shrub dunes in arid,semi-arid and semi-humid areas.In this paper,the Nitraria tangutorum shrub dunes of different decay types in Yanchi County,Ningxia province were studied,which is located in the southwestern margin of Mu Us sand land.By using the time-replacement space method of plant ecology and field observation and investigation methods,the vegetation and soil characteristics of four decay types of Nitraria tangutorum shrub dunes were studied to reveal the response of vegetation on the decay process of the dune and the main soil factors affecting vegetation growth.The main conclusions are as follows.(1)According to the number of wind erosion surfaces and vegetation growth conditions,Nitraria tangutorum shrub dunes can be divided into four different types,which are wind erosion type of slope toe,one side wind erosion type,two-side wind erosion type and multifaceted wind erosion type.According to the proportion and weight of decline types in the sample,the Nitraria tangutorum shrub dunes were divided into four decline grades:mild,moderate,severe and extremely severe.(2)In the four types of Nitraria tangutorum shrub dunes,there original face succession mode for Nitraria tangutorum communities——Nitraria tangutorum communities+Bassia dasyphylla community——Nitraria tangutorum communities+Bassia dasyphylla+Artemisia frigida community——Euphorbia esula communities+Achnatherum splendens communities+Nitraria tangutorum community.There were almost no Nitraria tangutorum on the caving surface,only a few branches or roots that had been buried in the dunes were re-exposed.Accumulation surface(sediment re-accumulated at the foot of the slope after the collapse of the upper part of the dune)gradually replaced the Nitraria tangutorum community as a constructive species and dominant species with one-year and two-year-old herbaceous plant Bassia dasyphylla,perennial herbaceous plant Agropyron cristatum and subshrub Sophora alopecuroides.(3)Under the 0-40 cm soil depth,the soil moisture content of the accumulation surface of the same decay type is higher than that of the original surface of the dunes.Soil organic matter content in 0-40 cm soil layer showed an overall upward trend with the increase of sand dune decay degree,and there were significant differences among different parts of sand dunes.The soil total nitrogen content showed an overall upward trend with the increase of dune decay types at all soil depths,and the differences between various parts of dunes with different decay types were significant.The variation trend of soil available phosphorus at different soil depths showed‘M’ type with the increase of dune decay degree,and the soil available phosphorus content was the lowest at the original surface of two wind erosion dunes.Soil available potassium content decreased with the increase of dune decay,and there were significant differences among different parts of the dune.Total nitrogen content in the 3-20 cm soil layer was the main soil factor affecting vegetation characteristics in the original dune surface.The content of available k in 20-40 cm soil layer was the main soil factor affecting vegetation characteristics on dune accumulation surface.(4)The original morphology of a single declining Nitraria tangutorum shrub dune was changed due to wind erosion,which showed that the dune caved and piled up again to form an accumulation surface,which changed the soil characteristics of the dune and affected the vegetation status of the dune.In terms of the whole dune area,the decline of multiple dunes results in the decrease of the original surface area of the dune and the rapid increase of the accumulation surface area(the proportion of the whole sample area increases).It will lead to a fundamental change in the plant community of the whole dune,from the community type of Nitraria tangutorum as the dominant species and dominant species to the community type of perennial herbaceous plants(Agropyron cristatum+Sophora alopecuroides+Bassia dasyphylla)as the dominant species and dominant species.