Vegetation and Soil Characteristic of Artificial Haloxylon Ammodendron Forest with Different Degradation Grades in Minqin Oasis-desert Ecotone
Vegetation degradation of Oasis-desert Ecotone concerns the expansion of desertification in arid area and semi-arid area,therefore,it is extremely significant focus in the process of combating desertification.Haloxylon ammodendron was introduced to Minqin regions from Xinjiang province in the 1960 s,combined with local clay barrier and became the primary windbreak and sand-fixation mode.The total area of artificial H.ammodendron plantations in Minqin county is about 0.035 million hm2,occupies 51.5% of the total area of artificial forestry.Because of the impact of climatic change,human activities and other factors,the water table is falling and soil moisture content is gradually declining in Minqin Oasis-desert Ecotone,this have lead to degradation of artificial H.ammodendron community,and seriously threats to the survival and development of Minqin Oasis.In this dissertation,we focused on the research of the artificial H.ammodendron community and its soil in Minqin Oasis-desert Ecotone,based on the investigation of vegetation,collecting and testing of soil samples,firstly divided all communities into four degradation grades according to vegetation data,secondly explored vegetation and soil characteristic of communities with different degradation grades as well as the relation between vegetation degradation and change of soil characteristic,provided scientific basis for monitoring and evaluation of soil quality,restoration of degraded vegetation and sustainable construction of desert ecosystem in desert area.The results were as follows:(1)With the aggravation of degradation of artificial H.ammodendron community,rate of dead branch,shoot length,growth power and proportion of survival of H.ammodendron had significant differences,rate of dead branch increased,shoot length shortened,growth power got poor,and dead plants began to appear in SD.The importance value of H.ammodendron was considerably greater than that of other species in all degradation grades.With the aggravation of degradation,the importance value of Calligonum mongolicum had no significant differences,that of H.ammodendron,Bassia dasyphylla and Limonium aureum as a whole decreased,that of Halogeton glomeratus and Agriophyllum squarrosum gradually increased,that of Artemisia desertorum first increased and then decreased.(2)With the aggravation of degradation,species α diversity indexes of shrub layer had no obvious change,number of species of herb layer increased;the order of Simpson index,Shannon-Wiener index and Pielou index of herb layer and whole was MD>LD>SD>ND.Species replacement rate among communities of contiguous degradation grades decreased,similarity increased with the aggravation of degradation,and the order of similarity was shrub layer>whole>herb layer.H.ammodendron was negatively correlated to L.aureum,C.mongolicum,Artemisia desertorum and Salsola collina in artificial H.ammodendron community,and had little correlation with B.dasyphylla,H.glomeratus and A.squarrosum.(3)The differences of soil mechanical composition,moisture content,total salt content,bulk density and total porosity among three soil layers on the whole was not significant,but moisture content and total salt content decreased with the increase of soil depth;the difference of pH among soil layers altered from significant to non-significant with the aggravation of degradation.Soil mechanical composition,bulk density,total porosity and total salt content of all soil layers had no obvious change with the aggravation of degradation,but soil mechanical composition of ND was relatively fine,that of MD was relatively coarse;moisture content and pH among different degradation grades had significant differences,moisture content of SD and pH of MD were lowest.(4)Soil available N,available P,total P and cation exchange capacity had no significant differences among soil layers,the difference of total N and organic matter among soil layers changed from significant to non-significant with the aggravation of degradation,various nutrient content decreased with the increase of soil depth as a whole.With the aggravation of degradation,available N and total P had no obvious change in all soil layers,but available N of SD was lowest,total P of soil layer 0-5cm gradually decreased;available P,organic matter,total N and cation exchange capacity had significant differences,available P and organic matter gradually decreased,total N of all soil layers of SD on the whole was minimum,cation exchange capacity of soil layer 10-20 cm of ND was greater than that of SD.(5)With the increase of soil depth as a whole,the number of soil total microbes and three kinds of microbes was decreased,proportion of bacteria was decreased,proportion of actinomycetes was increased.Number of bacteria occupied first place,number of actinomycetes came second,number of fungi was few in all soil layers.With the aggravation of degradation,significant change of number of fungi was not observed;number of bacteria,actinomycetes and total microbes decreased in 0-5cm and 5-10 cm soil layer,but no obvious change in 10-20 cm soil layer.Proportion of bacteria was maximum in three soil layers of MD,but proportion of actinomycetes was minimum.(6)Overall,enzyme activities decreased with the increase of soil depth.Enzyme activities of 0-5cm soil layer decreased with the aggravation of degradation,but not had significant change in other soil layers.The difference of proteinase activity and urease activity among soil layers changed from significant to non-significant with the aggravation of degradation,catalase activity was the opposite,invertase activity remained unchanged.The impact of degradation of artificial H.ammodendron community on proteinase activity,catalase activity,urease activity was greater than invertase activity,and the former had higher sensitivity to the change of resource and environment.(7)There was strong correlation among species α diversity indexes of artificial H.ammodendron community,but correlation between them and soil indexes was extremely feeble.Correlation among other soil physicochemical indexes was feeble except soil mechanical composition.There was extremely strong correlation between soil biological indexes and silt,pH,total salt content,total N,available P,organic matter,cation exchange capacity;the correlation between soil biological indexes and chemical indexes was stronger than the correlation between soil biological indexes and physical indexes as a whole.Correlation among other soil biological indexes was extremely strong except number of actinomycete.(8)The order of soil fertility of different layers in artificial H.ammodendron community with different degradation grades was ND 0-5cm>ND 5-10cm>MD 0-5cm>LD 0-5cm>ND 10-20cm>LD 5-10cm>MD 5-10cm>SD 0-5cm>MD 10-20cm>LD 10-20cm>SD 5-10cm>SD 10-20 cm,the order of soil fertility of different degradation grades was ND>LD/MD>SD,the order of soil fertility of different layers was 0-5cm>5-10cm>10-20 cm.