Evolution Of Vegetation And Soil System On Secondary Grassland Of Abandoned Land Area In Minqin

Minqin oases due to deterioration of the natural environment, there are large areas of farmland back/ abandoned, natural development form true xerophytic plays a significant role of vegetation secondary grassland of perennial herbs and less than half shrubs, it is vulnerable to desert grassland transition type of biochemical grassland ecological system, along with the increasing length of farmland, has to gradually exposed or the trend of the development of the desertification, to carry out the abandoned land soil ecosystem evolution law and drive mechanism research, is conducive to the region secondary protection, restoration and sustainable utilization of grassland. This paper chose the middle and lower reaches of Shiyang river basin, Minqin oases abandoned land in different years of secondary grassland as the research object, using the method of space instead of time, on soil physical properties,chemical properties, biological characteristics and soil enzymes were measured and the analyzed system, reveals the abandoned land secondary grassland soil system change rule,driving factors and its mechanism, to probe human intervention to restore soil system technical countermeasures, secondary conservation and utilization of grassland is returning abandoned land provides a theoretical basis and technological support, obtains the following main conclusions:(1) Secondary grassland vegetation in Minqin abandoned land during plant communities in the process of natural succession, species richness and diversity index show first increases,fluctuations decreased after the law, and the law of evenness index increased in a fluctuant way. In experimental investigation of 1 ~ 31 a in 9 samples, plants that appear there are 34,14 families, including 6 species of herbaceous plants, 28 species of shrubs plants; of plant in most kinds of minority, a few species belong to the majority, and a large proportion of species as a single genus and single branch. The succession process can be divided into fourperiods: the first stage(grain for 1-3a) for the annual and perennial root herbs, are rapid recovery stage. The second stage(grain for 3 to 8a) for the annual herbaceous plant to perennial herbaceous succession stage. The third stage(grain for 8 to 24a) perennial herbaceous turn to the perennial shrub succession stage. The fourth stage(grain for 24 to 31 a)perennial shrub plant is stable stage.(2) Within the scope of different years, abandoned land 0 to 40 ㎝ soil moisture content with the increase of soil depth after first increased a change to the reduction process; In 20 to40 ㎝ low soil moisture content levels remained low, levels did not change obviously, and soil moisture content change is largely a v-shaped cloth. All kinds of fine sand are above 50%,the proportion of the clay content is less than 10%, soil type is sandy soil. With the extension of the period of returning farmland to forests, soil clay reduced, it increases, and the biggest change for 0 to10 cm of soil. With returning years of extending, the porosity of the soil is a smaller degree of change, but the difference is not obvious. Composed of farmland soil nutrient loss, after converting the mechanical roughening of Minqin area wind, accelerate soil wind erosion, exacerbate soil erosion.(3) The vegetation succession is closely related to the soil chemical properties and restricted each other. The study area soil nutrient content on the whole is not high, and uneven distribution, shown as "rich K, high N, low P", general characteristics, low organic matter content, significant differences in soil fertility, soil salinity is higher, generally, as the alkali saline harm is serious, soil texture is relatively heavy, porous permeability is poor. In secondary grassland soil system evolution, most of the nutrients in the soil(effective potassium, slowly available potassium and ammonium nitrogen, effective phosphorus, total nitrogen, available phosphorus and organic matter as the area decreases with returning years of extending of the secondary grassland vegetation restoration, and the salt and PH accompanied by increase evidently with returning years of extending of vegetation restoration,especially the soil salt content at the beginning of the farmland is appropriate, along with the increasing length of farmland, in the five years after returning farmland soil salt content will gradually rise, accumulation phenomenon is obvious. It shows that the salinization of soil is serious, and soil quality has a tendency to decline.(4) The three main types of soil microorganisms with the natural recovery of abandoned land are: bacterial > actinomycetes > fungi. The highest number of bacteria that is given toillustrate the advantage bacterium are bacteria. May be because the soil microbial quantity is concerned with nature of soil, such as the Minqin oasis is neutral alkaline soil, suitable for bacterial growth and reproduction, and inhibits fungal survival, therefore, bacteria number greater than the fungi. The same duration of abandoned land, the soil microbial biomass C,N and P along with the deepening of soil cohesion showed obvious table. Soil microorganism quantity and soil microbial biomass general trend and species richness and diversity index changed in law is consistent, with returning years of extending, soil microorganism quantity and soil microbial biomass decreased gradually; Soil microbial biomass, and vegetation restoration is an interactive process between them give back to the response.(5) With returning years of extending of returning abandoned land secondary natural recovery of grassland, soil peroxidase, sucrase, urease and phosphatase activities was showed decreased gradually with increasing of soil depth;0 ~ 10 cm of soil enzyme activity in 4 layers occupies a large proportion in the total enzyme activity. Research tested enzyme activity,although the change law is different, but the overall trend is along with the increasing length of returning abandoned land secondary grassland restoration, falling in a fluctuant way. Early farmland soil enzymatic activity is larger, as the growth of the abandoned land secondary returning years of the grassland vegetation restoration, soil enzyme activity was gradually reduced. Therefore, soil enzyme activity is closely related to the recovery of vegetation. This shows that in the process of returning abandoned land secondary grassland vegetation restoration and soil enzymatic activity with the increase of vegetation species richness and diversity index rises. This is due to the conversion of 1 a, soil moisture and nutrients conditions led to field weed grow, resulting in diversity, so as to create large accumulation of residual, rotting humus in the soil and increase organic matter content, after the microbe growth metabolism active, improve the capacity of producing enzyme and enhance the soil enzyme activity. Returning 8 to 15 a, due to lack of rain, the Minqin oasis annual rainfall less than evaporation, the sand is big, the special natural habitat only survived a few hardy species,returning 24 to 31 a, poor drought tolerance of plants gradually disappear, plant biomass decreased gradually, the residual branches leaves less, humus and root secretion reduced,which is formed by the organic matter content in soil is less, to reduce the number of microorganisms, soil enzyme activity also gradually decline. Combination of the above factors, made the soil enzyme activity with the extension of the natural recovery of thesecondary grassland vegetation waning. Soil enzyme activity and natural vegetation restoration are extremely complicated and slow process, not only its enzyme activity and natural succession of vegetation species richness, species diversity, and in the process of growing grain for the last year, also with the soil of organic fertilizer and other physical and chemical properties such as particle size, granular structure, temperature.(6) The evolution of the soil system is a complex process, in the different ecological system, soil physical characteristics, chemical characteristics, microbial characteristics and seasonal dynamic changes of soil enzyme activities have great differences. In Minqin oasis different duration of abandoned land in the process of secondary grassland succession,soil-related factor in the spring, summer, autumn and winter seasons replace in law, which can be divided into the following two forms: the first is higher in summer and autumn, winter and spring are lower. Such as: soil moisture content, PH value, conductivity, total nitrogen, total phosphorus, total potassium, available phosphorus, nitrate nitrogen, organic matter, Mg2+,Ca2+, microbial biomass C and microbial biomass N, microbial biomass P, catalase and phosphatase activity, etc. This is due to the rapid March of vegetation germinating quickly, it is necessary to absorb a lot of nutrient from the soil, soil dissolved organic carbon, nitrogen and phosphorus decreased sharply, to a certain extent, formed between the microorganisms and the nutritional needs of competition, the amount of microbial nutrition available shrink,constraints on microorganism growth and reproduction, at this point each of these factors is reduced. Entered 5 to 6 months, due to rising temperatures and a large number of plant root activity enhancement, the mineralization of soil organic matter increased, soil nutrients are enough, abundant roots secretion, the ground relative lush vegetation growth, promoted the underground soil respiration, combined with high surface temperature, soil microbial biomass and enzyme activity increased. In September, because the surface plant gradually into recession, a lot of litter input, root decay and carbohydrates from the surface to the deep constantly infiltration, surface vegetation wither and decay, which fade into the soil, soil root gradually decayed at the same time, the biological available in the soil organic matter increased, soil physical and chemical factors, microorganisms and soil enzyme activity, in turn, appear increase to a certain extent, soil above related factors will appear larger peak.Winter when soil temperature is low, inhibit the growth of the soil microbes and enzyme activity, make its metabolism reduced a lesser value. The second is the spring and winter inthe soil content is higher, low content of summer and autumn. Such as: soil available phosphorus, available potassium, available potassium and slowly available potassium, HCO3-,Cl- and SO42-, etc. The main reason may be the soil available phosphorus, available potassium,available potassium consumption is mainly in the middle of plant growth in 5-9 mid month,from spring plants lost to fall withered plant growth, absorb a lot the above elements from the soil; coupled with the region’s precipitation patterns can well explain the phenomenon, the area rainfall mainly concentrated in the summer and fall(from June to September), soil slowly available potassium is fixed by the layered clay minerals mainly potassium ion, and moisture can promote the slowly available potassium into rapidly-available potassium ion state, therefore, summer and fall are in high precipitation, soil slowly available K due to the dissolution of water content decreased, the lowest value in the maximum rainfall in September, the maximum value appeared in rainfall at least in winter and spring(December to March).So in late September soil available phosphorus, available potassium, available potassium and slowly available potassium is minimized, into the winter plants stop growing, the elements of abundant accumulation, reach maximum until next March.(7) Abandoned land in Minqin, soil physical, chemical and microbiological characteristics of the change are mainly caused by water in the soil and fertilizer factors, the lack of rain in the region, soil is barren, farmland in the early because before putting into the water and fertilizer in agricultural production has residual, therefore, abandoned lands soil properties is being to the development, but with returning years of extending, the remnants of water and fertilizer gradually depletion, cut down the content of water and fertilizer and soil microbial quantity is relatively small, slow decomposition of plant residues, soil properties began to show a negative development. Characterization of the main factors of the change of soil PH value, salt content, microorganism quantity, microbial biomass C, N, sucrase, catalase,urease.(8) Through the downstream of Shiyang river basin, Minqin returning abandoned land secondary grassland 1 ~ 31 years natural succession process of research, we believe that:after returning farmland, 5 years, 8 years, 15 years, species richness and diversity index decreased obviously, soil nutrient loss, high salinization degree and mechanical composition coarsening, microorganism quantity and microbial biomass, enzyme activity and theanti-interference ability weakened a lot of ecological problems such as soil environment,abandoned land using simple sealing education measures, to promote vegetation restoration,and improve the regional ecological environment there is still a big problem, in order to ensure the good ecological protection, need appropriate intervention and regulation.Combining with the characteristics of vegetation evolution, certain measures can be taken,artificial directly introduce late succession of plant varieties, quicken the process of abandoned land vegetation restoration. In improving the ecological environment at the same time can’t ignore the development of the economy, but if only consider the economic benefits,it is difficult to guarantee abandoned land secondary grassland natural succession this good land resources.Therefore,4 to 5 years after returning farmland is a critical period, adding appropriate economic measures, the study of late alternative species succession which has economic benefits and introduces, the win-win of ecological and economic type should be used artificial intervention, reasonable utilization and protection, prevent to develop in the direction of desertification and salinization, so that the soil secondary grassland soil test system to get better.