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Effects Of Biological Nitrogen Fixation On Soil Environment And Vegetation Development In Bare Land Of Glacier Retreat

Posted on:2021-05-23Degree:MasterType:Thesis
Country:ChinaCandidate:X F WangFull Text:PDF
GTID:2370330605965069Subject:Physical geography
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Biological nitrogen fixation(BNF)is a biochemical process in which organisms transform molecular nitrogen(N)into ammonium N in the atmosphere under the effects of nitrogenase.At the early stage of primary succession where N is commonly as a key limiting nutrient,BNF becomes the main source of N in soils.The high-altitude region in the eastern Tibetan Plateau features the unique climate,geological and ecological environment.Under such conditions,engineering activities,geological disasters and glacier retreat have caused a large number of bare land exposed to air.How to manage these degraded ecosystems scientifically and realize the rapid and stable recovery of the bare land has become an important issue at present.Understanding the ecological and environmental effects of BNF on the processes of bare land restoration will provide a guidance for the ecological restoration of bare land with the perspectives of habitat improvement,nutrient turnover and community succession trajectory.This can not only quickly establish a stable community based on the law of primary succession,but also provide a theoretical support for the rational ecological restoration of bare land.In this study,the retreated area of Hailuogou Glacier on the Gongga Mountain was selected to collect the soil,biological soil crust,litter and plant samples according to the soil age(0,9 and 39 years).The spatial and temporal distribution characteristics of BNF and its influencing factors at the early stage of primary succession in the glacier retreat region were delineated according to in situ measurement of BNF activities,laboratory physical and chemical property analysis,laboratory control experiment and statistical analysis,etc.Meanwhile,the effects of BNF on the soil properties and other non-N fixing plants at the early stages of primary succession were explored.The main results and conclusions are as follows:1.The seasonal variation in the nitrogenase activities in the soils,biological soil crust,and litter in the retreated area showed an order of May > November > July.Theactivities of nitrogenases in various substrates decreased in the order of Astragalus mongholicus rhizosphere/ non-rhizosphere > Hippophae rhamnoides litter >biological soil crust > other soils In May,Astragalus mongholicus rhizosphere/non-rhizosphere > Hippophae rhamnoides litter > biological soil crust> other soils in July,and biological soil crusts > Astragalus mongholicus rhizosphere >Hippophae rhamnoides litter > other soils in November.With the primary succession progressed during the 39 years,the BNF showed an increase at the earliest stage and then decreased at the relatively later one.The nitrogenase activity was highest at the9-year site(1.04 ± 0.09 nmol ? g-1 ? h-1)and smallest at the new exposure site,likely because of the only existence of free-living N-fixing microorganisms at the 0-year site with a small amount of fixed N and the low N-fixing efficiency.The N-fixing plants and microorganisms dominated at the 9-year site,which contributed to the high nitrogenase activity.Soil organic matter and soil N accumulation increased markedly at the 39-year site,which resulted in the BNF without competitive advantage and subsequently the nitrogenase activity decreased significantly.There was a significant positive correlation between nitrogenase activity and water content,DOC,DON,MBC,MBN,NH4+-N,P and Bio-P(P < 0.05).2.The results of indoor experiment showed that the nitrogenase activity was 5?< 15 < 25? under the temperature controlled condition,and the nitrogenase activity of each substrate increased significantly with the temperature(p < 0.01).Under the light controlled condition,except for the soil in A and C horizon at the 39-year point,the nitrogenase activity of all the other samples showed A trend of increasing first and then decreasing with the increase of light intensity.The reason for the low nitrogenase activity under dark conditions is that organisms cannot synthesize carbohydrates,and the energy required for BNF is suppressed.However,under strong light conditions,photosynthesis is extremely strong,which consumes a large amount of ATP and reducing substances and releases a large amount of oxygen,so that a large amount of energy is consumed in the organism and the nitrogenase is irreversibly inactivated when it encounters oxygen.The addition experimental results of N,phosphorus(P),molybdenum(Mo)nutrient and their combination showed that the activity ofnitrogenase in 0-year point did not reach the detection limit of the instrument after nutrient addition.After the addition of P and P + Mo,the nitrogenase activity of most of the samples at the 9-year site increased significantly,while N addition and its combination had no effect or inhibition.This showed that there was no N limit,but there were P and Mo + P limits in 9-year site.For the 39-year site,the addition of N,P,Mo and their combined additions had no effect or inhibitory effect on nitrogenase activity.One of the reasons may be that the nutrient elements in the 39-year soil have met the needs of biological growth and development.Organisms do not need to obtain N through a high cost BNF process,but directly choose to obtain N from soil(low cost).3.BNF significantly improves soils structure and nutrition,including soil water content,grain,soil p H,and C,N,and P nutrients.Changes in soil structure and increased nutrient elements increase the soil microbial enzyme activity,which in turn promotes increased soil nutrients.N-fixing plant Astragalus mongholicus had a greater effect on the surface and rhizosphere soil properties and nutrient availability,while another N-fixing plant,Hippophae rhamnoides,had a smaller effect and had no significant difference from non-N-fixing plants.Astragalus mongholicus significantly improved the N nutrient status of the non-N-fixing plants associated with it,and played a significant role in promoting the development of non-N-fixing plants.Hippophae rhamnoides had no significant effect on the associated non-N-fixing plants.Astragalus mongholicus has high application potential in the ecological restoration of bare land in sub-high mountainous areas.
Keywords/Search Tags:Biological nitrogen fixation, Glacial retreat area, Nitrogen fixing plants, Soil improvement, Vegetation development
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