Research On Characteristics And Controlling Factors Of Chemical Weathering And Element Migration With The Fractionation Of Altitude In Mount Huangshan Granite Critical Zone

Silicate chemical weathering is an important carbon sink process of global atmospheric CO₂.It is also one of the main sources of mineral nutrients in the forest ecosystem and an important link in the geochemical cycling of elements in earth critical zone.The fractionation,enrichment and migration of elements in soil profile of silicate critical zone during weathering and pedogenesis have always been concerned.A thorough understanding of the controlling factors of chemical weathering and element migration in silicate critical zones is a significant prerequisite to fully reveal its impact on global climate change.Mount Huangshan is a world geopark which consisted of Mesozoic granite.Due to the tectonic movement,Mount Huangshan has formed unique topographic and geomorphic features and local mountain microclimate,making it an ideal place to study the characteristics of chemical weathering and element migration in silicate critical zone.In this study,Mount Huangshan is selected as typical area for study,and the soil samples of representative weathering profiles from the top to the bottom was collected.Meanwhile,the topography,climate,hydrology,and vegetation distribution information were obtained.The major elements,trace elements and rare earth elements geochemistry characteristics in soil samples were analyzed to reveal the chemical weathering processes and element migration of the Mount Huangshan granite critical zone,and discussed the main control factors combined with molecular biology methods.It would help to understand the sources and cycle of nutrient under the extreme environmental condition of Mount Huangshan and reveal the income and expenditure of mineral nutrient to provide supporting scientific data for environmental protection of Mount Huangshan geological ecosystem.Meanwhile,it is helpful to thorough understand the chemical weathering process of silicate under the background of multiple influences of tectonic movement and climate.The main conclusions are as follows:(1)Based on the analysis of chemical weathering intensity,it is concluded that there are different weathering characteristics in different soil layers of five weathering profiles which at different altitudes in Mount Huangshan granite critical zone.The result of model calculation indicated that the order of chemical weathering rate of the profiles is:high altitude>medium altitude>low altitude.Therefore,it is more difficult to providing mineral nutrients for forest ecosystem through soil chemical weathering in the process of elevation of Mount Huangshan.The fractionation of soil chemical weathering in the Mount Huangshan granite critical zone reflects the regulating effect which brought by the slight change of parent material composition structure of the soil.Meanwhile,the difference of structure,climate,biology and other factors caused by the uplift of elevation also has a certain control effect on the chemical weathering.(2)The standardized analysis of elements shows that Mg O,Na₂O,Ca O,Cu,Sr and Ba are in deficit.Fe₂O₃is enriched in low altitude area and deficit in middle and high altitude area,but K₂O is opposite.Co,Rb,U and Th are enriched,and the high field-strength elements are stable.The LREE shows enrichment of HREE.The negative anomaly of Eu at low altitude is weaker than that at medium and high altitude.According to the results of migration rate calculation,the migration characteristics of soil elements in Mount Huangshan are as follows:Si O₂,K₂O,Na₂O,Ca O,Rb,the high-field strength elements and rare earth elements are deficient;Al₂O₃,Fe₂O₃,Mg O,Co,Cr and other elements are enriched.Chemical weathering and parent material play an significant role in Mount Huangshan granite critical zone.(3)According to analysis of the controlling factors of chemical weathering and element migration in Mount Huangshan granite critical zone,it can be safely concluded that soil chemical weathering is significantly controlled by climate.Temperature plays a positive role in chemical weathering by regulating soil microbial respiration and accelerating mineral dissolution.But rainfall is negatively regulated because of short residence time and intense physical erosion due to the steep geomorphology of the Mount Huangshan.Soil microbial community structure and chemical weathering show a mutual regulation mechanism.Microorganisms can effectively decompose organic matter and enzymes to promote chemical weathering,whereas strong weathering provides a more advantageous environment to promote microbial growth and metabolism.Meanwhile,mineral composition and its adsorption effect combine p H and organic matter to affect chemical weathering process.Under the background of multiple influences,the characteristics of chemical weathering and element migration with the fractionation of altitude are significant,and the carbon sink capacity of granite chemical weathering decreases with the increment of altitude.