Effects Of Land Use Change On Carbon,Nitrogen And Phosphorus Export To Karst Groundwater

Carbon cycle has always been the focus topic of global change.Global imbalance of atmospheric carbon dioxide budget showed the presence of the“missing carbon sink”.Previous studies suggest that carbonate weathering can not form long-term carbon sink,which leads to the neglect of its importance in regulating climate.However,the latest progress in the study of rock weathering carbon sink shows that not only the weathering rate of carbonate rocks is fast,but also the dissolved inorganic carbon（DIC）generated by the weathering process of carbonate rocks can be transformed into autochthonous organic carbon（AOC）by the biological carbon pumping（BCP）effect,forming a stable carbon sink.Excessive import of nitrogen and phosphorus will lead to outbreak of phytoplankton such as cyanophyta and chlorophyta in the water body,and the water quality will become worse.The effect of biological carbon pump can improve the water quality.Land use change is the most direct way for human activities to influence the carbon cycle of karst process,and it can be regulated by human beings.Therefore,it is of great significance to study the export of carbon,nitrogen and phosphorus from land use change to karst water for studying the importance of karst carbon cycle in the global carbon cycle and improving the water environment.In this study,the temporal and spatial variations of hydrology,hydrochemistry,carbon sink fluxes,total nitrogen flux,total phosphorus flux in Shawam Simulation Test Site are investigated.The results show that:（1）The discharge（Q）,runoff deep（RD）,water temperature（T）,soil CO₂ concentration,p CO₂,HCO₃^-concentration and electrical conductivity（EC）of karst springs exhibited temporal variations that are higher in summer and autumn,and lower in winter and spring,contrary to p H change.Changes in soil CO₂ concentration,p CO₂,HCO₃^-concentration and EC under different land uses,from large to small,are grassland,shrub land,agricultural land,bare soil land and bare rock land,which are contrary to the changes of Q,RD and p H.The temporal variation of total nitrogen（TN）concentration is not obvious,and total phosphorus（TP）concentration is the highest in autumn and the lowest in summer.The concentrations of TN and TP in the land use with vegetation growth are significantly lower than those in the land use without vegetation growth.（2）The rainfall is the limit of carbonate weathering-related carbon sink.RD of karst systems under lands covered by vegetations decreased sharply when the rainfall is inadequate,and thus the carbon sink flux.While the rainfall is plentiful and vegetations grow well,more soil CO₂produced,and then there is higher HCO₃^-concentration and carbon sink flux.In 2016-2017 hydrologic year,the carbon sink flux is mainly controlled by RD,with the largest in bare rock land and the smallest in grassland.In 2015-2016 and 2017-2020 hydrologic years,the carbon sink fluxes are mainly controlled by HCO₃^-concentration,with the largest in grassland.In 2019-2020 hydrologic year,the carbon sink flux exhibited temporal variations that are higher in summer and autumn,and lower in winter and spring.（3）The TN flux exhibited temporal variations that are higher in summer and autumn,and lower in winter and spring,the TP flux exhibited temporal variations that are the highest in autumn and the lowest in spring.The flux of TN and TP in the land use with vegetation growth are significantly lower than those in the land use without vegetation growth,the TN flux is mainly controlled by the TN concentration,with the shrub land being the smallest;the TP flux is mainly controlled by Q,with the grassland being the smallest.（4）When the land use without vegetation growth is converted into the land use with vegetation growth,carbon sink flux increased,TN and TP flux decreased,with the grassland change being the most significant.Therefore,we can adjust the land use to grassland to increase the carbon sink flux and reduce the nitrogen and phosphorus flux to a greater extent,which can not only increase the carbonate weathering-related carbon sink,but also improve the water environment,so as to achieve a win-win situation.