Migration And Hydrological Process Differentiation Of Carbon And Nitrogen In The Slope Of Karst Critical Zone

Karst critical zone is an innovative issue in the field of resource and environment in China.Karst landform is forced by surface and subsurface water erosion on carbonate rocks,and characterized by sinking streams,caves,enclosed depressions,fluted rock outcrops,and large springs.Such a fragile ecological zone suffers soil erosion,degradation of ecosystems,poor land resources,and frequent drought and flood disasters,which have seriously influence of human living environment.Water and nutrients circulation have a closed relationship with evolution of regional ecosystem.It is important to apply new methods and ideas to karst critical zone research,whose object is rock-soil-water-biology-atmosphere continuum.Karst soil and rock could redistribute water and nutrient during rainfall-runoff conversion process.Amount of carbon released by carbonate rock weathering gets into water and forms a CO₂-H₂O-CaCO₃ system.Dissolved inorganic carbon in runoff was the key factor of carbon sink formation in karst area.Karst area is approximately 344.7×10⁴ km² and bare carbonate distribution area is 90.7×10⁴ km²,which comprises more than one-third and one-seventh of territorial area in China,respectively.The southwestern karst zone of China is one of the most concentrated karst areas,where characterized as subtropical monsoon climate zone.Water and nutrient restriction during karst ecological restoration has drawn great attention of ecologists,and water and nutrient loss urgently needs a quantitative study.The structure of soil upper covered different types of carbonate rock is different,so the chemical composition of runoff on karst slopes is also various.How does carbon sink generate and what influence of such differential characteristics have on ecological balance and ecosystem services need to be futher studied.In situ simulated rainfall experiments were conducted at concentrated karst area in China,Puding Country of Guizhou province.We selected 3 sites?two with different carbonate matrixes and one without carbonate matrix?and built 3 duplicate runoff plots on each sites,measured surface and subsurface runoff discharge under 5 rainfall intensities,and analysed chemical composition of runoff samples.The major results show that:1.Subsurface runoff is the main form of runoff production and surface runoff yielding mechanism is dominated by saturation-excess runoff.The soil water conservation capacity of karst slope is weaker than that of non-karst area,because most of precipitation discharged by surface runoff and subsurface runoff at karst slope.Carbonate rocks have important influence on the redistribution process of precipitation infiltrated into soil,and different types of rocks have diverse effects on the underground runoff process.2.The first flush effect of runoff under simulated rainfall was obvious.Carbonate rock weathering in karst areas released a large amount of inorganic carbon into surface runoff and subsurface runoff,which resulted in different dynamics of DIC content and NO₃^-content in runoff.The DIC migration along with runoff may be the initial source of carbon sink formation in karst area,and the imbalanced chemistry composition of runoff may deteriorate the fragile karst ecosystem.3.The dynamic of C/N and the content of base ions in both surface and subsurface runoff demonstrated that carbonate rock weathering in karst area was the significant factor contributed to the chemical composition of runoff.The DIC content showed no decreased trend with the increase of rainfall intensity,which meant the DIC loss load increased with the increase of rainfall intensity.The poor soil water conservation ability in karst area leads to serious N loss,considering N restriction was one of the main reasons of sustainable ecosystem restoration in karst area.4.The solute load method widely used to evaluate the rock weathering carbon sinks in karst area is controversial at present,because of the influence of exogenous acid and other factors have great uncertainty.A correction factor??=0.65?was put forword by this study for better evaluation of the carbon sinks,and the result of the carbon sink in the karst area of China is 4.74 Tg C yr^-1.This value is about 53%of previous result and reduces the error range of carbon sink evaluated by different methods to only 7%.5.We obtained the runoff and N-loss parameters of both dolomite area and limestone area by the simulated rainfall experiments.Combining with daily precipitation data of three karst districts recorded in 1961-2014,the N loss load of 344.7×10⁴ km² karst region in China was estimated as 1.05-1.67 Tg yr^-1.The karst area of South China is the most serious district among three districts,and about 90%of N loss occurred during the rainy season.Understanding the processes and estimating N-loss precisely is highly valuable in determining long-term soil security and sustainability in karst regions.