Temporal And Spatial Variations Of Carbon And Nitrogen Output In The Karst Critical Zone And Its Environmental Significance

The critical zone of the earth has received extensive attention from multidisciplinary fields in the past decade.It refers to the heterogeneous near-surface environment,which vertically covers various circles including plant canopy,soil layer,vadose zone and aquifer.Karst springs are a key area to maintain human survival and the function of the earth’s ecosystem,and karst springs are a part of the karst key zone that cannot be ignored.The karst area in southern China with the Guizhou Plateau as the core has more than 600,000 km~2 of carbonate rocks.The unique geological structure and complex underground structures make groundwater reserves abundant.Karst spring water is the direct manifestation of karst groundwater,and it is also an important output end element of the surface-subsurface material cycle.Therefore,an in-depth understanding of the nutrient migration characteristics in karst springs will help to clarify the nutrient output laws and mechanisms of karst key zones.Strong karstification and the destruction of surface vegetation have led to aggravation of soil and water leakage in karst areas.Carbon and nitrogen elements enter groundwater with the process of soil and water leakage under the erosion and leaching of rainfall,resulting in the leakage（loss）of surface nutrients.Aiming at the problem of nutrient loss in karst areas,this paper,based on the spatial distribution of karst rocky desertification,selects rocky desertification degrees in Guizhou,the core area of karst in southern China,as no-potential Shibing,potential-slight Bijie,and moderate-The Guanling-Zhenfeng Huajiang of the intensity is the study area.Heichong Spring and Gushu Spring in Shibing Heichong Research Area,Xujia Spring and Zhulin Spring in Bijie Salaxi Research Area,and Yanshang Spring and Yongfang Spring in Guanling-Zhenfeng Huajiang Research Area were selected as research objects.From July 2020 to June2021,6 karst springs will be monitored for one hydrological year.Through the methods of hydrochemistry and Pearson correlation analysis,the variation characteristics of carbon and nitrogen output from karst springs and their ecological environment significance were comprehensively discussed,so as to provide a scientific reference for the restoration of surface rocky desertification ecosystems.The specific conclusions are as follows:（1）The p H,EC,and anion and cation contents of karst spring water in different study areas are different,and the water chemistry types are also different,mainly because the water chemistry is affected by the weathering of different carbonate rock types.The lithology in Shibing Heichong study area is dominated by dolomite,HCO₃^-,Ca²⁺and Mg²⁺ions are dominant ions,and the water chemical type is HCO₃-Ca-Mg;Limestone is the main lithology in the Salaxi study area of Bijie,with high content of HCO₃^-and Ca²⁺ions,and the hydrochemical type is HCO₃-Ca;In the Guanling-Zhenfeng Huajiang study area,the lithology is dominated by dolomitic limestone,with high content of HCO₃^-and Ca²⁺ions,and the hydrochemical type is HCO₃-Ca.The p H and EC of the karst spring water showed a variation pattern of"Shibing Heichong study area>Guanling-Zhenfeng Huajiang study area>Bijie Salaxi study area".The chemical weathering of carbonate rocks is the main factor affecting the chemical characteristics of water.（2）In the three study areas,the variation characteristics and overall variation trends of different forms of carbon and nitrogen content in karst springs are different,mainly due to the influence of rainfall and flow,but also due to human influences such as phytoplankton and agriculture.The carbon and nitrogen output showed clear spatial differences.The carbon and nitrogen output of karst springs is mainly affected by rainfall.The output forms of carbon and nitrogen in karst springs are mainly in the form of DIC and NO₃-N.After a hydrological year of monitoring,it was found that the content of carbon and nitrogen in karst springs was higher in the dry season,and lower in the rainy season.The content of carbon and nitrogen in karst springs was greatly diluted by rainfall,showing a negative correlation with it.In the Shibing Heicong research area and the Guanling-Zhenfeng Huajiang research area,the carbon and nitrogen in the karst springs increased abnormally,which was due to the influence of local agricultural fertilization,phytoplankton and microorganisms.In the three study areas,the rainfall showed the variation characteristics of"Shibing Heichong study area>Bijie Salaxi study area>Guanling-Zhenfeng Huajiang study area".The carbon and nitrogen content of karst spring water showed the difference between"Shibing Heichong study area>Bijie Salaxi study area>Guanling-Zhenfeng Huajiang study area".The relationship between the annual output of carbon and nitrogen in karst springs is reflected as"Shibing Heichong study area>Bijie Salaxi study area>Guanling-Zhenfeng Huajiang study area";The carbon and nitrogen output of karst spring water has a very significant positive correlation with flow,and also has a significant positive correlation with carbon and nitrogen content,and rainfall has a significant impact on the carbon and nitrogen output of karst spring water.（3）The carbon and nitrogen content and output in karst springs are the highest under the conditions of mountain and canyon without potential rocky desertification level;Under the potential-mild rocky desert level plateau mountain conditions,the carbon and nitrogen content and output in the karst spring water were relatively high;under the medium-strength rocky desert level,the carbon and nitrogen content and output in the karst spring water were the lowest.No-potential rocky desertification mountain canyon Shibing Heichong research area,with high vegetation coverage,can maintain soil nutrient content,litter can maintain soil water holding capacity,well developed underground,excreted in the form of groundwater,Shibing Heichong.The spring water in the study area has higher carbon and nitrogen output.Potential-Mild rocky desertification plateau mountain Bijie Sala Creek study area,the vegetation coverage is relatively high,which ensures the input of nutrients and the water holding capacity of the soil,can effectively reduce or avoid the formation of surface runoff,the groundwater level is shallow and stable,Groundwater is mainly excreted by spring water,and the carbon and nitrogen output of spring water is relatively high.In the Guanling-Zhenfeng study area,the plateau canyon of medium-strength rocky desertification,the vegetation is mainly economic crops,the rock exposure rate is high,and surface runoff is easy to form.This resulted in the lowest carbon and nitrogen output of spring water in the Guanling-Zhenfeng Huajiang study area.（4）Through the analysis of the annual output of carbon and nitrogen in karst spring water and the carbon and nitrogen storage of the overlying soil,it is found that the higher the carbon and nitrogen storage in the overlying soil environment of the spring water,the greater the carbon and nitrogen output of the spring water,the soil nutrient loss is also relatively high.On the basis of estimating the carbon and nitrogen storage in the rainy and dry seasons of each spring,comparative analysis of the annual output of carbon and nitrogen in spring water and the carbon and nitrogen content of the soil overlying the spring water,it was found that the overlying carbon and nitrogen storage of the karst springs in the Shibing Heichong study area was relatively high,the annual carbon and nitrogen output of the spring water was the highest,and the soil nutrient loss was also the highest;The overlying carbon and nitrogen storage of the karst spring in the Salaxi study area of Bijie is relatively high,the annual carbon and nitrogen output of the spring is relatively high,and the soil nutrient loss is also relatively high;In the Guanling-Zhenfeng Huajiang study area,the overlying carbon and nitrogen storage of spring water is low,the annual carbon and nitrogen output of spring water is the lowest,and the soil nutrient loss is also the lowest.In conclusion,in the three study areas,under the condition of low degree of rocky desertification,although the overlying soil environment of karst springs has high carbon and nitrogen storage,affected by the continuous supply of soil with high carbon and nitrogen content,the karst spring water shows a relatively high annual output of carbon and nitrogen.The resulting soil nutrient loss is also relatively high,and the potential-mild rocky desertification area faces a greater risk of soil nutrient loss than other rocky desertification levels.