| Earth's Critical Zone is the key zone to maintain ecosystem function of earth and human survival.Karst Critical Zone is one type of Earth's Critical Zone.Karst area is dominated about 540×10 km2 in Southwest China,where is the largest area in the three concentrated distribution karst areas in the world.Due to the special physicochemical properties and geomorphic characteristics of carbonate rocks,thin and discontinuous soil,mismatch of soil and water process?soil is upstairs,water is downstairs?are the two main characteristics in vertical interface in Karst Critical Zone.The atmosphere,plant,soil/rock,groundwater continuum layer in Karst Critical Zone has more complex interaction relationship because of the weakened or missing of soil,which cause more acute environmental problems,such as soil erosion,rocky desertification,low biodiversity,low production and environmental capacity and difficulty in ecological rehabilitation.Vegetation and underground water interface importantly affect other various processes in Karst Critical Zone.Vegetation and underground water interface provides an important breakthrough point for people to control Karst Critical Zone.However,canopy interception process in Karst Critical Zone is still unclear.There are still many uncertainties in interpretation of plant water sources and its control factors in bare karst region,and how plant water source and plant sap flow respond to groundwater depth are not reported in bare karst region.Therefore,water and hydrochemical characteristics of precipitation,throughfall,stemflow,borehole water and epikarst spring water under forest and shrub were observed from field observation station of Ministry of land and resources,Guilin,Guangxi.Hydrogen and oxygen stable isotopes are used to analyze its response to precipitation.Using hydrogen and oxygen stable isotope technology and heat ratio method,the spatial and temporal characteristics of water absorption and transpiration under four different groundwater depths and response to groundwater level are studied.We got some new knowledge as the following.1.Influence of canopy interception on epikarst water and chemistry and its response to precipitationPrecipitation,throughfall,stemflow,borehole and epikarst spring water?PTSBE profile?from two different vegetation canopies,forest and shrub,were investigated from July to Octorber 2015 in Southwest China.Canopy can change precipitation recharge pattern and quantity to epikarst zone.Precipitation is partitioned into throughfall and stemflow because of canopy interception.Vegetation interception can reduce total supply of precipitation to epikarst zone,but increase the effective supply.Compared with Caesalpinia decapetala,Toona sinensis vegetation would increase the effective recharge to epikarst zone.Significant difference was found for pH,EC and DO concentrations among precipitation,throughfall,stemflow and epikarst water.Stemflow was an acidic input and throughfall was a less acidic input to epikarst zone.Results indicated that precipitation become acid and nutrient enrichment after canopy interception.Canopy also changes hydrologic cycle processes and increases precipitation solute input to epikarst zone.Our studies suggested that precise interpretation of mechanisms of carbonate rock dissolution and deposition should use throughfall and stemflow.2.Variation of total organic carbon flux in forest hydrological processesTotal organic carbon concentration?TOC?and its flux in precipitation,throughfall,stemflow,borehole water and epikarst spring water were monitored in two typical vegetation?Toona sinensis and Caesalpinia decapetala?in karst rocky mountain area,Guilin Guangxi.The results indicated that in the process of precipitation through the atmosphere-vegetation-soil/rock profile,variation trend of TOC concentration is,stemflow>throughfall>borehole water>epikarst spring water>rainfall.The TOC flux under shrub rain(204.86 kg·hm-2)was 1.3 times higher than that in Toona sinensis(153.48 kg·hm-2).The TOC flux from precipitation input is63.06 kg·hm-2,with 48.29 kg·hm-2 TOC flux from epikarst spring output.The TOC difference between input and output is 14.77 kg·hm-2,which indicate that TOC in system is positive balance.When precipitation enters vegetation,vegetation canopy,as"TOC live reservoir",increase TOC flux and was a carbon“source”.However,soil-rock system in epikarst zone,as"TOC dead reservoir",was carbon“sink”with absorption,filtration and fixing of TOC.3.Response of plant water sources to groundwater depth in bare karst region,Southwest ChinaThis study attempted to quantify plant water uptake pattern responding to grounwater depth based on stable hydrogen and oxygen isotopic methods.There was a significant negative correlation between total precipitation in first ten days and groundwater depth.Average oxygen isotopic values indicate that soil water recharge spring water,and then soil water and spring water recharge rock water together in China bare karst region.Rock water is the main water absorption source of plants in bare karst area of Guangxi.Total precipitation in first two days,total precipitation in first fifteen days and underground water depth are the main controlling factors for plant to absorb rock moisture.Karst groundwater storage space in Southwest China,especially epikarst zone,supply abundant water for plant growing in rainy season and maintain plant basic transpiration during dry season.Composition of karst water bearing medium controls the proportion of vegetation absorption to rock water,with the shallower karst develops,the more benefical for vegetation to obtain rock water.4.The spatio-temporal characteristics of sap flow and its respond to groundwater depth in bare carbonate region.Sap flow of Toona sinensis growing in carbonate rocky outcrop in Karst region of southwest China were measured using heat ratio method techniques.Meteorological factors and groundwater depth in four boreholes were also monitored.The results showed that:1)the average daily sap flow of Toona sinensis was low in bare karst area.Average amount of sap flow in four samples showed significant difference in different karst aquifer medium?P<0.05?;RDA analysis and correlation analysis between sap flow and meteorological factors showed that the main meteorological driving sap flow of Toona sinensis were photosynthetic effective radiation,water vapor pressure deficit,gust velocity and wind direction.Plant sap flow rate in carbonate rocky outcrop did not decrease with groundwater depth,but it related with karst aquifer medium,shallow karst development.The more development of shallow karst area,the more convenient for plant to absorb water form rock.The study coincides with the results of plant water sources.In addition,vegetation underground water consumption is calculated using sap flow method and isotope mixing ratio method.Total transpiration of Toona sinensis is lagging behind the maximum precipitation and the highest monthly underground water level,which the maximum precipitation and the highest underground water level are between April to June,while the highest plant monthly total transpiration is mainly in June to Aguest.In Southwest China's bare karst region with abundant groundwater resources,shortage of surface water,shallow groundwater depth,hard and pure dense carbonate rock et al,three vegetation water use model and its environmental response strategies are summarized from difference of carbonate water bearing medium:?1?Under the habitats of dissolution pore and dissolution seam,plants mainly absorb soil moisture for transpiration,which plant utilization ratio of rock water decreases with the increase of grounwater depth;?2?Under the habitats of dissolution pore and fissure,plants use soil moisture and rock water for transpiration,which plant utilization ratio of rock water raised with the increase of groundwater depth;?3?under the carbonate fissure and conduit habitat,rock water was the dominant water source for plant and plants utilization ratio of rock water decreases with the increasing of groundwater depth.Our results will help technological progress of vegetation restoration in rocky desertification fragile ecosystem,and also to evaluate the ecological and hydrological functions of vegetation under climate change. |
PDF Full Text Request