Variations of soil CO2 concentration and pCO2 in a cave stream on different time scales in subtropical climatic regime

Abstract

In karst regions, soil CO2 is a major chemical driving force for the karst processes and finally has a significant impact on the hydrochemical processes of karst underground river. Hydrochemical features, soil and climatic parameters with a high-temporal resolution have been monitored on different scales (daily scale, seasonal scale and interannual scale, storm scale) in the Xueyu Cave watershed from 2009 to 2015. The aim of this study is to understand how cave stream pCO2 and hydrochemistry respond to overlying soil carbon recharge on different time scales. The results show that the variational amplitudes of the hydrochemistry in Xueyu Cave underground river (XUR) tend to be in the order: storm-scale > seasonal > interannual > daily scale. Soil CO2, pCO2 (CO2 partial pressure in the XUR) and Spc (specific conductivity) were higher in summer and autumn than those in winter and spring. The synchronous variations of XUR pCO2 with soil CO2 concentrations in the same order of magnitude confirm the “soil CO2 effects” on the formation of XUR hydrochemical features. The storm-scale fluctuations of hydrochemistry in XUR water are especially depending on the intensities of rainfalls that determine whether the “dilution effects” or the “CO2 effects” are predominant in the stream during rainfall events. At the same time, soil moisture and soil CO2 work as important factors for controlling pCO2 variations in the XUR. The identified relationship of soil-XUR pCO2 suggests a temperature control on carbonate weathering on daily and seasonal scale but a rainfall/soil moisture control on storm and annual scale. The combined effect of point and diffused infiltration that delays the arrival of storm flows determines the fluctuations of the discharge and pCO2 variation. Defining relationships between CO2 from overlying soils and groundwater offers the chance to explore the processes at different time scales, potentially increasing our ability to understanding the carbon dynamics in karst systems.