As global warming becomes more and more serious due to the increase of greenhouse gases, especially CO2, the global carbon cycle research has attracted increasing attention. Karst processes-related carbon cycle, as a key part of the global carbon cycle and a result of the water-carbonate rock-CO2 gas-aquatic organism interaction, significantly affects the global carbon budget. Analyzing the relevant karst hydrochemical changes could not only enhance the understanding of the karst processes but also reveal the driving forces and intensity of the karst processes-related carbon cycle.

Corrosive CO2 (CO2 aqueous solution), as one of the essential conditions of karst processes, determines the hydrochemical characteristics of karst groundwater due to the dissolution of carbonate rock. Currently, there have been many studies focusing on the hydrochemical changes and its driving forces. However, most of them mainly based on theoretical inferences, lacking of high-resolution soil CO2 data support. Because of the complexity of epikarst system, reliability of the theoretical inferences is greatly limited, implying that real-time high-resolution monitoring of both hydrochemistry and soil CO2 is of significance in karst process research.

The Research Group on Karst Processes-related Carbon Cycle led by Professor LIU Zaihua in the State Key Laboratory of Environmental Geochemistry (SKLEG), Institute of Geochemistry, Chinese Academy of Sciences (IGCAS), recently published their study results on the basis of high-resolution field monitoring of both karst hydrochemistry and soil CO2.

During July 2010 – December 2011 covering a complete hydrologic year, the researchers monitored both the concentration of soil CO2 and hydrochemical parameters at high-resolution (every 15 min) in an epikarst spring system at Chenqi, Puding, SW China. They also investigated the response of hydrochemical changes to soil CO2 and weather conditions. They found that both soil CO2 and rainfall are the major driving forces for the epikarst hydrochemical variations.

Their study showed that the soil CO2 effect on hydrochemical variations was reflected in all seasonal, diurnal and storm-scales. For example，in spring-summer growing season, there was an increase in pCO2 and electrical conductivity (EC) but a decrease in pH caused by the increase in soil CO2; In autumn-winter dormant season, on the contrary, a decrease in pCO2 and EC but an increase in pH caused by the decrease in soil CO2 happened.

In addition, during rainy seasons, hydrochemical changes in epikarst groundwater were regulated by both dilution and soil CO2 effects. Under high-intensity rainfall, the dilution effect was dominant, indicated by a quick decrease in EC, pH and calcite saturation (SIc) but a quick increase in pCO2. In contrast, under low-intensity rainfall, the soil CO2 effect was dominant, indicated by an increase in EC and pCO2 but a decrease in pH and SIc.

This study has shown the high sensitivity and variability of epikarst processes to the environmental change, suggesting that the role of karst processes in the global carbon cycle needs to be reappraised based on high-resolution monitoring strategy.

The paper was published in Journal of Hydrology, please visit the link below for more information: http://www.sciencedirect.com/science/article/pii/S0022169412007081

 (By Professor LIU Zaihua’s group)