The carbon isotope fractionation in the atmosphere–soil–spring system associated with CO2-fixation bacteria at Yaji karst experimental site in Guilin, SW China

Abstract

In the epikarst-diffusing infiltration layer, the water–carbonate rock–CO2 gas interaction takes place under the driving force of biotic/abiotic CO2. Therefore, it is necessary to study the changes of carbon isotopic composition relating to biotic/abiotic CO2 in the karst system. This paper presents some hydrogeochemical and biological characterization in the Yaji karst experimental site of SW China. Based on the hydrogeochemical studies, it was found that spring 31 (S31) mainly flowing out from doline 1, was primarily characterized as a HCO3 −–Ca type, with a mean magnesium/calcium ratio of only 0.87 %, and a positive relationship (R 2 = 0.9591) existed between calculated PCO2 and SI C in S31 which generally originates from the complex water–carbonate rock–CO2 gas interactions. The difference of soil CO2–δ13C in two soil layers (0 to −10 and −20 to −30 cm) from doline 1 is 6.6 ‰ and the CO2-fixation bacteria has the significant positive correlation with soil organic carbon (SOC) in doline 1(P < 0.0003). Moreover, the difference between soil CO2–δ13C in doline 1 and δ13CDIC in S31 is 10.93 and 76 % CO2 in S31 comes from soil biotic CO2 based on O’Neil equation. The above results indicate that when atmospheric CO2 transfers through the epikarst-diffusing infiltration layer, 12CO2 will be requested by the CO2-fixation bacteria with high priority, and soil CO2 probably originates from heterotrophic microbial respiration in the bulk soil. Then, under the intervention of CO2-fixation bacteria, the epikarst-diffusing infiltration layer will produce the “placenta” function to spur the stable isotopic equilibrium fractionation between soil CO2 and spring DIC, which could be used to predict the value of soil CO2–δ13C or δ13CDIC when we only have one aspect result of the samples, especially for monitoring drip water in the cave. Moreover, the future research on the isotopic fractionation of CO2-fixation bacteria could provide more surprising information to explain the function of epikarst-diffusing infiltration layer.