Effects of rainfall on the karst-related carbon cycle due to carbonate rock weathering induced by H2SO4 and/or HNO3

Abstract

To determine the mechanism underlying the effect of rainfall on the karst-related carbon cycle while controlled by that of H2SO4 and/or HNO3, high-resolution monitoring was conducted in Lanhuagou Spring, which is a typical epikarst spring in southwest China. During the monitoring period, the signals from major elements and stable isotopes showed that the CO2 generated by H2SO4 and/or HNO3 entirely escaped from the water. Therefore, the CO2 consumption induced by H2CO3 and CO2 emission induced by H2SO4 and/or HNO3 jointly affected the karst-related carbon cycle. The CO2 consumption induced by H2CO3 dropped following rainfall events and recovered gradually thereafter, which was only affected by the retention time of infiltrating water (dilution effect of rainfall). However, the CO2 emission induced by H2SO4 and/or HNO3 had showed a downward trend from the first rainfall event (R1) to the end of monitoring. The effects of rainfall on CO2 emission induced by H2SO4 and/or HNO3 were related to both the dilution effect and the removal of H2SO4 and/or HNO3. Although the average CO2 consumption induced by H2CO3 dropped from 2.8 mmol/L before rainfall to 2.21 mmol/L thereafter (decreased by 21.07 %), the average CO2 emission induced by H2SO4 and/or HNO3 dropped from 1.29 to 0.49 mmol/L (decreased by 62.02 %); the range of CO2 emission induced by H2SO4 and/or HNO3 decline (0.8 mmol/L) was larger than that of CO2 consumption induced by H2CO3 (0.59 mmol/L), resulting in higher net CO2 consumption (CO2 consumption induced by H2CO3 minus CO2 emission induced by H2SO4 and/or HNO3) after rainfall than before rainfall in the monitoring period. Therefore, in the model used to predict karst-related carbon flux caused by global climate change, it is necessary to consider the impact of rainfall on CO2 emission induced by H2SO4 and/or HNO3. Importantly, the net CO2 consumption rather than CO2 consumption induced by H2CO3, as well as runoff should be considered as two multipliers when carbonate weathering is interfered by H2SO4 and/or HNO3.