Difference in the relationship between soil CO2 concentration and the karst-related carbon cycle under different land use types in southwest China

As an important driving force for karstification, soil CO2 has a close relation with karst-related carbon cycle. However, their relationship may be disturbed when H2SO4 and HNO3 participate in karstification. Here, soil CO2 and spring water samples were collected from two catchments to examine the dynamics of carbon under different land use types. The net CO2 consumption at the Baishuwan spring catchment (BSW; range 1.7-2.69 mmol/L, average of 2.21 mmol/L) was higher than it was at the Hougou spring catchment (HG; range -0.63 to 0.02 mmol/L, average of -0.24 mmol/L). Due to the participation of H2SO4 and HNO3, CO2 was released from bedrock and reduced net CO2 consumption when this portion of CO2 escaped from the water. With regard to soil CO2 concentration, a bidirectional gradient of CO2 concentration occurred at BSW, while alternating bidirectional and unidirectional gradients occurred at HG. However, the delta C-13 of soil CO2 could not confirm whether the vertical changes in soil CO2 concentration as well as net CO2 consumption were related to the CO2 released from bedrock to soil. With regard to seasonal changes, net CO2 consumption was consistent with soil CO2 concentration at BSW, while the reverse relationship was found at HG. These observations indicated that soil CO2 concentration was not the dominant factor controlling net CO2 consumption at HG, which was affected by H2SO4 and HNO3, and more CO2 escaped from the water due to the reduced water-rock reaction time in the rainy season.