4 Discussion
4.1 Impactmechanism ofdroughts on karst dissolution
Karst dissolution is essentially the physical and chemical action of water on soluble rocks (Ren & Liu, 1983; Ford & Williams, 2007). In other words, water is the most crucial factor in karst dissolution. Critically, the water in karst areas mainly comes from atmospheric precipitation, particularly in karst areas of Southwest China. That is the key to the effect of drought on karst dissolution. Theoretically, drought leads to a decline of precipitation and, in turn, reduces the soil water, weakens the karst dissolution process, and finally causes the loss of karst dissolution. That is the primary mechanism of the impact of drought on karst dissolution.
To better understand the impact mechanism, three factors, soil water, soil pH, and soil organic matter, which are most closely related to karst dissolution, were selected to study the influence of drought on karst dissolution. Among the three factors, soil water is the most critical prerequisite for karst dissolution and is also the carrier of CO2 forming carbonic acid in the soil. Soil carbonic acid is a direct cause of karst dissolution, which is generally measured by soil pH value. In theory, the more soil water, the lower the soil pH, and then the higher the karst dissolution, and vice versa. In contrast, soil organic matter is an indicator of the soil environment, which significantly impacts soil water and soil pH levels. For example, high soil organic matter means that the soil structure is better and could maintain more water and CO2 in the soil. Naturally, more soil carbonic acid can be formed. There is no doubt that many other factors can affect karst dissolution, but they are mainly achieved by soil water and soil carbonic acid. And absolutely, it is of great significance to consider the influence of other factors in future research.
Data collection and field experiments were carried out simultaneously. Moreover, each data is composed of no less than three samples. In the experiment, soil water was measured by a portable soil water meter of Spectrum (TDR 300, accuracy is 0.1% in Volume content). Soil pH was investigated by a portable soil acidity meter (FieldScout pH 400) with an accuracy of ±0.01 in value. Soil organic matter was analyzed by potassium dichromate sulfuric acid solution titration in a laboratory. The results show that the soil water decreased obviously under the influence of drought, while soil pH showed an upward trend (Figure 5). Interestingly, there is no apparent change trend of soil organic matter under the influence of drought except in Site A (Figure 5). These results suggest that drought directly affected soil water and soil pH, but the impact on soil organic matter was not clear.
To further understand the mechanism, we applied a Pearson correlation analysis to research the correlation between karst dissolution loss and soil water, soil pH, and soil organic matter. To exclude the influence of time, season, and other factors on soil water, soil pH, and soil organic matter, all the data used for analysis were from the drought control group. As expected, there was a clear negative correlation between karst dissolution loss and soil water, but a positive correlation between karst dissolution loss and soil pH (Table 3). Meanwhile, there has a negative correlation between soil water and soil pH (Table 3). Besides, there is no consistent correlation between karst dissolution loss and soil organic matter or soil organic matter and soil water and soil pH, except for Site A (Table 3). The results show that soil water plays a crucial role in the karst dissolution process; soil carbonic acid results from the interaction between soil water and soil environment. Finally, the effect of soil organic matter on karst dissolution is uncertain.
Although only three factors are used in our study, the results clearly described the critical influence process of drought on karst dissolution. Simultaneously, we also noticed that continuous field observation is crucial to understand the effect of drought on the karst dissolution process. Whatever the outcome, this experiment had demonstrated that soil water was the center of the impact of drought on karst dissolution, and the soil carbonic acid is an indirect control factor. As for soil organic matter, its effect is still uncertain, but in most cases, increasing soil organic matter is beneficial to mitigate the impact of drought on karst dissolution.