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.