2.4 Data collection
A total of nine hundred standard tablets were buried in four types of land cover (forest, cropland, shrubland, and grassland) in three experiment sites for different periods. The specific buried pattern of the standard tablet was shown in Figure 2. In forest and cropland, three layers (-5 cm, -25 cm, and -35 cm of underground) of standard tablets are buried for five experiment periods (30 days, 60 days, 90 days, 120 days, and 365 days), respectively. While in shrubland and grassland, due to the thickness of the soil layer is too thin, only two layers (-5 cm and -25 cm of underground) of standard tablets were buried. Of the three experiment sites, Site A experimented from August 6, 2017, to August 5, 2018, Site B experimented from August 13, 2017, to August 12, 2018, and Site C experimented from August 1, 2017, to July 31, 2018, respectively. When the burial time reaches 30 days, the first batch of standard tablets shall be taken back for three sites, and so on for the rest of the set of tablets.
To protect the safety of field experiments, we hired local people to manage the experiment sites. After the experiment, 892 out of 900 standard tablets were retrieved from three field experiment sites. Of the eight lost tablets, six of them are broken, and two of them were not found. After the treatment of retrieved tablets, the first data we get is the weight of the standard tablet after the experiment, and the following formula could calculate the karst dissolution amount (Yuan & Cai, 1988).
\(D_{w}=(W_{2}-W_{1})\times 1000\times S^{-1}\) (1)
Where \(D_{w}\) refers to the karst dissolution amount (mg cm-2), \(W_{1}\) is the weight of after experimenting with the tablet (g), \(W_{2}\) is the initial weight of tablet (g),\(S\) is the superficial area of the tablet (cm2).
The karst dissolution loss caused by different intensity droughts are calculated:
\(D_{L}=D_{\text{wn}}-D_{\text{wt}}\) (2)
Where \(D_{L}\) is the amount of karst dissolution loss caused by drought (mg cm-2), \(D_{\text{wn}}\) is the karst dissolution amount of normal state (mg cm-2),\(D_{\text{wt}}\) is the amount of karst dissolution of drought treatment (mg cm-2).
Removed the broken and missing tablets, each soil layer comprises three tablets in one experiment time. Every dissolution data we obtained from different periods is contained of three or two tablets on average. This method can eliminate some noise of experiment data to a certain extent. Furthermore, it is important to note that the present study is focused mainly on the karst dissolution loss caused by different intensity droughts, rather than accurately measuring the dissolution amounts of karst.