Dynamics of soil profile water content in peak-cluster depression areas in karst region

Soil water is a critical factor for vegetation rehabilitation and eco-environmental construction in karst mountain regions. The dynamics of water content in soil profiles with different geological backgrounds were analyzed in peak-cluster depression zones in karst region of Northwest Guangxi. The study was conducted in maize-soybean rotation fields in Guzhou Village with pure limestone geology and in mulberry tree fields in Mulun Nature Reserve with dolomite limestone geology. The parameters of rainfall, wind speed, wind direction, temperature, humidity and atmospheric pressure were recorded in meteorological stations with multi-functional meteorological sensors. Volumetric soil water content was monitored at soil depths of 5 cm, 10 cm, 20 cm, 30 cm, 40 cm, 50 cm, 70 cm, and 90 cm by automatic gauge systems and data collected every 30 min using CR1000 data acquisition device. Soils were sampled three times during the study using circular knife (AZ-HSS-100-24) at corresponding water monitoring depths. Then bulk density, capillary water capacity and saturated water content were determined using the drying method. Also saturated conductivity for fixed water level and soil water storage were calculated. Dynamic variations in soil water at different soil layers (0-90 cm) in the farmland ecosystems in karst peak-cluster depressions in Guzhou Village and Mulun Nature Reserve were then discussed in relation to atmospheric precipitation, soil water content and other variables in 2009 and 2010. The results showed that while soil bulk density, capillary water capacity and saturated water content increased, non-capillary porosity and saturated conductivity decreased with increasing soil depth. Also higher soil bulk density corresponded with lower saturated conductivity. The physical properties of 0-30 cm soil layer in peak-cluster depression zones did not favor water storage and regulation functions at both investigated sites. Soil water content was not only high, but also increased with increasing soil depth. Soil water contents in 2009 and 2010 showed evident layer trends for the active layer, sub-active layer and stable layer. This was attributed to the soil clayey texture of peak-cluster depression areas. Variations in soil water content were little different for the two study sites. In pure limestone geology, the below 40 cm soil layer was relatively stable in 2009 while the below 30 cm soil layer was relatively stable in 2010. In dolomitic limestone geology, the below 20 cm soil layer was relatively stable in both 2009 and 2010. The storage and regulation functions of soils in peak-cluster depression zones could had been overestimated due to heavy nature of the soil layers. Because soil water storage was influenced by rainfall, evaporation and plant transpiration, it showed variations characteristic of these factors. Periods of relative stabilization, consumption and supplement of soil water storage were noted in 2009 and 2010, but with one month difference in occurrence between two years. The time difference was mainly attributed to rainfall distribution characteristics in the regions. Soil water deficit compensation and rehabilitation was most effective under moderate rainfall intensity and long, heavy rainfall conditions. Light and heavy rains had less effect on soil water because they hardly infiltrated down to lower soil layers. Long and heavy rains were, however, sufficient to replenish soil water in both the upper and lower soil layers.