喀斯特地区洼地剖面土壤含水率的动态变化规律

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

本文基于连续2年土壤水分的定位监测数据,分析探讨了喀斯特地区不同地质背景(纯灰岩与白云质灰岩)洼地剖面(0～90 cm)土壤含水率的动态变化规律。结果表明:洼地剖面土壤含水率总体较高,且从表层到深层表现为增长型;2009年和2010年土壤含水率的变化均具有明显的分层现象,从上到下依次为活跃层、次活跃层、相对稳定层,但均无速变层,不同地质背景的具体分层略有差异;活跃层和次活跃层集中分布在浅层土壤层,相对稳定层较厚,对应着较差的水文调蓄功能,洼地土壤的水分调蓄功能可能会因其相对较深厚(80～100 cm)的土层而被高估。受降雨、蒸发及植物蒸腾等因素的影响,土壤储水量具有明显的动态变化特征,一年中可分为相对稳定期、消耗期和补给期3个阶段,而土壤水分亏缺的补偿和恢复,主要依靠强度适中、历时较长且雨量较大的降雨,微雨和暴雨的作用较小。

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 bothinvestigated 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.