亚热带岩溶平原区土壤CO2系统对地下水面升降的响应──活塞效应一例

地下水面是土壤生态系统内部的一个重要界面，使土壤CO²系统形成固有的垂直分 带性及分带结构。地下水面的季节性升降能够引起土壤CO²系统的强烈响应。首先使土壤空 气发生机械性整体对流。同时，还通过引发一系列其它物理、化学和生物变化，激发产生一个 局部扩散层，从而改变CO²剖面垂直分带结构。粘性土壤、土质不均和土壤液相部分有较大碳 酸盐容量是产生局部扩散层的主要内在原因。在中国南方岩溶平原区高台型地下水动态条件 下，地下水面季节性升降的活塞效应和气温年周期变化叠加，使土壤CO²剖面形态变化形成有 规律的序列演化旋回。这一规律可以帮助对土壤CO²进行中长期预测。

RESPONSE OF SOIL CO2 TO GROUND WATER TABLE CHANGES IN A SUBTROPICAL KARST PLAIN REGION: A CASE STUDY ON PISTON EFFECT

Profile of soil CO² concentration (SCC), soil moisture, air and soil temperatures and ground water level in wells were monitored from Sept. 1999 to Jan. 2000 at the experiment site of the Institute of Karst Geology in a karst plain area in central Guangxi, China. Significant spatial and temporal variations in SCC and a strong dependence of SCC variation on ground wator table (GWT) fluctuation were observed. To GWT changes different parts of the profile respond in different manner. To a GWT decline with an average rate of 4- 6cm/ day during an early winter time, the SCC in the upper part did not respond essentially but dropped with atmosphere temperature. Meanwhile the SCC in the lower part rose from 1% to 7% despite that air temperature had decreased by more than 15℃ in average. An extended version of piston effect concept has been proposed to explain the phenomenon. GWT is considered to be the most critical boundary plane in the soil ecosystem and divides it into aeration and saturation belts, which are, as are known, distinguished by significantly different conditions in soil CO² production and transportation. In our case for example SCC in aeration belt exceeded that in saturation one by a factor of 4-6. The distinct features of boundary framework result in a 2 layered zonation of soil CO² within the aeration belt itself: an upper zone with upward CO² flux pointing to the ground surface and a lower zone with downward flux pointing to GWT. For this zonation a single-peak curve of CO² profile is characteristic. When GWT moves up and down, at first a mass flow is induced in aeration belt. It should be a quick gas movement process with a lifetime no more than a few hours. However the shifts will then lead to a series of other physical, chemical and also biological responses from the soil CO² system, which could last for a few months when coupled with water table decline process and will induce a new zone (zone of local diffusion) to occur between the two existing zones, which typically charasterized by a moderate oscillatory regime of CO² concentration. For the new zonation multi-peak features of CO² profile are characteristic. As karst residual soils are heavily clayey and highly inhomogeneous, and possess essential double porosity, in repeated cycles of moistening and drying the inhomogeneity in air-filled porosity and gas permeability will be enhanced, and in general air-filled porosity remains less than 3% in a long period (up to 5 months and more) after dewarering began. It should be noted that, as is known, repeated cycles of moistening and drying as well as aerobic and anaerobic cycles increase soil respiration rate greatly. It is reasonable to postulate that large pores would encourage more intensive respiration thus promoting growth of high CO² spots in the local diffusion zone, which were actually recorded in monitored profiles during this study. A large carbonals volumetric capacity of liquid phase is thought to be another important factor facilitating the process. Thus the heterogeneity of soil structure, especially irregular distribution of air-filled porosity, the clayey soil and large carbonals capacity of liquid phase are believed to be the 3 major internal factors allowing a local diffusion zone with intensive variations in CO² concentration to occur at the initial stage after dewatering process began and helping it exist for quite a long time. In general, the piston effect along with annual temperature cycle makes the feature of CO² profile evolve in a regular consequence: curve with almost increasing concentration with depth→single-peak curve→multi -peak curve→single-peak curve→ curve with almost increasing concentration with depth.