表层岩溶系统中土-气-水界面碳流通的短尺度效应——以贵州茂兰国家喀斯特森林公园的秋季日动态监测为例

CARBON FLUXES IN THE INTERFACES OF SOIL-AIR-WATER OF EPIKARST ECOSYSTEM IN SHORT TIME SCALE:AN EXAMPLE OF DAILY DYNAMICS IN AUTUMN AS MONITORED IN MAOLAN KARST PARK, GUIZHOU

在 2 0 0 1年秋季对贵州省荔波县茂兰国家喀斯特森林公园林地、草地植被下土壤CO2 呼吸释放及岩溶表层泉水HCO3 及其δ13 C值的变化进行了日动态的野外监测 ,表明岩溶系统中土 -气 -水界面间存在着碳交换的日动态变化以及所伴随的同位素交换的变化 ,这种变化与土温的日动态有密切的关系。林地植被显示了平抑这种日动态幅度的效应 ,而草地植被则显示响应于温度变化的较灵敏的动态变化。这种短尺度的变化构成了表层岩溶系统对外界条件的灵敏响应 ,进一步揭示了在生物作用下岩溶地质作用在碳循环过程及其同位素交换上的灵敏性和动态性。

Recent studies on karst process and terrestrial carbon cycling have concentrated on dynamics of carbon fluxes among soil, air, and water in association with the carbon isotopic equilibrium. This investigation aims at depicting daily change of such carbon fluxes in epikarst under different vegetations. The authors conducted a whole day onsite monitoring of soil respiration emission of CO 2, bicarbonate discharge of karst water and analysis of δ 13 C composition in a karst terrain of Maolan Karst Reserve, Guizhou, China in autumn of 2001. The main results are as follows: (1) Compared with the significant daily variation of temperature of free air and of surface soil, soil temperature at different depths varies in the range of 22.0～24.3℃ under forest and of 14.5～19.5℃ under grass respectively. More gentle variation of soil temperature is found under forest than that under grass. While variation of soil temperature at 20cm is still prompt to response air temperature under grass, air temperature hardly affects soil temperature at soil depth lower than 10cm. (2) Soil CO 2 fluxes to air by respiration emission varies in the range of 21.21～41.79mg·m -2 ·h -1 and 24.76～51.84 mg·m -2 ·h -1 under forest and grass respectively. The highest emission appears at 4∶00 am under forest and at 6∶00 pm under grass, with the lag effect behind soil temperature at 10cm depth being 10h under forest and only 4 hrs under grass respectively; The δ 13 C values of soil respired CO 2 varies in negative response to soil respiration emission intensity, despite of heavier carbon under grass than under forest by 1.0‰～1.5‰. The range of δ 13 C of soil respired CO 2 in forest is -21.78‰～-19.65‰ and -20.57‰～-18.83‰ in grass respectively. (3) The pattern of HCO - 3 discharge daily dynamics of the associated karst springs differs from that of CO 2 emission both under forest and under grass. While only minor variation of bicarbonate concentration in water can be found in the spring under forest, the HCO - 3 discharge fluctuates under grass and the variation of the δ 13 C values is up to 2‰. (4) Taking into account of the lag effect, both the soil respiration emission intensity and the δ 13 C of respired CO 2 and/or discharged bicarbonate is well correlated to soil temperature at 10cm under forest or at 10cm or deeper under grass. However, the forest vegetation tends to eliminate the magnitude of respiration intensity and the variation of carbon isotopic composition. The heavier carbon both of respired CO 2 and discharged bicarbonate than that reported on Guilin under similar vegetations in summer is attributed to lower temperature herein in autumn. Nevertheless, the soil under grass vegetation is still active in prompt response to daily temperature changes both on CO 2 emission and the isotopic fractioning by the fluxes. In conclusion, there exists daily variation both of carbon fluxes and the isotopic exchange between the interfaces of soil air and water in karst ecosystem even in autumn. This is largely controlled by the soil temperature regimes and soil carbon lability under different vegetation conditions. More significant variation is likely to occur of soil carbon fluxes and the associated isotopic composition under grass due to the sensitivity to temperature changes by the limited vegetation cover and probably the high lability of soil carbon. This illustrates the dynamics of carbon fluxes in the karst system in small time scale (intra year level),which is implicated by the microlamination found in the stalagmites in caves. Again this dynamic nature of soil carbon fluxes reinforces the awareness about sensitivity and kinetics of epikarstification under soil biota in relation to terrestrial carbon cycling.