华北低丘山区栓皮栎生态系统氧同位素日变化及蒸散定量区分

Variation of vapor oxygen isotopic composition and partitioning evapotranspiration of oak woodland in the low hilly area of north China

利用稳定同位素技术对华北低丘山区栓皮栎生态系统氧同位素日变化及蒸散定量区分进行研究,为华北低丘山区森林生态系统水汽交换研究提供基础。试验采用离轴积分腔输出光谱技术(OA-ICOS)连续测定生态系统不同高度水汽浓度和δ18O值,同时采用真空提取和液态水同位素分析仪测定枝条和土壤的δ18O值。结果显示,4个晴天中大气水汽浓度日变化复杂,变化趋势差异大,而δ18O日变化均成高-低-高的"V"型变化,最小值出现在12:00—18:00。Keeling方程在10:00—12:00的相关系数R2均大于0.71,方程达到极显著水平,表明此时段蒸腾速率较高,满足植物蒸腾的同位素稳定态假设。利用Keeling方程估算的栓皮栎生态系统δET值有相似的低-高-低日变化,与大气的δv值变化趋势相反。同位素分割结果显示栓皮栎生态系统中蒸腾占蒸散比例日变化呈现低-高-低的趋势,10:00—14:00蒸腾占蒸散比例达到90%以上,尽管6:00—10:00和14:00—18:00的蒸腾占蒸散比例下降,但平均值仍高达69.38%,表明华北低丘山区栓皮栎生态系统的蒸散主要来源于植物蒸腾。

The relative contributions of plant transpiration （T） and soil evaporation to total evapotranspiration （ET） were determined by using stable isotope measurements in a semiarid oak woodland in the rocky mountainous area of north China. Off-Axis Integrated Cavity Output Spectroscopy （OA-ICOS） was used to obtain continuous atmospheric vapor concentration and its δ18 O value at different height gradients in the stand. Cryogenic vacuum distillation and liquid water isotope analyzer were used to determine the δ18 Ovalue of the branch and the surface soil. The results from 4 sunny days showed that water vapor concentrations at 2 m and 11 m above ground were lower than that on the ground （0.1 m） and higher than that in the background atmosphere （ 18 m）, but there was no significant difference of water vapor concentration between 2 m and 11 m. Whereas the diurnal change of atmospheric water vapor concentration exhibited ＂V＂, ＂U＂ and ＂W＂ pattern on four sunny days, the diurnal change of δ18 O showed a ＂V-shaped＂ pattern, and the minimum δ18 O value appeared at the time between 12：00 and 18：00. The atmospheric water vapor δ18 O value was greatest at 0.1 m height above ground, followed by 2 m, 11 m and 18 m height, while the diurnal variations of δ18 O for all sampling heights had similar trends. The δE values, which were calculated based on the Craig-Gordon model, were all smaller than δs on May 25, June 1, June 6, and June 14, indicating considerable 18 O depletion in the water vapor of evaporation. The isotopic compositions in 10：00 -- 14：00 intervals were significantly correlated with the reciprocal of water vapor concentration （ P〈0.001 ） and the R2 values were higher than 0.71, which suggested that transpiration rate was faster at that period of a day and satisfied the steady state hypothesis of isotope in plant transpiration. The 8ST was determined from the Keeling plots and regression analyses showed the intercepts for the three intervals were different. The δET value in the oak woodland increased initially and decreased thereafter. The maximum enrichment of δET appeared during 10：00 --14：00. The isotope partitioning result showed that the diurnal change of the contribution of transpiration to total ET in the oak woodland was a convex pattern. The percent of T! ET increased from the morning, reached maximum values during 10：00 -- 14：00 interval, with maximum values of 98.68%,96.93%,93.81% and 91.34% for May 25, June 1, June 6 and June 14, respectively. The contributions of transpiration to total ET in 6 ： 00 --10 ： 00 interval were 80.67% ,85.81% ,56.94%, and 54.98%, in 14 ： 00--18 ： 00 interval were 81.70%, 72.23%, 61.72%, and 60.94%, respectively. Although the contributions of T/ET in 6 ： 00 --10 ： 00 and 14：00- 18：00 intervals were smaller than those in 10：00 --14：00 interval, the averaged percent was still more than 69.38%, indicating that plant transpiration played a major role in the oak woodland ET in the low hilly area of north China. The approach would improve the understanding of water exchange of forestry ecosystem.