不同动力学分馏系数对北京山区侧柏叶片水δ^18O的模拟

Simulation ofδ18 O in Platycladus orientalis leaf water with different kinetic fractionation coefficients

利用稳定同位素技术对植物叶片水18O同位素组成(δL,b)进行研究,可以为植物叶片生理及森林水文的研究提供理论参考。本研究连续监测北京山区侧柏人工林生态系统冠层大气水汽浓度(W_a)和大气水汽18O同位素值组成(δ_v),结合测定的侧柏枝条水18O同位素组成(δ_x)和δL,b,分析了动力学分馏系数εk1(32%)和εk2(28%)对δL,b的预测效果。结果表明:侧柏人工林生态系统W_a日变化无明显规律,大气相对湿度(RH)日变化呈"V"型,气孔导度(g_s)在日尺度上先增大后减小;同位素接近稳态时(正午前后),δL,b略有增加,W_a、RH、g_s与δL,b均呈显著负相关关系;同位素接近稳态条件下,将不同动力学分馏系数εk1、εk2应用于Craig-Gordon模型,预测δL,b,εk2的预测值更接近δL,b的实测值,表明εk2应用于模型更符合北京山区侧柏叶片水同位素富集情况。研究结果将加深对叶片水同位素富集模型、蒸散拆分模型的认识。

Clarifying 18O isotope composition of leaf water(δL,b)would provide theoretical refe-rence for the study of leaf physiology and forest hydrology.We continuously monitored the concentration of atmospheric water vapor(Wa)and 18O isotope composition of atmospheric water vapor(δv)at the canopy of Platycladus lateralis plantation in the mountain area of Beijing.We analyzed the effects of kinetic fractionation coefficientsεk1(32%)andεk2(28%)on the prediction ofδL,b by combining the measured leaf water 18O isotope(δx)andδL,b of P.lateralis.The results showed that the diurnal variation of Wa was irregular.Atmospheric relative humidity(RH)showed a"V"shape of diurnal variation,and stomatal conductance(gs)increased first and then decreased at the diurnal scale.Wa,RH,and gs showed a significant negative correlation withδL,b when isotopes approached a steady-state equilibrium around noon.The kinetic fractionation coefficientεk1 andεk2 were applied to the Craig-Gordon model to predictδL,b under the isotopic quasi-steady-state condition.The results showed that the predicted values ofεk2 approached the observed values ofδL,b.This result indicated that the application ofεk2 to the model was more consistent with the change of water isotope concentration in the leaves of P.lateralis in the mountain area of Beijing.These results would improve our understanding of water isotope enrichment model and evapotranspiration resolution model in leaves.