华北平原农田土壤蒸发δ^(18)O的日变化特征及其影响因素

Diurnal variations of soil evaporation δ^(18)O and factors affecting it in cropland in North China

土壤蒸发δ18O(δE)是影响大气水汽δ18O(δv)变异的重要因素,也是农田生态系统蒸散组分土壤蒸发和植物蒸腾拆分的核心科学问题之一。δE主要基于Craig-Gordon模型计算,主要受地表大气水汽δv、相对湿度(h)、平衡和动力学分馏系数以及土壤蒸发前缘液态水δ18O(δs)的影响。该研究以华北平原冬小麦(Triticum aestivum)-夏玉米(Zeamays)生态系统大气水汽δv的原位连续观测数据为基础,同时结合不同深度的土壤日变化采样,综合探讨了δE的日变化特征及其影响因素。结果表明:冬小麦和夏玉米生长季δE的日变化表现为双峰曲线,分别在6:00和15:00左右达到峰值。h强烈影响农田生态系统δE,特别是在h>95%的高相对湿度环境条件下Craig-Gordon模型并不适用。大气水汽δv的原位连续观测技术克服了传统的降水平衡预测大气水汽δv方法的不确定性,可以显著提高δE的准确性。不同的平衡分馏系数对δE的结果无显著影响。不同的动力分馏系数尤其是考虑湍流扩散对动力分馏系数的影响会显著影响δE的模拟结果。土壤蒸发前缘的确定直接影响δs和标准化到土壤蒸发前缘温度下的h,显著影响δE的准确性。结合动态箱或静态箱与稳定同位素红外光谱连续观测技术直接测定δE,从而避免模型参数化过程引入的不确定性是未来研究的重要方向。

Aims The δ18O of soil evaporation （δE） is an important factor controlling the variations of atmospheric δ18O （δv）, and it is also one of the key challenges of partitioning evapotranspiration into evaporation and transpiration components. δE is mostly simulated by the Craig-Gordon model, which is constrained by the δv of water vapor, the relative humidity （h）, the equilibrium and kinetic factors and the δ18O of soil water （δs） at the evaporating front. Our objective is to investigate the diurnal variations of δE and factors affecting it. Methods We determined the δ18O of water vapor in a winter wheat-summer maize cropland based on the in-situ and continuous water vapor isotope ratio measurement system. We sampled soil water at different depths and analyzed it using the cryogenic vacuum distillation technique to acquire the δ18O of soil water at the evaporating front. Important findings During the growing period of winter wheat-summer maize, the diurnal variation of δE exhibited a bimodal pattern with peaks at 6：00 and 15：00. The h has a significant effect on the diurnal variation of δE in cropland ecosystems, and causes the Craig-Gordon model to be invalid under high humidity condition of h〉95%. The in-situ and high resolution measurement of δv overcomes the uncertainty of using the local precipitation equilibrium method to evaluate δv, which improves the accuracy of δE. Different equilibrium factors have no significant influence on the accuracy of δE. Different kinetic factors, especially the canopy scale kinetic factor, influence the accuracy of δE significantly. The location of the evaporating front determines the h normalized to soil temperature and the δ18O of soil water directly and also influences the accuracy of δE significantly. Further research is needed to attain direct measurement of δE by combining isotope ratio infrared spectroscopy （IRIS） with the static chamber or dynamic chamber.