基于氢氧稳定同位素的中亚热带人工林土壤水运移过程与控制机制研究

Soil water plays an important role in the links of precipitation infiltration, runoff formation and evapotranspiration. Within the framework of traditional translatory flow, infiltrating precipitation and water at any soil depth is well mixed and plants extract the same water that eventually enters the stream. Recent researches show that the water taken up by plant and mobile water entering streamflow are surprisingly distinct (ecohydrological separation), and there is a partial mixture between the two water pools (hydrologic connectivity). The qualitative and quantitative assessment on ecohydrological separation and hydrologic connectivity of soil water is the limiting factor of understanding rainfall infiltration, runoff formation and evapotranspiration. In this research, the field experiments will be conducted in Songtang watershed of CERN Qianyanzhou station by building in-situ runoff plots, simulated precipitation runoff plots and hydrological monitoring and sampling platforms of the small watershed. δD and δ18O of precipitation, soil water, xylem water, surface runoff, subsurface runoff, groundwater and reservoir water will be observed based on rainfall progress, before and after storm events and monthly. Simultaneously, the fluxes of evapotranspiration and the fluxes and ratios of its δD and δ18O in the ecosystem will be measured. Ecohydrological separation and hydrologic connectivity of soil water and their effects on evapotranspiration partitioning will be analyzed qualitatively and quantitatively based the dual isotope approaches and water balance and isotope mass balance equations. At last, the processes and control mechanisms of soil water movement in a mid-subtropical plantation will be elucidated systematically based on the above research results.

土壤水是连接降水入渗、径流形成和蒸散发等过程的枢纽。以往通常认为降水与土壤水完全混合后形成径流。最近研究表明降水进入土壤后可分为植物吸收的水及自由移动形成径流的水(生态水文分离)，且存在着部分混合(连接性)。土壤水生态水文分离过程及连接性的定性与定量解析是理解降水入渗、径流形成和蒸散发的限制因素。以中亚热带江西千烟洲松塘小流域人工林为研究对象，建立原位径流小区、模拟降水径流小区及小流域水文监测与采样平台；开展降水过程、事件及月尺度的降水、土壤水、茎秆水、地表径流、壤中流、地下水、水库水等δD与δ18O的采样和分析，辅以生态系统蒸散及其δD与δ18O比值和通量等配套观测；基于δD和δ18O相关性及水量平衡与同位素质量守恒方程定性和定量解析土壤水生态水文分离过程及连接性，定性和定量解析土壤水生态水文分离对土壤蒸发和植物蒸腾拆分的影响，综合揭示中亚热带人工林生态系统土壤水运移过程及其控制机制。

土壤水是连接降水入渗、径流形成和蒸散发等过程的枢纽。以往通常假设降水与土壤水完全混合后形成径流，而基于δD-δ18O关系研究表明，降水与土壤水混合存在生态水文分离现象，即土壤水可分为由土壤无效水和可供植物吸收的有效水构成的束缚水及自由移动形成径流的自由水，且两个水库间存在着部分混合即连接性。在中亚热带江西千烟洲人工林，建立原位径流小区、模拟降水径流小区及小流域水文监测与采样平台；开展降水过程及月尺度的降水、土壤水、茎秆水、土壤溶液等δD与δ18O的采样和分析，辅以生态系统蒸散及其δD与δ18O比值和通量等配套观测，研究结果发现：1）目前不同时间及空间尺度上的完全分离及连接性的定量解析是理解生态水文分离现象的限制因素。而土壤束缚水与自由水δD和δ18O测定在时间、空间分辨率上的加强，将有助于生态水文分离现象在不同时间及空间尺度上的研究；2）当降水量大于5-6 mm才能入渗进入土壤， 0-5 cm的土壤水主要来源于当季降水，然而15-20 cm及40-45 cm深度的土壤水主要来源于前几个月的降水；3）采样当天的降水以及上层土壤水的下渗将影响lc-excess指征土壤蒸发分馏信号；土壤蒸发损失在非干旱期主要受到相对土壤含水量的影响，在干旱期主要受到土壤积温的影响；4）整个研究期，水汽压亏缺的影响远高于相对土壤含水量；季节性干旱期，相对土壤含水量的影响增加，阐明了季节性干旱下土壤水分供给限制了植物蒸腾对大气需求的响应；5）0-20 cm、20-50 cm和50-100 cm土壤水δD和δ18O富集程度顺序在干旱时期与非干旱时期会呈现周期性的“翻转”变化，而土壤自由水与束缚水之间存在连接性。