绍兴淡水湿地森林土壤水氘同位素对降水的响应

Response of Deuterium Isotope in Soil Water to Rainfall in Freshwater Wetland Forests of Shaoxing, Zhejiang Province

【目的】以浙江省绍兴市汤浦水库库区3种淡水湿地森林(阔叶林、针阔混交林、针叶林)为研究对象,分析不同量级降水后其土壤水氘同位素的时空变化规律,定量阐明降水对各层土壤水的贡献率,为揭示降水在该淡水湿地森林生态系统水循环中的分配规律提供科学依据。【方法】利用稳定同位素技术研究3类淡水湿地森林土壤水氘同位素值在3次不同量级降水后的动态变化,分析比较土壤水与其潜在水源(大气降水、浅层地下水等)的氘同位素值(δD),判断林中土壤水来源;运用二元线性混合模型计算不同量级降水对这3种湿地森林中枯枝落叶层及各层土壤水的贡献率。【结果】绍兴汤浦水库库区淡水湿地森林中的土壤水δD在3次不同量级降水后均介于大气降水δD和浅层地下水δD之间,表明该库区土壤水主要来源于降水和浅层地下水;小雨(7.5 mm)后,3个林地0～60 cm土层土壤水δD值较雨前对照略有升高,而60～100 cm深层土壤水δD值变化很小,表明7.5 mm降水可入渗补给到0～60 cm土层;中雨(14.5 mm)后第1天,阔叶林、针阔混交林和针叶林0～100 cm土层土壤水δD均值分别降低了9.5‰±4.0‰,9.3‰±4.1‰和7.0‰±2.6‰,中雨后9天内,3个林地各层土壤水δD值随采样天数的增加逐渐升高并接近雨前对照,表明降水δD对土壤δD的影响逐渐减小;大雨(35.0 mm)后第1天,3个林地枯枝落叶水和表层(0～20 cm)土壤水δD值接近降水δD值;不同量级降水对3个林地枯枝落叶水的贡献率最大,0～20 cm表层土壤水次之,随着采样天数增加,降水对各层土壤水的贡献率皆呈现减小的趋势;降水事件(不同量级降水)是影响雨后土壤水δD以及该次降水对土壤水贡献率的主要因子。【结论】浙江绍兴汤浦水库库区不同类型淡水湿地森林中枯枝落叶层水δD对降水δD的响应最显著,0～20 cm表层土壤水δD的响应次之;单次降水量越大,降水对各层土壤水的影响越明显,贡献率也越大,大雨(降水量>20 mm)影响各层土壤水的时间超过9天;该地区淡水湿地森林对小雨(降水量≤10 mm)和中雨(10 mm<降水量≤20 mm)在土壤剖面的入渗具有一定的调控作用,且降水在针阔混交林土壤中存留时间最长,针叶林最短,可见,浙江绍兴汤浦水库混交林(阔叶林和针阔混交林)对小雨及中雨的调控作用略优于纯林(针叶林)。建议今后在长三角地区进行人工淡水湿地森林植被恢复和构建时应考虑多树种混交种植。

【Objective】 Three different types of freshwater wetland forests （broad-leaved forests, coniferous-broad-leaved mixed forests, and coniferous forests）were selected as research objects to analyze the temporal and spatial variations of deuterium isotope values in forest soil profiles after different magnitude precipitation,and quantitatively explain the contribution of precipitation to the soil water in each layer in Tangpu Reservoir Area of Shaoxing, Zhejiang Province. The result of this study provide a scientific basis for revealing the distribution of precipitation in the water cycle of forest ecosystems in the wetlands of the area.【Method】 In this study, we examined the dynamic variation of deuterium values （ δ D） in Soil Waterby using stable isotope technique under three different rainfall conditions inthree different types of freshwater wetland forests, analyzed the relationship of δ D values between soil water and the potential water sources （rainfall and shallow underground water, etc.）, and further determined the source of litter water and soil water and the distribution of one-timerainfall in the three wetland forests. The contribution rate of rainfall in litter and each layer of the soil profile was calculated according tobinary linear mixed model after the occurrence of different rainfall events.【Result】 The δ D values of soil water were between those δ D values of rainfall and shallow groundwater after three different magnitudes ofrainfall in the freshwater wetland forests in the Tangpu Reservoir Area of Shaoxing, indicating that the soil water and litter water mainly came from precipitation and shallow groundwater.After small rainfall （ 7.5 mm ）, the soil water δ D values in 0-60 cm increased slightly and were higher than the values before rain, however, there was almost no change and relatively stable in the δ D values of 60^-100 cm deep soil water, indicating that 7.5 mm precipitation could be infiltrated and recharged into the 60 cm soil layer in the three wetland forests. The first day after moderate rain （14.5 mm）, the soil water δ D average values of the soil layer of 0^-100 cm decreased by 9.5 ‰±4.0 ‰, 9.3 ‰±4.1 ‰, and 7.0 ‰±2.6 ‰ in the broad-leaved,mixed broadleaf-conifer, and coniferous forests, respectively. In 9 days after the moderate rainfall, the soil water δ D value of the three forests kept changing with increasing number of days after rainfall and approached to control before rain, indicating that the effect of precipitation δ D on soil δ D gradually decreased.The first day after the heavy rain （35.0 mm）, δ D value of the litter water and surface soil layer of 0-20 cm in the three forests was close to rainfall δ D value. The contribution rate of rainfall to the litter layer in the three forests was the highest, and the contribution rate to soil water was the second at soil layer of 0-20 cm.With the increase of sampling days, the contribution of precipitation to the soil water in each layer was presented with a decreasing trend.Precipitation events （precipitation of different magnitudes）were the main factors affecting the soil water δ D and the contribution of precipitation to soil water after rain.【Conclusion】 The response of δ D values of litter layer to precipitation was the most significant, soil water δ D （0-20 cm） to rainfall δ D was second significant in the wetland forests of Tangpu Reservoir Area in Shaoxing, Zhejiang Province.The greater magnitude of one-time rainfall, the more obvious the effect on soil water （0^-100 cm）, and the greater the rainfall contribution to the soil water,heavy rain （precipitation〉20 mm） affected soil water in each layer for more than 9 days. The result suggest that freshwater wetland forest played an important role in regulating the process of small rainfall （precipitation≤10 mm） and moderate rainfall（10 mm〈precipitation≤20 mm） infiltration in the soil profile.The retention time of precipitation （7.5 mm and 14.5 mm） was the longest in the soil of conifer-broadleaf mixed forest, and it was the shortest in the soil of coniferous forest. indicating that the coniferous and broad-leaved mixed forest （broad-leaved forest and coniferous and broad-leaved mixed forest） of Tangpu Reservoir in Zhejiang Province is slightly better than pure forest （coniferous forest）. It is suggested that multi-species mixed planting should be considered in future restoration and construction of artificial freshwater wetland forests in the Yangtze River Delta Region.