酸沉降背景下流域土壤碱饱和度变化特征及湖泊响应研究

China is the third largest acid rain impacted area following Europe and America, and the Taihu basin is a typical region that is impacted by acid rains. Our previous study showed that the average pH of the regional rainfall was <5 in the recent years. This has caused the accelerated erosion of carbonate minerals widely distributed in the basin, resulting in a larger amount of Ca and Mg delivered to Taihu lake, and the hardness of the water body increased by 3 folds compared with that in 1950s. However, what is the effect on base cations and base saturation of the soils in the basin caused by this process, what is the response of the lake water, and what is the calcium and magnesium transportation and transformation between land and water under the impact of acidic deposition? The scientific issues have not been studied yet. Based on our previous study, this research will apply field sampling and monitoring, in-situ experiments, isotopic method, model computation and statistical analysis to conduct comprehensive investigation into the temporal-spatial distribution and trend of base saturation of the soils in the basin, evolution of acid neutralizing capacity of the macrophyte and algae dominant lake area, the transportation and transformation of a larger amount of Ca and Mg delivered to the lake between water and solid phase. And, these researches will be implemented from multiple aspects including spatial variation, vertical variation, and historical trends. Also, the acidification possibility of the regional soils and the lake water will be assessed and predicted using scenario analysis and statistical calculation. This study will reveal the transportation characteristic of base elements under the intensive human impact and its environment effects, and will provide scientific basis for an improved understanding of the aquatic environment evolution of Taihu lake and for the restoration of the lake ecosystem.

我国是继欧美之后的第三大酸雨区。以太湖流域为例，前期研究显示其近年平均降雨pH<5，流域广泛分布的碳酸盐矿物被加速侵蚀使大量钙镁淋失进入太湖，造成水体硬度较1950s年代升高了3倍以上。但这一过程对流域土壤中碱基离子含量、碱饱和度变化产生什么影响，水体的响应如何，酸沉降影响下的碱基元素陆水间迁移特征如何等科学问题尚无系统研究。本项目在前期研究基础上，通过野外采样监测、现场模拟实验、模型理论计算、同位素手段、数理统计分析等综合手段，从空间分布、垂向变化、历史演变等多维尺度上探寻酸沉降背景下太湖流域土壤（剖面）碱饱和度分布特征及变化趋势，太湖草藻型湖区酸中和能力的演变，大量入湖钙镁在不同介质间的迁移规律，并通过不同情景分析和统计计算预测流域土壤和湖泊发生酸化的条件。研究将揭示强烈人为干预下太湖流域碱基元素的转化特征及其综合环境效应，为完整理解太湖水环境演变和生态系统恢复提供科学依据。

虽然太湖流域是酸不敏感地区，但人为酸化已使太湖水化学发生显著变化。鉴于目前鲜有流域土壤碳酸盐和太湖沉积物对长期酸化响应的研究，本课题通过对流域不同土地利用土壤和湖泊沉积物剖面的碱基元素及碳酸盐分析，对比了陆域不同酸化强度土壤中碳酸盐流失程度，探讨了陆域碱基离子和碳酸盐流失与湖泊沉积物中变化的联系，模型预测了不同酸化情景下太湖水体碱基元素变化特征及酸化趋势。结果表明，受氮肥和酸沉降双重影响的麦稻土壤表层pH显著低于菜地和湿地（p<0.05），而仅受酸沉降影响的湿地土壤碳酸盐浓度显著高于麦稻地和菜地土壤（p<0.05）。尽管目前太湖上游不同类型土壤平均pH为6.6，仍为碳酸盐缓冲体系，但表层碳酸盐含量已显著下降，证明了酸化对碳酸盐流失过程是累积性且不可逆的。总体上，人为酸化显著加速了流域风化过程，洮滆水系中碳酸盐和硅酸盐风化率分别达到67.2和11.4 t•km-2•a-1，与60年前相比分别增加4.1和2.7倍；同时，南河水系的碳酸盐和硅酸盐风化率分别增加了3.1和4.9倍，苕溪水系(TX)的碳酸盐和硅酸盐风化速率分别增加了5.2和3.4倍。而太湖沉积物剖面显示，碳酸盐从15 cm的深度（约为1980s）到表层呈上升趋势，这表明在过去几十年里由于太湖藻华频发造成的碱性湖泊环境下，湖泊沉积物是流域流失的碳酸盐Ca和Mg的主要汇。在较高的pH环境中，沉积物中的Ca/Mg比值与土壤中的不同，表明Ca和Mg在迁移和沉积过程中经历了不同的生物地球化学过程。本研究通过使用校准后的MAGIC模型评估预测了三种发展情景下对碱基离子的影响，结果表明尽管未来30年（至2050年）太湖流域基本不会出现明显的酸化，但酸化作用对流域化学风化（碱基离子流失）的加速和水化学变化的影响更加显著且不可逆，对相应的水生态系统的影响或更体现。本研究揭示的太湖流域在人为影响下的加速化学风化及对太湖水化学变化的影响，可为完整理解湖泊水化学和生态系统变化及生态恢复提供可能的新思路，为重新评估湖泊生态服务功能、保持流域可持续发展提供科研基础。