亚热带低丘坡地农业开发对壤中流及其氮素迁移过程的影响

Subsurface lateral flow usually transporting a larger number of nitrogen into water environment that have resulted in the increasing agricultural non-point source pollution. The vigorous agricultural exploitation on slopes in the subtropical low hilly region may cause a drastic change of the subsurface lateral flow and resulting nitrogen transport processes due to the alteration of land surface characteristics and the increase of nitrogen input by agricultural land uses, but its mechanism is still unclear. This research selects a representative slope with Masson pine forest and a tea cropped slope converted from Masson pine forest as objects in hilly region of subtropical China, the objectives are to the determine the characteristics of hydraulic effective macro-porosity and its affects on soil water movement flow and solutes transport by a combination of dye tracer method and computer image analysis technique; and to determine the affects of the agricultural exploitation on slopes on the dynamic change of velocity and quantity of subsurface lateral flow and resulting nitrogen transport by the long term monitoring and element tracer method; Based on the characteristics of hydraulic effective macro-porosity, using the Hydrus 2/3D dual region non-equibrium model, to simulate subsurface lateral flow and resulting nitrogen transport processes over a complicated terrain and at a large slope scale for understanding the dominant process and controlling mechanism of the agricultural exploitation on slopes on the processes. This research will improve our understanding on the theory on water-nitrogen transport in soils, the prevention and treatment of agricultural non-point source pollution, and the sustainable development of agricultural ecosystems in subtropical region.

壤中流常携带大量氮素养分进入水体环境从而加剧农业面源污染的风险。亚热带丘陵区大力开展的低丘坡地农业开发会改变陆地下垫面特征和导致较高的氮素投入，会造成壤中流及其氮素迁移过程的变化，但其控制机制不清楚。本项目选取亚热带丘陵区典型马尾松坡地和由马尾松林开垦的茶园坡地为研究对象，拟采用染色示踪方法和计算机图像分析技术，定量评价土壤水力学有效性孔隙特征，及其对土壤水分运动和溶质迁移的影响；利用长期监测和元素示踪方法，研究坡地壤中流及其氮素迁移速度和数量的动态变化规律，揭示农业开发的影响；最后，基于土壤水力学有效性孔隙特征，利用Hydraus2/3D两区非均衡模型，模拟较大坡面尺度和复杂地形下的壤中流及其氮素迁移过程，明确低丘坡地农业开发影响壤中流及其氮素迁移的主导过程和控制机制。开展本研究对于完善土壤水氮运移理论和促进区域农业面源污染防治与农业可持续发展有重要意义。

壤中流常携带大量氮素养分进入水体环境从而加剧农业面源污染风险。本研究采用染色示踪方法和计算机图像分析技术评价土壤水力学有效性孔隙特征；采用野外观测和模型模拟方法分析壤中流及其氮素迁移的动态变化规律；利用情景模拟的方法揭示影响农业开发坡地壤中流过程的控制机制。研究结果表明，经农业开发的茶园坡地红壤，土壤结构高度发育，且渗透性强，优先流发达，在降雨过程中能够使大量水分入渗进土体。由于土壤较薄，水分运动到弱透水性的母质层后，往往在入渗阻力的作用下改变运动方向而形成壤中流。长期监测结果表明农业开发的茶园中，壤中流约占降雨量的10%，且发生于降雨相对丰富的3-7月，表明茶园坡地壤中流过程主要受降雨和土壤前期湿润程度决定。农业开发带来的大量的氮素输入使得茶园壤中流系统中全氮浓度高达25.44 mg L-1，硝态氮高达20.64 mg L-1，远高于该地区土壤水中氮素平均浓度，从而造成了巨大的氮素流失风险。.数学模型能够较好地模拟土壤含水量和氮素浓度动态变化（R2=0.44, P<0.05），可以满足农业开发坡地中水氮运移过程模型模拟的需要。利用Hydrus-2D模型评估土壤前期含水量、降雨量和坡度对坡面壤中流过程影响的结果表明，土壤前期含水量越高，降雨量越大，坡面坡度越大，降雨事件中壤中流流量越大。尤其是降雨量对壤中流过程的影响最明显，当降雨量增加10%。壤中流流量增加139.9%，饱和水位增高86.4%。在土壤剖面结构中，坡体底部不透水层的水力学性质对壤中流过程的影响最明显。在土壤表层和亚表层土壤性质相同的情况下，坡体底层不透水层饱和导水率越小，壤中流流量越大，饱和水位高度越高。因此在进行低丘坡地农业开发时，应当尽量降低坡地坡度和优化氮肥施用来减少土壤氮素随壤中流流失的风险。