With the development of the binary structure of karst landforms, surface water is largely drained with rapid loss of nutrients. However, the pathway and mechanism of nutrient loss remain elusive. From a three-dimensional (vegetation-soil-epikarst system) perspective of a critical zone in karst area, this study conducted rainfall induced runoff and nitrogen loss monitoring during monsoon in karst shrub slopes. Isotope-based (D and 18O) hydrograph separation method was applied to partition the ratio of 'old' and 'new' water in main hydrological path. The main results were summarized as follows. Deep percolation and interflow were the dominant hydrological pathways, accoun-ting for 71% and 9% of total rainfall amount, respectively. In contrast, surface runoff occupied less than 2%. Both deep percolation and interflow were dominated with 85% and 61% of old water, respectively. The highest nitrate concentration occurred in deep percolation (1.97 mg·L-1), while the highest ammonium nitrogen concentration occurred in interflow (1.18 mg·L-1). Deep percolation contributed 89.4% of total nitrogen loss, which was significantly higher than that of surface runoff and interflow. Old water ratio showed a significant positive correlation with nitrate nitrogen concentration, ammonium nitrogen concentration, and total nitrogen loss, suggesting it might be the main agent driving nitrogen migration for the whole soil-epikarst system in karst hillslopes. The results would provide scientific basis for rational allocating water resources and developing nutrient loss control technology in karst region of southwestern China.
喀斯特二元结构发育,地表水大量漏失,养分表现出快速流失的特点,然而养分流失的途径和机制尚不清楚.本研究从喀斯特关键带三维空间的视角,通过监测喀斯特灌丛坡地雨季典型降雨产流和氮素流失特征,基于氢氧稳定同位素技术对主要水文路径“新旧水”比例进行划分,并探讨氮素流失的主要途径和机制.结果表明:深层渗漏和壤中流是喀斯特灌丛坡地主要水文路径,两者分别占降雨量的71%和9%,地表径流仅占2%;深层渗漏、壤中流均以“旧水”为主,“旧水”比例分别为85%、61%;硝态氮浓度在深层渗漏中最高(1.97 mg·L-1),铵态氮浓度在壤中流中最高(1.18 mg·L-1),深层渗漏对氮素流失的贡献率达89.4%,显著高于地表径流和壤中流;“旧水”比例与硝态氮、铵态氮浓度和流失总量均表现出显著正相关关系,“旧水”可能是喀斯特坡地土壤-表层岩溶带系统携带氮素迁移的主要介质.以上研究能够为西南喀斯特地区坡地水资源合理配置和养分流失阻控技术研发提供理论支撑.
Keywords: hydrological pathway; isotope; karst; nutrient loss; old water.