红壤丘陵区旱地和水田对农业小流域N负荷贡献的水文机制研究

Co-existence of upland and paddy fields is an important feature of agricultural catchments in the hilly red soil region. Due to diverse farming systems, the differences in the time-variable contributions of nitrogen loading are large between the two ecosystems. Owing to limitations in research methods, it is a big challenge to distinguish the contributions of the two ecosystems from the mixture of a catchment. This has led to uncertainties about the hydrological mechanisms of nitrogen loading for upland and paddy ecosystems at catchment scale. This project will be carried out in a high intensive planting agricultural catchment located in hilly red soil region. It will address the challenges regarding separating the N loadings of the two ecosystems by a combined approach of endmember mixing analyses, i.e., taking stable isotope & chemical elements as tracers and traditional theory of soil physics, i.e., Richards Equation. Based on this approach, the temporal dynamics, the main loss pathway and the responses to natural factors, e.g., seasonal draught and extreme precipitation and human activities, e.g., fertilization and tillage will be identified. The driving mechanisms of runoff on nitrogen loading will be revealed. Finally, we will evaluate the risk of nitrogen loading from the two ecosystems under diverse hydrological conditions. These research results could provide scientific support for making more environmentally-friendly agricultural management systems to control the agricultural non-point pollution.

旱地和水田并存是红壤小流域的重要特点。不同的农业管理制度决定了这两种生态系统对流域N负荷的贡献存在巨大差异。过去由于技术手段的限制，难以将两者对流域N负荷的贡献区分，导致目前两种生态系统N负荷的水文驱动机制尚未完全明晰。针对这一问题，本项目拟以红壤丘陵区高度集约化种植模式下的农田小流域为研究对象，借助稳定性同位素-地球化学离子示踪和经典土壤物理理论相结合的技术手段，以追踪径流来源为切入点，提出区分旱地和水田系统对流域N负荷的方法；阐明两种生态系统对流域N负荷贡献的大小、主要路径、季节性动态以及对自然因素（季节性干旱和极端降水等）和人为干扰（施肥，翻耕等）的响应；揭示径流对N负荷的驱动机制。在此基础上，评估两种生态系统不同水文条件下N负荷风险。研究结果可为红壤区制定更加环境友好的农业管理制度，实现农业面源污染精准控制提供科学依据。

水田和旱地生态系统对流域氮负荷的贡献有显著差异，但是由于两者交错存在，导致区分水田和旱地生态系统对红壤流域径流和氮负荷的贡献十分困难。本研究利用水文监测和径流来源示踪的技术手段，明确了水田生态系统和旱地生态系统降水-产流特征，阐明了水田和旱地子流域径流中氮素浓度和氮负荷的季节动态，区分了水田和旱地子流域对流域氮负荷的贡献，揭示了流域氮负荷的水文驱动机制。研究发现，1）旱地生态系统的产流过程对降水的响应强于水田生态系统，并且降水类型对水田和旱地生态系统的影响差异明显；2）雨季是流域氮素流失的“热时”，水田生态系统是氮素流失的“热区”；3）灌溉水是水田生态系统径流的主要组成成分，雨水是旱地生态系统径流的主要驱动因素，地下径流对两种生态系统径流的贡献均不可忽视；4）无论是水田还是旱地生态系统，流域氮负荷的时间动态均受径流组分的控制，饱和带的地下径流是流域氮负荷的主要流失途径，其贡献比例在旱地生态系统高于水田生态系统。相关研究成果可用于区分不同生态系统对流域氮负荷的贡献，为亚热带季风气候区及水文条件类似地区的农业面源污染防控提供理论支撑。