Nitrogen Uptake, Retention and Cycling in Stream: An Intersite N-15 Tracer Experiment

了解调节溪流中氮的吸收、保留和循环的因素对于理解河流生态系统的结构和功能是不可或缺的。此外，由于河流是陆地和下游水生生态系统之间关键的水文和生物地球化学联系，了解气候或土地利用的变化将如何影响物质在景观和水质中的运动，需要控制河流结构和功能的工作模型。尽管关于河流中关键氮转化的过程水平研究有丰富的文献，但缺乏一个将河流中流体动力学、氮循环、代谢和食物网动力学联系起来的强大模型。在这个项目中，我们将使用模拟建模、现场示踪剂添加和站点间比较方法来增加我们对河流氮循环控制的理解，并开发一个生态系统中氮动力学的广义模型。我们将讨论的中心假设是：河流中氮的吸收、保留和循环的相当大的变异性是由关键的水动力学、化学和代谢特征控制的，这些特征决定了水的保留、氮的缺乏程度和河流生态系统中食物网的能量流动。从这一假设中，我们得出了一些具体的预测，包括铵和硝酸盐的吸收，氮的食物网转移和氮周转。这些预测将通过在纬度从热带到北极的10条河流中进行一套相同的实地实验来验证，这些河流在水动力、化学和代谢特征上存在很大差异。现场试验包括：(1)短期（几小时）注射一种保守示踪剂并应用瞬时储存模型来定义水动力特性；(2)短期注射营养物质（NH4、NO3、PO4）来确定不同营养物质的相对吸收长度和潜在的N缺乏率；(3)全流总初级生产力（GPP）和群落呼吸(R)测量来定义溪流代谢特性。(4)长期（6周）添加示踪剂水平的sNH4，以测量河流生态系统中氮吸收、保留和循环速率的时空（纵向）动态。使用15室流氮质量平衡模型对大多数流进行了sNH4添加的模拟。在开始实地实验之前，该模型将被更新并重新运行，以根据目前的了解提供生态系统中‘5N ’的时空分布的特定地点预测。来自田间氮试验的数据将用于测试特定地点的模型结果，以及提供氮吸收、循环和周转的测量，这些测量将用于场间分析，以测试与潜在控制因素之间关系的预测。氮动力学的潜在控制因素包括河流流体动力学（瞬时储存区的重要性）、化学c~（河流氮浓度和N:P比、碎屑c:N比）和代谢（GPP P:R比）。?？

Webster, et al 9628860 Knowledge of factors regulating the uptake, retention, and cycling of nitrogen in streams is integral to understanding lotic ecosystem structure and function. Further, because streams serve as key hydrologic and biogeochemical links between upland and downstream aquatic ecosystems, understanding how changes in climate or land use will affect movement of materials across the landscape and water quality requires working models of controls on the structure and function of streams. Although there is a rich literature of process-level studies of key nitrogen transformations in streams, a robust model linking hydrodynamics, nitrogen cycling, metabolism, and food web dynamics in streams is lacking. In this project we will use simulation modeling, field tracer additions and an intersite comparative approach to increase our understanding of controls on stream nitrogen cycling and develop a generalized model of nitrogen dynamics in lotic ecosystems. The central hypothesis we will address is: The considerable variability among streams in uptake, retention, and cycling of nitrogen is controlled by key hydrodynamic, chemical, and metabolic characteristics that determine water retention, degree of nitrogen deficiency, and energyflow through food webs in stream ecosystems. From this hypothesis we derive a number of specific predictions involving ammonium and nitrate uptake, food web transfer of nitrogen, and nitrogen turnover. These predictions will be tested by conducting an identical set of field experiments in each of 10 streams ranging in latitude from the tropics to the arctic and differing greatly in their hydrodynamic, chemical, and metabolic characteristics. The field experiments include: (1) short-term (several hours) injections of a conservative tracer and application of a transient storage model to define hydrodynamic characteristics, (2) short-term injections of nutrients (NH4, NO3, PO4) to determine relative uptake lengths of different nutrients and potential N defici ency, (3) whole-stream measures of gross primary productivity (GPP) and community respiration (R) to define stream metabolic characteristics, and (4) long-term (6 weeks) additions Of 'sNH4 at tracer levels to measure temporal and spatial (longitudinal) dynamics of nitrogen uptake, retention, and cycling rates through the stream ecosystem. Simulations of the 'sNH4 addition have been performed for most of the streams using a 15-compartment, stream nitrogen mass balance model. The model will be updated and rerun for all streams prior to the start of the field experiments to provide site-specific predictions of temporal and spatial distributions of '5N in the ecosystem based on current understanding. Data from the field N experiments will be used to test the site-specific model results, as well as to provide measures of N uptake, cycling, and turnover that will be used in intersite analyses to test predictions of relationships with potential controlling factors. Potential controlling factors on N dynamics include stream hydrodynamics ( importance of transient storage zones), chemistry c~ (streamwater N concentrations and N:P ratios, detrital C:N ratios), and metabolism (GPP P:R ratios). ??