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

Webster等人9628860关于调节溪流中氮的吸收、保留和循环的因素的知识对于理解热带生态系统的结构和功能是不可或缺的。此外，由于溪流是高地和下游水生生态系统之间的关键水文和生物地质化学联系，要了解气候或土地利用的变化如何影响物质在景观和水质中的移动，就需要对溪流的结构和功能进行控制的工作模型。尽管有大量关于河流中关键氮素转化的过程水平研究的文献，但缺乏将河流中的水动力学、氮循环、新陈代谢和食物网动力学联系起来的可靠模型。在这个项目中，我们将使用模拟建模、田间示踪剂添加和站点间比较的方法来增加我们对河流氮素循环控制的理解，并开发一个热带生态系统氮素动态的通用模型。我们将解决的中心假设是：溪流之间在氮的吸收、保持和循环方面的相当大的变异性由关键的水动力学、化学和代谢特性控制，这些特性决定了河流生态系统中的水分保持、缺氮程度和通过食物网的能量流动。根据这一假设，我们得出了一些具体的预测，涉及氨和硝酸盐的吸收，氮素的食物网转移，以及氮素的周转。这些预测将通过在从热带到北极的纬度范围内的10条河流中每条河流进行一套相同的现场实验来检验，这些河流在水动力学、化学和代谢特征上存在很大差异。田间试验包括：(1)短期(几小时)注入保守的示踪剂并应用瞬变储存模型来确定水动力学特征；(2)短期注入营养物质(NH4、NO3、PO4)以确定不同养分的相对吸收长度和潜在的N缺乏；(3)全流测量总初级生产力(GPP)和群落呼吸(R)以确定河流代谢特征；(4)长期(6周)在示踪剂水平上添加‘sNH4以测量河流生态系统中氮素吸收、保持和循环的时空(纵向)动态。使用15个隔室的溪流氮质量平衡模型对大多数溪流的sNH4加成进行了模拟。该模型将在实地试验开始之前对所有河流进行更新和重新运行，以根据目前的理解提供对生态系统中5N时空分布的特定地点的预测。来自田间N试验的数据将被用来测试特定地点的模型结果，以及提供将用于地点间分析的N吸收、循环和周转的测量，以测试与潜在控制因素的关系的预测。N动态的潜在控制因素包括河流水动力(暂态储存区的重要性)、化学c~(？)(流水N浓度和N：P比、碎屑C：N比)和新陈代谢(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). ??