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

韦伯斯特等人，9628860 了解河流中氮素的吸收、保留和循环的调控因素是理解河流生态系统结构和功能不可或缺的一部分。此外，由于溪流是高地和下游水生生态系统之间关键的水文和生态地球化学联系，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，需要对溪流的结构和功能进行控制的工作模型。虽然有一个丰富的文献流程级的研究，关键的氮转化流，一个强大的模型连接流中的流体动力学，氮循环，新陈代谢和食物网动态是缺乏的。 在这个项目中，我们将使用模拟建模，现场示踪剂的添加和站点间的比较方法，以增加我们的理解流氮循环的控制，并开发一个广义的氮动力学模型在lotic生态系统。我们将解决的中心假设是：流之间的吸收，保留和氮的循环相当大的变化是由关键的流体动力学，化学和代谢特性，确定水的保留，氮缺乏的程度，并通过食物网在流生态系统energyflow控制。 从这个假设中，我们得出了一些具体的预测，涉及铵和硝酸盐的吸收，食物网转移的氮，氮周转。这些预测将进行一组相同的实地实验，在每10个流的纬度范围从热带到北极，并在其流体动力学，化学和代谢特性有很大的不同。田间试验包括：（1）短期（几个小时）注入保守示踪剂和应用瞬态存储模型来定义流体动力学特性，（2）短期注入营养素（NH_4、NO_3、PO_4）测定不同养分的相对吸收长度和潜在的氮素缺乏，（3）全河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，和（4）长期（6周）添加示踪剂水平的NH_4，以测量河流生态系统中氮素吸收、保留和循环速率的时空（纵向）动态。已经使用15室流氮质量平衡模型对大多数流进行了'sNH 4添加的模拟。该模型将被更新，并在现场实验开始之前，提供特定地点的预测的时间和空间分布的生态系统中的'5 N的基础上，所有流。从现场N实验的数据将被用来测试特定地点的模型结果，以及提供的N吸收，循环和营业额的措施，将用于站点间分析，以测试预测的关系与潜在的控制因素。影响氮动态的潜在因素包括河流水动力学（瞬时储存区的重要性）、化学c~（河水氮浓度和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). ??