Nitrogen Dynamics in a Salt Marsh

像弗吉尼亚海岸保护区这样的沿海泻湖型河口通常是氮有限的。氮的有效性控制着河口水生初级生产力。农业径流对河口氮负荷的影响在很大程度上取决于潮间带沉积物中发生的氮循环过程。硝化-反硝化等过程导致沉积物中N的损失，而异化的硝酸盐还原成铵则倾向于保留沉积物中的N。由于难以测量湿地沉积物中各种氮循环过程的速率，开发模型以预测由于自然或人为过程而流入河口的氮的影响的努力受到了限制。目前用于测量这些过程速率的许多技术是破坏性的，使用有效性可疑的抑制剂，或者需要添加高于环境浓度的底物。这样的技术最多只能测量潜在的反应速率，而不是原位反应速率。该项目将开发将15N同位素池稀释技术应用于从植被和未植被的潮间带湿地生态系统中采集的完整沉积物岩心所需的方法。这些测量将允许确定硝化作用的相对重要性，然后是反硝化作用与异化硝酸盐减少对总体氮收支的影响。这项研究将在弗吉尼亚海岸保护区的一个长期生态研究地点进行。综合起来，这些信息将提高湿地和近岸沉积物中氮循环反应速率估计的准确性，并允许评估这些过程在沿海海洋生态系统生态学中的作用。

Coastal lagoon-type estuaries such as the Virginia Coast Reserve are often nitrogen-limited. The availability of nitrogen controls aquatic primary productivity in the estuary. The effects of N-loading on the estuary due to agricultural runoff depends to a large extent upon the N-cycling processes that occur in the intertidal sediments. Processes such as nitrification- denitrification result in losses of N from the sediment, whereas dissimilatory nitrate reduction to ammonium tends to conserve N in the sediment. Efforts to develop models to predict the effects of nitrogen influx to estuaries due to natural or anthropogenic processes have been limited by the difficulties involved in measuring the rates of the various N-cycling processes in wetland sediments. Many of the techniques currently used to measure the rates of these processes are disruptive, utilize inhibitors of questionable effectiveness, or require additions of substrates at higher than ambient concentrations. Such techniques may at best then measure potential rather than in-situ reaction rates. This project will develop the methodologies required to apply 15N isotope pool dilution techniques to intact sediment cores taken from vegetated and unvegetated intertidal wetland ecosystems. These measurements will allow the determination of the relative importance of nitrification, followed by denitrification v.s. dissimilatory nitrate reduction to the overall N-budget. The study will be performed at a Long Term Ecological Research site in the Virginia Coast Reserve. Taken together, this information will improve the accuracy of rate estimates of nitrogen cycling reactions in wetland and near shore sediments and permit an evaluation of the role of these processes in the ecology of coastal marine ecosystems.