Collaborative Research: Quantifying the effects of different nitrogen forms on marsh resilience to environmental change

氮从流域输送到海岸会损害水生生态系统，包括形成死区，其中氧浓度下降到可以支持大多数水生生物的水平以下。盐沼通过在氮进入河口之前拦截流域氮，提供了防止营养盐污染的重要防线。这些氮可以促进沼泽植物的生长，就像肥料促进花园的生长一样。然而，某些形式的氮也可以被微生物利用，微生物用它作为氧气的替代物来分解低氧沉积物中的植物有机物。了解氮输送的这两种结果中哪一种最有可能发生对环境很重要。如果沼泽植物吸收了这些氮并长得更大，这将导致沼泽通过捕获更多的沉积物并向沉积物中添加更多的植物物质而获得海拔。这一过程将提高盐沼对海平面上升的适应能力。然而，如果沼泽微生物能够胜过沼泽植物来竞争增加的氮，那么它可能会增强沉积物的分解并降低沼泽跟上海平面上升的能力。这项研究的结果将有助于预测沼泽将如何应对沿海系统的海平面上升，这些系统经历了大量的陆源氮输入，有助于创造更具弹性的沿海社区。这项研究还支持劳动力的发展，通过培训跨多个层次的教育（高中，本科和研究生），特别是从历史上被边缘化的群体的人。为了了解氮形态和植物遗传多样性如何影响盐沼跟上海平面上升的能力，本研究有三个目标：量化环境氮的可用性如何改变耦合植物-微生物系统的响应，确定这些响应如何在不同的洪水条件下变化，并评估不同群体的大米草如何响应不同的氮源。为了实现我们的第一个目标，我们正在美国东部海岸沿着的两个地点进行成对的地块级营养强化实验，在那里将以一系列浓度添加硝酸盐和铵，为期两年。我们正在测量氮添加对沼泽植物，微生物以及碳和氮循环的影响。为了实现我们的第二个目标，我们正在这两个地点进行实验，将氮的形式与海拔进行交叉，以评估海拔的变化如何改变植物和微生物群落对不同形式氮的反应。这些实验是有目的地设计的，以便结果可以用于新一代的沼泽平衡模型，该模型结合了未来氮输入将如何改变沼泽的能力，以跟上海平面上升的步伐。最后，温室氮吸收实验提供了在没有沉积物微生物的情况下两种氮形态的特定人群吸收率的基本数据。总之，这项研究机械地解决了不同形式的氮和植物遗传变异如何影响沼泽储存碳的能力，并与海平面上升保持同步。这些信息被纳入一个模型框架，使资源管理者能够预测沼泽将如何应对营养富集和海平面上升的综合影响。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Nitrogen delivery from watersheds to the coast harms aquatic ecosystems, including the formation of dead zones where oxygen concentrations drop below levels that can support most aquatic life. Salt marshes provide an important line of defense against nutrient pollution by intercepting watershed nitrogen before it enters estuaries. This nitrogen can enhance the growth of marsh plants, much as fertilizers enhance growth in gardens. However, some forms of nitrogen can also be used by microbes, who use it as a substitute for oxygen to decompose plant organic matter in low-oxygen sediments. Understanding which of these two outcomes of nitrogen delivery is most likely to occur is important to the environment. If marsh plants take up that nitrogen and grow larger, this will cause the marsh to gain elevation by trapping more sediment and adding more plant material to the sediments. This process will increase salt marsh resilience to sea-level rise. However, if marsh microbes can outcompete marsh plants for the added nitrogen, then it could enhance sediment decomposition and reduce the ability of marshes to keep up with rising sea levels. The results of this research will help predict how marshes will respond to sea-level rise in coastal systems that experience large inputs of land-derived nitrogen, helping to create more resilient coastal communities. This research also supports workforce development through training across multiple levels of education (high school, undergraduate, and graduate), in particular for people from historically marginalized groups. To understand how nitrogen speciation and plant genetic diversity affect the ability of salt marshes to keep pace with sea-level rise, this research has three goals: quantify how environmental nitrogen availability alters responses of the coupled plant-microbe system, determine how these responses vary under different flooding regimes, and assess how different populations of cordgrass respond to different nitrogen sources. To address our first goal, we are conducting paired plot-level nutrient enrichment experiments in two locations along the eastern US coast where nitrate and ammonium will be added at a range of concentrations for two years. We are measuring the effects of nitrogen addition on marsh plants, microbes, and carbon and nitrogen cycling. To address our second goal, we are performing experiments at both locations, where nitrogen form is crossed with elevation to assess how variation in elevation alters the responses of the plant and microbial communities to different forms of nitrogen. These experiments are purposefully designed so that results can be used in a new generation of the Marsh Equilibrium Model that incorporates how future nitrogen inputs will alter the capacity of marshes to keep pace with sea-level rise. Lastly, greenhouse nitrogen uptake experiments are providing essential data on population-specific uptake rates of both nitrogen forms in the absence of sediment microbes. Taken together, this research mechanistically addresses how different forms of nitrogen and plant genetic variation affect marsh ability to store carbon and keep pace with sea-level rise. This information is being incorporated into a modeling framework that allows resource managers to predict how marshes will respond to the combined effects of nutrient enrichment and sea-level rise.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.