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

从流域到海岸的氮素输送损害了水生生态系统，包括形成死区，那里的氧气浓度降到可以支持大多数水生生物的水平以下。盐沼通过在进入河口之前拦截分水岭的氮，提供了一条抵御营养物质污染的重要防线。这种氮可以促进沼泽植物的生长，就像化肥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，微生物将其用作氧的替代品，以分解低氧沉积物中的植物有机物。了解这两种氮素输送结果中的哪一种最有可能发生对环境很重要。如果沼泽植物吸收氮素并长得更大，这将导致沼泽通过捕获更多的沉积物和向沉积物中添加更多的植物物质来获得海拔。这一过程将增加盐沼对海平面上升的适应能力。然而，如果沼泽微生物在增加的氮方面能够胜过沼泽植物，那么它可能会促进沉积物的分解，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陆源氮的输入，有助于创建更具弹性的沿海社区。这项研究还通过多层次教育(高中、本科和研究生)的培训来支持劳动力发展，特别是针对历史上被边缘化群体的人。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，这项研究有三个目标：量化环境氮可获得性如何改变植物-微生物耦合系统的反应，确定这些反应在不同洪水制度下的差异，以及评估不同种群的线草对不同氮源的反应。为了实现我们的第一个目标，我们正在美国东部海岸的两个地点进行成对的地块水平的营养强化实验，在那里将以不同的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、微生物以及碳氮循环的影响。为了实现我们的第二个目标，我们正在两个地点进行实验，在这两个地点，氮素形态与海拔交叉，以评估海拔变化如何改变植物和微生物群落对不同形态氮素的反应。这些实验是有目的地设计的，这样结果就可以用于新一代沼泽平衡模型，该模型结合了未来的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，温室氮素吸收实验正在提供关于在没有沉积物微生物的情况下这两种氮素的群体特定吸收速率的基本数据。综上所述，这项研究从机械上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。这些信息正在被纳入一个模型框架，允许资源经理预测沼泽将如何对营养丰富和海平面上升的综合影响做出反应。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.