Salt marshes have the potential to intercept nitrogen that could otherwise impact coastal water quality. Salt marsh plants play a central role in nutrient interception by retaining N in above- and belowground tissues. We examine N uptake and allocation in two dominant salt marsh plants, short-form Spartina alterniflora and Distichlis spicata. Nitrogen uptake was measured using 15N tracer experiments conducted over a four-week period, supplemented with stem-level growth rates, primary production, and microbial denitrification assays. By varying experiment duration, we identify the importance of a rarely-measured aspect of experimental design in 15N tracer studies. Experiment duration had a greater impact on quantitative N uptake estimates than primary production or stem-level relative growth rates. Rapid initial scavenging of added 15N caused apparent nitrogen uptake rates to decline by a factor of two as experiment duration increased from one week to one month, although each experiment shared the qualitative conclusion that Distichlis roots scavenged N approximately twice as rapidly as Spartina. We estimate total N uptake into above- and belowground tissues as 154 and 277 mg N·m-2·d-1 for Spartina and Distichlis, respectively. Driving this pattern were higher N content in Distichlis leaves and belowground tissue and strong differences in primary production; Spartina and Distichlis produced 8.8 and 14.7 g biomass·m-2·d-1. Denitrification potentials were similar in sediment associated with both species, but the strong species-specific difference in N uptake suggests that Distichlis-dominated marshes are likely to intercept more N from coastal waters than are short-form Spartina marshes. The data and source code for this manuscript are available as an R package from https://github.com/troyhill/NitrogenUptake2016.

中文翻译：

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Spartina alterniflora 和 Dishhlis spicata 的氮吸收和分配估计

盐沼有可能拦截氮，否则会影响沿海水质。盐沼植物通过将 N 保留在地上和地下组织中，在养分截留中发挥核心作用。我们检查了两种主要盐沼植物，短型互花米草和 Distichlis spicata 的氮吸收和分配。使用 15N 示踪剂实验进行了为期 4 周的测量，并辅以茎水平生长率、初级生产和微生物反硝化试验来测量氮吸收。通过改变实验持续时间，我们确定了在 15N 示踪剂研究中很少测量的实验设计方面的重要性。与初级生产或茎水平相对增长率相比，试验持续时间对定量 N 吸收估计的影响更大。随着实验持续时间从一周增加到一个月，添加 15N 的快速初始清除导致表观氮吸收率下降两倍，尽管每个实验都共享定性结论，即 Dishhlis 根清除 N 的速度大约是 Spartina 的两倍。我们估计 Spartina 和 Distichlis 的地上和地下组织吸收的总氮分别为 154 和 277 mg N·m-2·d-1。推动这种模式的是 Dishhlis 叶片和地下组织中较高的 N 含量以及初级生产的显着差异；Spartina 和 Distichlis 产生了 8.8 和 14.7 g 生物量·m-2·d-1。与这两种物种相关的沉积物中的反硝化潜力相似，但 N 吸收的强烈物种特异性差异表明，与短型 Spartina 沼泽相比，以 Distichlis 为主的沼泽可能从沿海水域拦截更多的 N。本手稿的数据和源代码可从 https://github.com/troyhill/NitrogenUptake2016 以 R 包的形式提供。