Deciphering the role of dissolved organic nitrogen in stream nutrient cycling

许多地表水是黄色或棕色的，这是由于有机分子的存在，这些有机分子来自许多自然来源，如树叶、土壤和湿地。这些分子影响着河流中的许多特性，比如光穿透水的能力和水的酸度。有机分子还可以为溪流提供能量和必需的营养物质，如氮和磷。这些有机分子的一种特别重要的形式是溶解的有机氮，它在全球的森林、北极苔原、河口和溪流中都有发现。然而，对于溶解的有机氮在何时何地作为生物群落的能量来源或氮源，或两者兼而有之，我们知之甚少。该项目将研究溶解有机氮在河流中的作用，这些河流跨越了五个不同的森林区域，跨越了不同的环境，以阐明溶解有机氮的作用。这项研究与水资源管理有关，因为溶解的有机氮是造成河口有毒藻华的过量营养负荷的来源，当它在饮用水供应中被发现时，它可以在水处理过程中产生危害人类健康的消毒副产物。这项研究还将把代表性不足的群体纳入项目所需的实验室工作和实地工作，并为一名研究生和一名博士后科学家提供培训机会。在溶解有机氮（DON）的竞争作用之间进行区分是困难的，因为直接操纵水生生态系统中的DON池最容易通过添加特定化合物（如氨基酸）来实现，这些化合物可在商业上获得，但在环境条件下溪流和河流中发现的DON并不特别具有代表性。基于最初的实验结果，研究人员将在新英格兰、波多黎各、西班牙、捷克共和国和西伯利亚的河流中进行一系列新的野外操作，以更好地了解DON在全球河流中作为能量来源或氮源的程度。在新英格兰进行的初步实验观测表明，DON既可以作为氮源，也可以作为能量源，但其作用似乎随着时间和空间的变化而变化。在这里，研究人员将进行额外的实验，将硝酸盐脉冲添加到全球网络的溪流中，以评估DON主要作为氮源而不是能量（碳）源的频率和条件。这组实验将提供一些初步的实验证据，以评估这两个相互竞争的驱动因素在调节自然水域环境DON水平方面的重要性。

Many surface waters are yellow or brown in color due to the presence of organic molecules which come from many natural sources such as leaves, soils, and wetlands. These molecules affect a wide range of characteristics within streams such as the ability of light to penetrate the water and the acidity of the water. The organic molecules also can provide streams with energy and essential nutrients such as nitrogen and phosphorus. One particularly important form of these organic molecules is dissolved organic nitrogen, which is found globally in forests, arctic tundra, estuaries and streams. However, very little is known about when and where dissolved organic nitrogen serves as a source of energy, a source of nitrogen, or both to biological communities. This project will examine the role of dissolved organic nitrogen in streams across five distinct forested regions spanning a diverse set of environments in an effort to shed light on the role of dissolved organic nitrogen. This research is relevant to water resources management because dissolved organic nitrogen is a source of excessive nutrient loading causing toxic algal blooms in estuaries, and when it is found in drinking water supplies it can produce, during water treatment, disinfection by-products that can harm human health. This research will also integrate under-represented groups into the laboratory work and field work needed for the project and provide training opportunities for a graduate student and a post-doctoral scientist.Differentiating among the competing roles for dissolved organic nitrogen (DON) is difficult, as direct manipulation of the DON pool in aquatic ecosystems is most easily accomplished by addition of specific compounds such as amino acids, which are commercially available but are not particularly representative of the DON found in streams and rivers under ambient conditions. Based on initial experimental results, the researchers will conduct a series of novel field manipulations in streams across New England, Puerto Rico, Spain, Czech Republic, and Siberia to better understand the extent to which DON serves as an energy source or nitrogen source in streams globally. Initial experimental observations in New England show that the DON can serve as both a nitrogen source and an energy source, but its role appears to vary over space and time. Here the researchers will perform additional experiments where nitrate pulses are added to streams across a global network to assess how frequently, and under what conditions, DON serves predominantly as a nitrogen source, versus an energy (carbon) source. This set of experiments will provide some of the first experimental evidence to assess the importance of these two competing drivers in regulating the levels of ambient DON in natural waters.