Regulation of reactive nitrogen transport and transformation in contaminated environments

农业活动导致陆地流域内一些化学、辐射和生物活性氮(N)物种的释放迅速增加。这些活性氮化合物通过从土壤表面淋失含水的无机氮物种，包括氨(NH4)和硝酸盐(NO3-)，以及通过增加一氧化二氮(N2O)的排放而改变气候系统，这些活性氮化合物在水质恶化中发挥了重要作用。一氧化二氮是NH4和NO3-在流域中转化的产物，因此通过这些物种的损失来促进水质改善的分水岭地区可能会显示出这种气体的排放增加。当无机氮丰富时，这些氮的转化和交换受到氧化还原条件和温度的密切调控。 田间研究很少对淋溶N在农业流域中产生的N2O排放量进行量化，这些研究往往侧重于直接影响点(即土壤表面)。然而，通过人为土地利用活动释放到土壤中的很大一部分活性氮被淋溶到相互关联的水生系统中，在那里它随后被去除或转化。产生N2O的氮素转化过程可以通过其独特的同位素指纹(即d15N和d18O)来区分。由于稳定同位素是将N2O分配给实地来源和(或)转化过程的唯一手段，它们很可能成为今后十年的重要监测工具，特别是考虑到在开发现场便携式和高灵敏度N2O同位素分析仪方面的最新进展。 拟议的研究计划的目标是量化速率和表征反应性N转化的同位素模式，这些反应N转化导致温带农业为主的流域经历高N负荷的淋溶N排放N2O。这将通过以下方式实现：选择提供描述与农业活动有关的淋溶N负荷条件的模式系统的田间地点；调查这些地点作为N2O前体的无机N化合物的迁移、转化和同位素信号；研究这些地点在不同氧化还原状态、温度条件和N负荷水平下与N2O释放有关的速率和同位素特征；通过同时分析碳基温室气体，将N2O释放放在分水岭潜在全球变暖的背景下；并确定观察到的化学模式作为长期N2O排放监测和缓解工具的用途。

Agricultural activities have led to a rapid growth in the release of a number of chemically, radiatively, and biologically reactive nitrogen (N) species within terrestrial watersheds. These reactive N compounds play important roles in the deterioration of water quality through leaching losses from soil surfaces of aqueous inorganic N species, including ammonium (NH4+) and nitrate (NO3-), and through changes to the climate system via increased emissions of nitrous oxide (N2O), a powerful and long-lived greenhouse gas and stratospheric ozone depleting compound. Nitrous oxide is a product of the transformation of both NH4+ and NO3- in watersheds, so watershed zones that facilitate improved water quality through losses of these species may show enhanced emissions of this gas. These N transformations and exchanges are closely regulated by redox conditions and temperature when inorganic N is abundant. Emissions of N2O in agricultural watersheds arising from leached N are seldom quantified in field studies, which tend to focus upon the point of immediate impact (i.e., soil surfaces). However, a significant proportion of reactive N released to soils through anthropogenic land use activities is leached into linked aquatic systems, where it is subsequently removed or transformed. Nitrogen transformation processes that generate N2O can be differentiated by their unique isotopic fingerprints (i.e., d15N and d18O). As stable isotopes provide the only means by which N2O can be apportioned to sources and/or transformation processes in the field, they are likely to factor in as crucial monitoring tools over the next decade, particularly in light of recent advances in the development of field portable and high-sensitivity N2O isotopic analyzers. The objective of the proposed research program is to quantify rates and characterize isotopic patterns of reactive N transformations that lead to N2O emissions from leached N in temperate zone agriculturally dominated watersheds experiencing elevated N loading. This will be accomplished by: selecting field sites that provide model systems for characterizing leached N loading conditions associated with agricultural activities; investigating the transport, transformation, and isotopic signals of aqueous inorganic N compounds that are precursors to N2O at these sites; studying the rates and isotopic signatures associated with N2O released under varying redox states, thermal conditions, and N loading levels at these sites; placing N2O release in the context of overall global warming potentials for the watershed points of study through concurrent analysis of carbon-based greenhouse gases; and identifying the utility of the observed chemical patterns for use as longer-term N2O emission monitoring and mitigation tools.