Nitrogen (N) pollution remains a crucial environmental problem in coastal waters affected by eutrophication globally, but the sources of the pollution are not often well quantified locally. This study explored how land use influences N concentrations and sources along 400 km of the Potomac River – the second largest tributary of the Chesapeake Bay located in the Mid-Atlantic United States. Our results showed that total dissolved nitrogen and nitrate (TDN and NO₃⁻-N) in tributaries of the Potomac River were significantly correlated with watershed cropland percentage (R² = 0.68, n = 31). As a result, TDN and NO₃⁻-N concentrations along the Potomac River mainstem increased sharply from forest reaches to agricultural reaches followed by slowly decreasing downstream along urban reaches near Washington DC. NO₃⁻-N and oxygen isotope ratios (δ¹⁵N–NO₃^- and δ¹⁸O–NO₃^-), both of which were highest in tributaries draining urban land use and lowest in tributaries draining forest land use, generally increased along the Potomac River mainstem across the forest-agricultural-urban land use gradient. Source tracking using δ¹⁵N–NO₃^- and δ¹⁸O–NO₃^- suggested that nitrate inputs from agriculture were the main sources in the major segments of the Potomac River, and inputs from forest soils and wastewater treatment plants were important in the headwater and the section below Washington D.C., respectively. TDN load estimations indicated >66% of TDN load of the Potomac River was from the agricultural zone, and >36% of TDN from agricultural and forested areas was retained within the urban river section during the summer low-streamflow period. This study highlights the importance of nonpoint agricultural sources to the Potomac River, as well as N retention within river channel. Given that some agricultural sources may persist further downriver even as urbanization increases, isotope source tracking can be used to prioritize more effective source reduction strategies in the Chesapeake Bay Watershed.

在受全球富营养化影响的沿海水域，氮 (N) 污染仍然是一个关键的环境问题，但污染的来源在当地往往没有得到很好的量化。本研究探讨了土地利用如何影响波托马克河（位于美国中大西洋切萨皮克湾的第二大支流）400 公里沿线的氮浓度和来源。我们的结果表明，波托马克河支流中的总溶解氮和硝酸盐（TDN 和NO ₃ ^- -N）与流域农田百分比显着相关（R ²  = 0.68，n = 31）。因此，TDN 和 NO ₃ ^-- 沿波托马克河干流的 N 浓度从森林河段到农业河段急剧增加，然后在华盛顿特区附近的城市河段下游缓慢下降。NO ₃ ⁻ -N与氧同位素比值（δ ¹⁵ N–NO ₃ ^-和δ ¹⁸ O–NO ₃ ^-），城市土地利用支流最高，林地利用支流最低，总体呈上升趋势。波托马克河干流跨越森林-农业-城市土地利用梯度。使用δ ¹⁵ N–NO ₃ ^-和δ ¹⁸ O–NO 进行源追踪₃ ^-表明农业的硝酸盐输入是波托马克河主要河段的主要来源，森林土壤和废水处理厂的输入分别在源头和华盛顿特区以下部分很重要。TDN 负荷估计表明，波托马克河超过 66% 的 TDN 负荷来自农业区，而在夏季低流量期间，超过 36% 的来自农业和林区的 TDN 保留在城市河段内。本研究强调了非点农业源对波托马克河的重要性，以及河道内的氮保留。鉴于即使城市化程度提高，一些农业来源仍可能继续向下游流动，