Milldams and their legacies have significantly influenced fluvial processes and geomorphology. However, less is known about their effects on riparian zone hydrology, biogeochemistry, and water quality. Here, we discuss the potential effects of existing and breached milldams on riparian nitrogen (N) processing through multiple competing hypotheses and observations from complementary studies. Competing hypotheses characterize riparian zone processes that remove (sink) or release (source) N. Elevated groundwater levels and reducing soil conditions upstream of milldams suggest that riparian zones above dams could be hotspots for N removal via denitrification and plant N uptake. On the other hand, dam removals and subsequent drops in stream and riparian groundwater levels result in drained, oxic soils which could increase soil nitrification and decrease riparian plant uptake due to groundwater bypassing the root zone. Whether dam removals would result in a net increase or decrease of N in riparian groundwaters is unknown and needs to be investigated. While nitrification, denitrification, and plant N uptake have typically received the most attention in riparian studies, other N cycle processes such as dissimilatory nitrate reduction to ammonium (DNRA) need to be considered. We also propose a novel concept of riparian discontinuum, which highlights the hydrologic and biogeochemical discontinuities introduced in riparian zones by anthropogenic structures such as milldams. Understanding and quantifying how milldams and similar structures influence the net source or sink behavior of riparian zones is urgently needed for guiding watershed management practices and for informed decision making with regard to dam removals.

米尔达姆斯（Milldams）及其遗留遗产极大地影响了河床过程和地貌。然而，关于它们对河岸带水文学，生物地球化学和水质的影响知之甚少。在这里，我们通过多种相互竞争的假设和补充研究的观察结果，讨论了现有和突破的米尔丹对河岸氮（N）加工的潜在影响。相互竞争的假说表征了去除（下沉）或释放（来源）N的河岸带过程。地下水位的升高和米尔达姆上游上游土壤条件的减少表明，大坝上方的河岸带可能是通过反硝化作用和植物吸收N去除氮的热点。另一方面，大坝的拆除以及随后溪流和河岸地下水水位的下降导致排水，由于地下水绕过根区，可能会增加土壤硝化作用并减少河岸植物吸收的含氧土壤。拆除大坝是否会导致河岸地下水中氮的净增加或减少尚不清楚，需要进行调查。在河岸研究中，虽然硝化，反硝化和植物吸收氮通常受到最多的关注，但还需要考虑其他氮循环过程，例如异化硝酸盐还原为铵盐（DNRA）。我们还提出了一种河岸间断的新概念，该概念强调了人为结构（如米尔达姆斯）在河岸带引入的水文和生物地球化学间断。