Nitrogen Sinks or Sources? Denitrification and Nitrogen Removal Potential in Riparian Legacy Sediment Terraces Affected by Milldams
Nitrogen Sinks or Sources? Denitrification and Nitrogen Removal Potential in Riparian Legacy Sediment Terraces Affected by Milldams
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DOI：
10.1029/2022jg007004
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发表时间：
2022-09
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影响因子：
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通讯作者：
E. Peck;S. Inamdar;M. Sherman;Johanna Hripto;M. Peipoch;A. Gold;K. Addy
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文献类型：
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作者：
E. Peck;S. Inamdar;M. Sherman;Johanna Hripto;M. Peipoch;A. Gold;K. Addy
河岸带是重要的生态交错带，通过反硝化等过程去除氮（N）来缓冲水生生态系统。然而，大坝如何改变河岸氮循环和缓冲能力知之甚少。在这里，我们假设，地下水升高和缺氧，由于备份的溪流以上milldams可能会增强反硝化作用。我们评估了反硝化速率（使用反硝化酶测定）和潜在的控制因素在河岸沉积物在不同深度的上游和下游的两个残留的美国中大西洋milldams。反硝化作用在上游和下游之间没有显着差异，尽管考虑到更深和更宽的几何形状，上游每公里河流的反硝化作用更大。此外，反硝化作用通常发生在水文变化的浅层沉积物中，其中硝酸盐-N和有机物最集中。在低于1米的深度，反硝化和硝态氮下降，而铵态氮浓度大幅增加，表明抑制铵消耗或异化硝酸盐还原为铵。这些结果表明，反硝化作用发生在动态地下水位导致硝化和矿化速率较高的地方，而另一种产生铵氮的氮过程与反硝化作用竞争更深，更停滞/混合不良的深度有限的硝酸盐氮。最终，虽然目前还不清楚是否残留milldams的N源，有限的反硝化速率表明，他们并不总是有效的汇，因此，milldam去除，特别是伴随着去除铵氮丰富的遗留沉积物，可能会提高河岸氮缓冲。
Riparian zones are key ecotones that buffer aquatic ecosystems through removal of nitrogen (N) via processes such as denitrification. However, how dams alter riparian N cycling and buffering capacity is poorly understood. Here, we hypothesized that elevated groundwater and anoxia due to the backup of stream water above milldams may enhance denitrification. We assessed denitrification rates (using denitrification enzyme assays) and potential controlling factors in riparian sediments at various depths upstream and downstream of two relict U.S. mid‐Atlantic milldams. Denitrification was not significantly different between upstream and downstream, although was greater per river km upstream considering deeper and wider geometries. Further, denitrification typically occurred in hydrologically variable shallow sediments where nitrate‐N and organic matter were most concentrated. At depths below 1 m, both denitrification and nitrate‐N decreased while ammonium‐N concentrations substantially increased, indicating suppression of ammonium consumption or dissimilatory nitrate reduction to ammonium. These results suggest that denitrification occurs where dynamic groundwater levels result in higher rates of nitrification and mineralization, while another N process that produces ammonium‐N competes with denitrification for limited nitrate‐N at deeper, more stagnant/poorly mixed depths. Ultimately, while it is unclear whether relict milldams are sources of N, limited denitrification rates indicate that they are not always effective sinks; thus, milldam removal—especially accompanied by removal of ammonium‐N rich legacy sediments—may improve riparian N buffering.