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Groundwater nitrification and denitrification are not always strictly aerobic and anaerobic processes, respectively: an assessment of dual-nitrate isotopic and chemical evidence in a stratified alluvial aquifer
Biogeochemistry ( IF 4 ) Pub Date : 2020-01-01 , DOI: 10.1007/s10533-020-00637-y
Ahamefula U. Utom , Ulrike Werban , Carsten Leven , Christin Müller , Kay Knöller , Carsten Vogt , Peter Dietrich

Nitrification and denitrification are traditionally assumed to occur under aerobic and anoxic conditions, respectively. However, new and interesting alternatives challenge the traditional assumption. Along this line, we provide dual-nitrate isotopic and chemical evidence for the occurrence of denitrification linked with heterotrophic nitrification in an upper oxic region and nitrification in an underlying low-oxygen (sub-oxic/anoxic) layer of a stratified and channelized alluvial aquifer. Particularly significant is the source of the oxidant required for nitrification within the deeper low-oxygen layer. Combined with the existence of steep geochemical gradients, the introduction of a favorable manganese oxide oxidant from the upper oxic layer into the underlying low oxygen region during diffusive mixing resulted in the higher concentrations of nitrate observed (due to the anoxic reoxidation of nitrite to nitrate) as well as the strong positive correlation of nitrite/nitrate with manganese (II) ion concentrations (in the deeper anoxic layer compared with the shallower oxic layer). The observations and findings presented herein have implications for not only reconciling the discrepancies in such unconventional pathways of nitrogen metabolism between groundwater ecosystems and river/stream/soil/marine systems (where such processes have most commonly been reported) but also for devising effective nitrogen nutrient remediation and complex nitrogen-cycling modeling strategies.

中文翻译:

地下水硝化和反硝化并不总是严格的好氧和厌氧过程:分层冲积含水层中双重硝酸盐同位素和化学证据的评估

传统上假设硝化和反硝化分别发生在有氧和缺氧条件下。然而,新的有趣的替代方案挑战了传统的假设。沿着这条路线,我们提供了双重硝酸盐同位素和化学证据,证明反硝化作用的发生与上层好氧区的异养硝化作用和分层和通道化冲积含水层的底层低氧(亚氧/缺氧)层中的硝化作用有关. 特别重要的是在更深的低氧层内硝化所需的氧化剂的来源。结合陡峭的地球化学梯度的存在,在扩散混合过程中,将有利的氧化锰氧化剂从上部好氧层引入下面的低氧区域导致观察到的硝酸盐浓度更高(由于亚硝酸盐再氧化为硝酸盐的缺氧)以及亚硝酸盐的强正相关/硝酸盐与锰(II)离子浓度(与较浅的好氧层相比,在较深的缺氧层中)。本文提出的观察和发现不仅对调和地下水生态系统与河流/溪流/土壤/海洋系统(此类过程最常被报道的地方)之间这种非常规氮代谢途径的差异有影响,而且对设计有效的氮养分也有影响。修复和复杂的氮循环建模策略。
更新日期:2020-01-01
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