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
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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.
KeywordsStratified aquifer Nitrogen biogeochemical cycling Steep concentration gradients Oxic denitrification Anoxic nitrification Manganese oxide reduction
This research was supported by the German Academic Exchange Service (DAAD) for a doctoral study program and in part by the Collaborative Research Center 1253 CAMPOS (Project 3: Floodplain Hydrology) funded by the German Research Foundation (DFG, Grant Agreement SFB 1253/1 2017). We thank Helko Kotas and Andreas Schoßland from the Department of Monitoring and Exploration Technologies, Helmholtz Center for Environmental Research (UFZ), Leipzig, for their valuable assistance with the water sample collection. We are also grateful to the following for their assistance in coordinating the laboratory activities: Dr. Sybille Mothes (Analytical Chemistry Department, UFZ), Petra Blümel and Martina Neuber (Catchment Hydrology, UFZ, Halle), and Dr. Marc Schwientek (Center for Applied Geoscience (ZAG), Eberhard Karls Universität Tübingen).
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Conflict of interest
The authors declare that they have no competing interests.
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