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1. Draining the Landscape: How Do Nitrogen Concentrations in Riparian Groundwater and Stream Water Change Following Milldam Removal?

   https://doi.org/10.1029/2021JG006444  

   Lewis, Evan; Inamdar, Shreeram; Gold, Arthur J.; Addy, Kelly; Trammell, Tara L. E.; Merritts, Dorothy; Peipoch, Marc; Groffman, Peter M.; Hripto, Johanna; Sherman, Melissa; et al (August 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract Dam removals are on the increase across the US with Pennsylvania currently leading the nation. While most dam removals are driven by aquatic habitat and public safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals decrease the stream base level, which results in dewatering of the riparian zone. We hypothesized that this dewatering of the riparian zone would increase nitrification and decrease denitrification, and thus result in nitrogen (N) leakage from riparian zones. This hypothesis was tested for a 1.5 m high milldam removal. Stream, soil water, and groundwater N concentrations were monitored over 2 years. Soil N concentrations and process rates andδ¹⁵N values were also determined. Denitrification rates and soilδ¹⁵N values in riparian sediments decreased supporting our hypothesis but no significant changes in nitrification were observed. While surficial soil water nitrate‐N concentrations were high (median 4.5 mg N L⁻¹), riparian groundwater nitrate‐N values were low (median 0.09 mg N L⁻¹), indicating that nitrate‐N leakage was minimal. We attribute the low groundwater nitrate‐N to denitrification losses at the lower, more dynamic, groundwater interface and/or dissimilatory nitrate reduction to ammonium (DNRA). Stream water nitrate‐N concentrations were high (median 7.6 mg N L⁻¹) and contrary to our dam‐removal hypothesis displayed a watershed‐wide decline that was attributed to regional hydrologic changes. This study provided important first insights on how dam removals could affect N cycle processes in riparian zones and its implications for water quality and watershed management. 

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2. Nitrate respiration and diel migration patterns of diatoms are linked in sediments underneath a microbial mat

   https://doi.org/10.1111/1462-2920.15345  

   Merz, Elisa; Dick, Gregory_J; de_Beer, Dirk; Grim, Sharon; Hübener, Thomas; Littmann, Sten; Olsen, Kirk; Stuart, Dack; Lavik, Gaute; Marchant, Hannah_K; et al (December 2020, Environmental Microbiology)

   Summary Diatoms are among the few eukaryotes known to store nitrate (NO₃⁻) and to use it as an electron acceptor for respiration in the absence of light and O₂. Using microscopy and¹⁵N stable isotope incubations, we studied the relationship between dissimilatory nitrate/nitrite reduction to ammonium (DNRA) and diel vertical migration of diatoms in phototrophic microbial mats and the underlying sediment of a sinkhole in Lake Huron (USA). We found that the diatoms rapidly accumulated NO₃⁻at the mat‐water interface in the afternoon and 40% of the population migrated deep into the sediment, where they were exposed to dark and anoxic conditions for ~75% of the day. The vertical distribution of DNRA rates and diatom abundance maxima coincided, suggesting that DNRA was the main energy generating metabolism of the diatom population. We conclude that the illuminated redox‐dynamic ecosystem selects for migratory diatoms that can store nitrate for respiration in the absence of light. A major implication of this study is that the dominance of DNRA over denitrification is not explained by kinetics or thermodynamics. Rather, the dynamic conditions select for migratory diatoms that perform DNRA and can outcompete sessile denitrifiers. 

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3. Microbial communities (bacteria, archaea and eukaryotes) in a temperate estuary during seasonal hypoxia

   https://doi.org/10.3354/ame01982  

   Santoferrara, LF; McManus, GB; Greenfield, DI; Smith, SA (February 2022, Aquatic Microbial Ecology)

   Eutrophication and hypoxia markedly alter trophic dynamics and nutrient cycling in estuarine water columns, but little is known about the microbial communities that drive and interact with these changes. Here we studied microbial plankton (bacteria, archaea, protists and micro-metazoans) in a large temperate estuary where bottom hypoxia occurs every summer due to warmer temperatures, stratification, and oxidation of organic matter fueled by nutrient enrichment. We used high-throughput sequencing of the 16S and 18S rRNA genes (V4 region) and quantified multiple abiotic and biotic factors in surface and bottom waters during the summer of 2019. The conditions associated with the intensification of hypoxia in bottom waters as the summer progressed were linked to significant changes in the diversity, community structure and potential functioning of microbial communities. Under maximum hypoxia (dissolved oxygen concentration: 0.9-3.1 mg l -1 ), there were increased proportions of ammonia-oxidizing archaea (AOA), bacterivorous and parasitic protists, and copepod nauplii. Sequence proportions of AOA ( Nitrosopumilus) and nitrite-oxidizing bacteria ( Nitrospinaceae) were significantly correlated with the concentration of oxidized N species (nitrite plus nitrate, which peaked at 14.4 µM) and the proportions of nauplii DNA sequences and biomass. Our data support a tight coupling of biogeochemical and food web processes, with rapid oxidation of ammonia and accumulation of oxidized N species as hypoxia intensifies during the summer. 

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4. Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

   https://doi.org/10.1029/2022JG007138  

   Inamdar, Shreeram P.; Peck, Erin K.; Peipoch, Marc; Gold, Arthur J.; Sherman, Melissa; Hripto, Johanna; Groffman, Peter M.; Trammell, Tara L. E.; Merritts, Dorothy J.; Addy, Kelly; et al (December 2022, Journal of Geophysical Research: Biogeosciences)

   Abstract The compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial‐scale legacies of milldams and decadal‐scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid‐Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a 2 year period. Clay and silt‐rich legacy sediments with low hydraulic conductivity, stagnant and poorly mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium‐N accumulation in near‐stream sediments and groundwaters. Riparian groundwater ammonium‐N concentrations upstream of the milldams ranged from 0.006 to 30.6 mgN L⁻¹while soil‐bound values were 0.11–456 mg kg⁻¹. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25–1,504 mg L⁻¹) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium‐N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant. 

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5. Well-Aerated Southern Appalachian Forest Soils Demonstrate Significant Potential for Gaseous Nitrogen Loss

   https://doi.org/10.3390/f10121155  

   Baas, Peter; Knoepp, Jennifer D.; Mohan, Jacqueline E. (December 2019, Forests)

   null (Ed.)

   Understanding the dominant soil nitrogen (N) cycling processes in southern Appalachian forests is crucial for predicting ecosystem responses to changing N deposition and climate. The role of anaerobic nitrogen cycling processes in well-aerated soils has long been questioned, and recent N cycling research suggests it needs to be re-evaluated. We assessed gross and potential rates of soil N cycling processes, including mineralization, nitrification, denitrification, nitrifier denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) in sites representing a vegetation and elevation gradient in the U.S. Department of Agriculture (USDA) Forest Service Experimental Forest, Coweeta Hydrologic Laboratory in southwestern North Carolina, USA. N cycling processes varied among sites, with gross mineralization and nitrification being greatest in high-elevation northern hardwood forests. Gaseous N losses via nitrifier denitrification were common in all ecosystems but were greatest in northern hardwood. Ecosystem N retention via DNRA (nitrification-produced NO3 reduced to NH4) ranged from 2% to 20% of the total nitrification and was highest in the mixed-oak forest. Our results suggest the potential for gaseous N losses through anaerobic processes (nitrifier denitrification) are prevalent in well-aerated forest soils and may play a key role in ecosystem N cycling. 

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