Effects of Ferrous Iron and Hydrogen Sulfide on Nitrate Reduction in the Sediments of an Estuary Experiencing Hypoxia

Abstract

Hypoxia is common feature of eutrophic estuaries and semi-enclosed seas globally. One of the key factors driving hypoxia is nitrogen pollution. To gain more insight into the effects of hypoxia on estuarine nitrogen cycling, we measured potential nitrate reduction rates at different salinities and levels of hypoxia in a eutrophic temperate microtidal estuary, the Neuse River Estuary, North Carolina, USA. We also tested the effect of hydrogen sulfide and ferrous iron additions on the nitrate reduction pathways. Overall, DNRA dominated over denitrification in this periodically hypoxic estuary and there was no correlation between the potential nitrate reduction rates, salinity, or dissolved oxygen. However, when hypoxia lasted several months, denitrification capacity was almost completely lost, and nearly all nitrate added to the sediment was reduced via DNRA. Additions of hydrogen sulfide stimulated DNRA over denitrification. Additions of ferrous iron stimulated nitrate consumption; however, the end product of nitrate consumption was not clear. Interestingly, substantial nitrous oxide formation occurred in sediments that had experienced prolonged hypoxia and were amended with nitrate. Given expanding hypoxia predicted with climate change scenarios and the increasing nitrate loads to coastal systems, coastal sediments may lose their capability to mitigate nitrogen pollution due to DNRA dominating over denitrification during extended hypoxic periods.

Nitrogen fixation and other biogeochemically important features of Atacama Desert giant horsetail plant microbiomes inferred from metagenomic contig analysis

“…Our finding of plant microbiome genes associated with nitrification, nitrate reduction and denitrification was consistent with evidence for extensive colonization of the sandy soils by aerenchymatous Equisetum rhizomes, as such sites are unlikely to experience extended periods of hypoxia known to inhibit nitrification and reduce denitrification ( Jäntti et al , 2021 ). Our study also detected genes such as hdrA , sox , dsr and soe associated with S oxidation or other S metabolism pathways ( Watanabe et al , 2014 , 2017 , 2019 ), though our observations suggested that S oxidation and S reduction may be performed by different bacteria at the two sites.…”

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

“…Jantti et al. (2021) reported that soil denitrification capacity was almost completely lost and DNRA dominated in estuarine soils when hypoxia lasted for several months. Additionally, differences in hydrologic variation directly influence microbial community composition, with less variable conditions being preferred by DNRA and sulfate reducing bacteria (Chen et al., 2021).…”

“…Early studies attributed this ammonium‐N accumulation to ammonification and suppression of nitrification (e.g., Doussan et al., 1998; Duval & Hill, 2007), whereas more recent studies increasingly attribute it to dissimilatory nitrate reduction to ammonium (DNRA; Chen et al., 2022; Covatti & Grischek, 2021; Liang et al., 2020; Murphy et al., 2020; Pandey et al., 2020; Su et al., 2022; Wang et al., 2020; G. Yin et al., 2017; Zhao et al., 2021). Stagnant and/or poorly mixed hydrologic conditions or flow paths with long residence time coupled with very low redox potentials and higher concentrations of electron donors such as organic carbon, dissolved iron (Fe), and sulfide have especially been shown to favor DNRA over denitrification (Burgin & Hamilton, 2007; Covatti & Grischek, 2021; Jantti et al., 2021; Reverey et al., 2018; Robertson & Thamdrup, 2017; Rutting et al., 2011; Sgourdis et al., 2011; Wang et al., 2020; G. Yin et al., 2017). Denitrification typically occurs for low organic C to nitrate‐N ratios (OC limiting) while DNRA has been reported for high organic C to nitrate‐N ratios (nitrate‐N limiting; >10–15; Chen et al., 2022; Pandey et al., 2020; Wei et al., 2022; S. Yin et al., 1998).…”

“…However, persistent hypoxic or anoxic (hypoxic <2 mg L −1 , anoxic ∼0 mg/L; Diaz & Rosenberg, 2008) and reducing soil conditions can increase N concentrations in riparian and wetland soils through accumulation of ammonium‐N (Duval & Hill, 2007; Jantti et al., 2021; Liang et al., 2020; Reverey et al., 2018; Wang et al., 2020). Early studies attributed this ammonium‐N accumulation to ammonification and suppression of nitrification (e.g., Doussan et al., 1998; Duval & Hill, 2007), whereas more recent studies increasingly attribute it to dissimilatory nitrate reduction to ammonium (DNRA; Chen et al., 2022; Covatti & Grischek, 2021; Liang et al., 2020; Murphy et al., 2020; Pandey et al., 2020; Su et al., 2022; Wang et al., 2020; G. Yin et al., 2017; Zhao et al., 2021).…”

The role of organic matter and microbial community controlling nitrate reduction under elevated ferrous iron concentrations in boreal lake sediments

“…Both analyzes were optimized for 1 ml sample volumes. The 15 N 2 , 15 N 2 O, and 15 NH 4 + were analyzed as per Jäntti et al (2021; described in detail in Supplement 1).…”