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.
Highlights
IntroductionMicrobes in coastal sediments provide an important ecosystem service by converting reactive N through microbial pathways to dinitrogen (N2) and nitrous oxide (N2O) gases (Seitzinger 1988; Dalsgaard et al 2005)
The amount of reactive nitrogen (N) in the environment has increased dramatically during the past 150 years, which has Communicated by Marco Bartoli Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.promoted eutrophication of coastal waters (Paerl and Piehler 2008)
Based on the results of this and other experiments, we conclude that the length of hypoxia has a substantial effect on the N cycling processes in carbon and SO42− rich sediments and that the role of Fe2+ has to investigated more thoroughly in hypoxic estuarine sediments where H2S accumulation occurs
Summary
Microbes in coastal sediments provide an important ecosystem service by converting reactive N through microbial pathways to dinitrogen (N2) and nitrous oxide (N2O) gases (Seitzinger 1988; Dalsgaard et al 2005). Under hypoxic conditions, N removal in coastal sediments decreases, since nitrate (NO3−) is reduced to ammonium (NH4+), instead of to N2 and N2O, via the dissimilatory nitrate reduction to ammonia (DNRA) pathway (An and Gardner 2002; Gardner et al 2006; Dong et al 2011). Chemolithoautotrophic DNRA in the other hand is not dependent on the availability of organic carbon (An and Gardner 2002; Gardner et al 2006; Dong et al.2011). Sedimentary nitrification, which typically provides most substrate for the NO3− reducing processes (i.e., Hietanen and Kuparinen 2008), ceases and DNRA rates are inhibited due to the low availability of electron acceptors. Hypoxia creates favorable conditions for DNRA; the presence of this process is still poorly quantified in estuarine systems
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