Effects of experimental warming and carbon addition on nitrate reduction and respiration in coastal sediments
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Climate change may have differing effects on microbial processes that control coastal N availability. We conducted a microcosm experiment to explore effects of warming and carbon availability on nitrate reduction pathways in marine sediments. Sieved continental shelf sediments were incubated for 12 weeks under aerated seawater amended with nitrate (~50 μM), at winter (4 °C) or summer (17 °C) temperatures, with or without biweekly particulate organic C additions. Treatments increased diffusive oxygen consumption as expected, with somewhat higher effects of C addition compared to warming. Combined warming and C addition had the strongest effect on nitrate flux across the sediment water interface, with a complete switch early in the experiment from influx to sustained efflux. Supporting this result, vial incubations with added 15N-nitrate indicated that C addition stimulated potential rates of dissimilatory nitrate reduction to ammonium (DNRA), but not denitrification. Overall capacity for both denitrification and DNRA was reduced in warmed treatments, possibly reflecting C losses due to increased respiration with warming. Anammox potential rates were much lower than DNRA or denitrification, and were slightly negatively affected by warming or C addition. Overall, results indicate that warming and C addition increased ammonium production through remineralization and possibly DNRA. This stimulated nitrate production through nitrification, but without a comparable increase in nitrate consumption through denitrification. The response to C of potential DNRA rates over denitrification, along with a switch to nitrate efflux, raises the possibility that DNRA is an important and previously overlooked source of internal N cycling in shelf sediments.
KeywordsAnammox Climate change Continental shelf Denitrification Dissimilatory nitrate reduction to ammonium Warming
Thank you to Michaeline Nelson, Amber Hardison, Heather Leslie, Stephen Porder and Ruby Ho for help in the lab and the field, and to Elise Heiss, Lindsey Fields, Shelley Brown, Wally Fulweiler, Scott Nixon, Bethany Jenkins, Jane Tucker, Steve Granger, Rodman Sykes, Rebecca Robinson, Catherine Luria, Marshall Otter, David Murray, Joe Orchardo and Karen Kidd. This material is based upon work supported by the National Science Foundation by OCE-0852289 to JJR and OCE-0852263 and OCE-0927400 to AEG, and Rhode Island Sea Grant to JJR.
Conflict of Interest
The authors declare that they have no conflict of interest.
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