Effects of experimental warming and carbon addition on nitrate reduction and respiration in coastal sediments

Background/Question/Methods

Warming ocean temperatures associated with climate change may affect biogeochemical cycling in coastal ecosystems, with consequences for ecosystem nitrogen (N) availability. In coastal sediments, denitrification and anammox (anaerobic ammonium oxidation) reduce NO₃^- or NO₂^- to N₂, removing biologically available N from the system. In contrast, DNRA (dissimilatory NO₃^- reduction to ammonium) recycles N within the sediments. Differential effects of warming on these microbially-mediated processes could affect the balance of ecosystem N availability. Temperature effects may also be mediated by carbon availability. To explore potential effects of warming and carbon availability on NO₃^-/NO₂^- reduction, we conducted a microcosm experiment with continental shelf sediments, using a system that allowed for coupled aerobic-anaerobic processes to occur in the sediments. After a 2-week pre-incubation period, sediments were incubated at ambient winter (4°C) and warmer summer (17°C) temperatures for 12 weeks, with and without regular carbon additions comparable to in situ C deposition, yielding 4 treatments in a full-factorial design. Throughout the incubation, we measured sediment O₂ profiles to determine O₂ consumption, and water column concentrations of dissolved inorganic N to determine net fluxes across the sediment-water interface. At the end of the experiment, potential NO₃^-/NO₂^- reduction rates were measured at 4 and 17°C. 

Results/Conclusions

Carbon addition and warming both increased O₂ consumption, although carbon addition had a greater effect. C addition also caused effluxes of NH₄⁺ from sediments early in the 12-week incubation. All sediments displayed net NO₃^- consumption at the beginning of incubations, but NO₃^- consumption decreased with C addition and warming, and switched to net efflux in sediments receiving both treatments. Along with data from potential NO₃^-/NO₂^- reduction rate measurements, these results suggest that warming and C addition increased NH₄⁺ production by remineralization and possibly DNRA, resulting in a stimulation of nitrification and thus NO₃^- production, without a comparable increase in denitrification and anammox. This interpretation was supported by the finding that warming and C addition together stimulated DNRA potential rates, but not denitrification potential rates. Taken together, these results suggest that DNRA may be an important and previously overlooked process in continental shelf sediments, and that warming and increased organic matter availability or deposition could shift the role of shelf sediments from N removal to N recycling through the unexpected link of DNRA and nitrification.