N cycling in the world’s oceans

Nitrogen N is an essential nutrient for all living organisms (nucleic acids and amino acids) N has many oxidation states, which makes speciation and redox chemistry very interesting NH4+ is preferred N nutrient

Marine N Bioavailable/Fixed (oxidation state) NO3- 5.7*105 Tg N (+5) NH4+ 7.0*103 Tg N (-3) Organic N 5.3*105 Tg N (-3) Libes, 1992 Non-bioavailable N2O 200 Tg N (+1) N2 2.2*107 Tg N (0)

Marine Fixed N Budget Marine Reservoir: 6.3*105 Tg N Codispoti et al. (2001) Marine Reservoir: 6.3*105 Tg N Sources: 287 Tg N/yr Sinks: 482 Tg N/yr Atmospheric deposition: 86 Tg N/yr N2 fixation: 125 Tg N/yr N2O loss: 6 Tg N/yr Water Column denitrification: 150 Tg N/yr River Input: 76 Tg N/yr Organic N export: 1 Tg N/yr Benthic denitrification: 300 Tg N/yr Sedimentation: 25 Tg N/yr

N2 NH4 NO3 N2 NH4 NO3 Water column Sediment Phytoplankton Chlorophyll Fixation N2 Nitrification Mineralization NH4 NO3 Uptake Phytoplankton Grazing Chlorophyll Zooplankton Mortality Large detritus Water column Susp. particles Nitrification N2 NH4 NO3 Denitrification Aerobic mineralization Organic matter Sediment

Nitrogen Cycle http://www.petsforum.com/personal/trevor-jones/nitrogencycle.gif

Organic Matter Oxidation Sequence Morel & Herring, 1993 Respiration ΔG° (kJ/mol) -119 Denitrification -113 MnO2 reduction -96.9 Fe oxide reduction -46.7 Sulfate reduction -20.5 Methanogenesis -17.7

Alternative pathways to N2 Microbially mediated Nitrification Anammox Heterotrophic Denitrification OLAND Nitrogen Fixation Chemical Reactions MnO2 Reduction Mn2+ Oxidation

Marine Fixed N Budget Unbalanced WHY?????????????????????? N Fixation may have been underestimated Limited data on Trichodesmium and other N fixers; variability in abundances and fixation rates of organisms Recent estimates of N fixation rates have increased (Gruber and Sarmiento, 1997; Karl et al., 1997) Denitrification may have been overestimated Stoichiometric and model-based estimates used; limited data on direct denitrification measurements

My research Denitrification describes the removal of fixed N, mostly NO3-, resulting in the formation of non-biologically available N, primarily N2 gas Continental shelf sediments are responsible for up to 67% of marine denitrification estimates Sandy sediments comprise 70% of continental shelves; global estimates of denitrification are mostly based on muddy sediments Sands contain less organic matter and nutrients, and high oxygen concentrations in overlying water

Benthic primary production (BPP) Sandy sediments have low organic matter content, substrate for heterotrophic denitrification BPP supplies reactive organic matter through remineralization Organisms compete with microbes for nutrients such as NH4+ Organisms also produce oxygen during photosynthesis Role of BPP remains unclear

Isotope tracer experiments 15NH4+, 14NO3- 1A 15NO3- 14N15N, 15N15N 1B 15N15N 1C 14N15N 1D 14N14N 1E POM 1F A. Experiment 1 15NO3-, 14NH4+ 2A 14N15N, 14N14N 2B 2C 2D 2E B. Experiment 2 2F Possible outcomes of amendment experiments. 1A = Aerobic nitrification of 15NH4+; 1B = Heterotrophic denitrification with 14NO3- and/or 15NO3-; 1C = OLAND with 15NH4+ or partial nitrate reduction to nitrite followed by anammox with 15NH4+; 1D = Same as 1C except with standard nitrate; 1E = Heterotrophic denitrification with standard nitrate; 1F = Assimilation. 2A = Aerobic nitrification of standard ammonium; 2B = Heterotrophic denitrification with 14NO3- and/or 15NO3-; 2C = OLAND with standard ammonium or partial nitrate reduction to nitrite followed by anammox with standard ammonium; 2D = Same as 2C except with 15NO3-; 2E = Heterotrophic denitrification of 15NO3-; 1F = Assimilation

Sampling Sampling

Membrane Inlet Mass Spec. (MIMS)

Results W27 and Experiment 2 results suggest the presence of denitrification Experiment 1 results suggest that within the 48-hr timescale of the experiment, no alternative pathway to N2 exists in these sediments

Denitrification Rates W27 Experiment provided a rate of 21.6 µmole N m-2 d-1 R4-Experiment 2 provided rates of 22.8 & 23.2 µmole N m-2 d-1 Rates obtained from other continental shelf studies of denitrification yielded 700-3200 µmole N m-2 d-1 Other continental shelf sites studied contain higher organic matter content than Georgia sediments Georgia continental shelf sediments are oxic to at least 1-cm depth, thus inhibiting higher rates of denitrification

Discussion of results Sandy, continental shelf sediments are potentially important sites of denitrification that may have been overlooked These environments may have similar rates to current study site and if so, similar techniques can be used to measure such low rates of denitrification Denitrification was not completely inhibited by low organic matter content or benthic primary production BPP varies seasonally and spatially, yet denitrification rates were very close between two different stations during different seasons

Future work Impact of BPP can be explored further by monitoring nutrient and dissolved O2 concentrations and benthic primary production rates (monitored by SABSOON) Compare rates to Gulf of Mexico shelf denitrification rates (Nov. – Dec. 2004) Further explore the presence of alternative pathways in salt marsh sediments by using isotope tracers, 15N isotopic analyses, and HgCl2 (Oct. – Nov. 2004)

Future work (cont’d)