Search for: All records

Many estuaries experience eutrophication, deoxygenation and warming, with potential impacts on greenhouse gas emissions. However, the response of N₂O production to these changes is poorly constrained. Here we applied nitrogen isotope tracer incubations to measure N₂O production under experimentally manipulated changes in oxygen and temperature in the Chesapeake Bay—the largest estuary in the United States. N₂O production more than doubled from nitrification and increased exponentially from denitrification when O₂was decreased from >20 to <5 micromolar. Raising temperature from 15° to 35°C increased N₂O production 2- to 10-fold. Developing a biogeochemical model by incorporating these responses, N₂O emissions from the Chesapeake Bay were estimated to decrease from 157 to 140 Mg N year⁻¹from 1986 to 2016 and further to 124 Mg N year⁻¹in 2050. Although deoxygenation and warming stimulate N₂O production, the modeled decrease in N₂O emissions, attributed to decreased nutrient inputs, indicates the importance of nutrient management in curbing greenhouse gas emissions, potentially mitigating climate change. 

Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here, we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO2-) as a product, reactant, or intermediate, in the eastern tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruisespresent additional evidence that NO3- reduction is the predominant OMZ N flux, followed by NO2- oxidation back to NO3-. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200 times) thanthe combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic–oxic interface. We also showthat NO2- oxidation can occur when O2 is maintained near 1 nM by a continuous-purge system, NO2-oxidation and O2 measurements that further strengthen the case for truly anaerobic NO2- oxidation. We also evaluate thepossibility that NO2- dismutation provides the oxidative power for anaerobic NO2- oxidation. The partitioning ofN loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact,N loss rates at many depths were entirely due to anammox. Our new NO3- reduction, NO2- oxidation, dismutation, andN loss data shed light on many open questions in OMZ N cycling research, especially the possibility of truly anaerobicNO2- oxidation. 

Abstract The ocean is a net source of N 2 O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N 2 O via microbial N 2 O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N 2 O consumption measurements from anoxic zones to the sharp oxygen gradient above them, and experimentally determine kinetic parameters in both oxic and anoxic seawater for the first time. We find that the substrate affinity, O 2 tolerance, and community composition of N 2 O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N 2 O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N 2 O cycling. Microbes from the oxic layer consume N 2 O under anoxic conditions at a much faster rate than microbes from anoxic zones. These experimental results are in keeping with model results which indicate that N 2 O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O 2 and N 2 O gradients right above the ODZ is a previously ignored potential gatekeeper of N 2 O and should be accounted for in the marine N 2 O budget.