Aerobic Denitrification: Implications for Nitrogen Fate Modeling in the Missouri-Ohio-Mississippi (MOM) River Basin Robert Thomas1, John Washington1, Lidia Samarkina2 1US EPA ORD/NERL/ERD, Athens, GA; 2Senior Service America, Inc. Introduction Methods ¦ N-compunds are the cause in -50% of impaired water bodies. • 1.6 million metric tons of N compounds discharged annually from the MOM Basin. ¦ NO3- is dominant N species; understanding its behavior is essential for N-fate modeling. ¦ NO3- has risen 3 -10 fold since the early 1900s in major US rivers. ¦ High NO3- renders groundwater impotable, rivers degraded & coastal productivity impacted. • Denitrification is an important process for removing N03- from ecosystem because inert N2 is the final product; however denitrification is a step-wise biogeochemical reduction of N03_: NO, MO, NO n2o N, Moreover, convention holds that denitrification is a strictly anaerobic process. pfiesteria? • Four microcosm reactors were constructed. • 800 ml_ of filtered (0.2|im) river water added to each jar and mixed with a magnetic stir bar. • Added Ca-nitrate to 10 mg/L as N final concentration • 1 jar as control; 3 jars inoculated with 3 ml_ of agricultural wetland sediment slurry (0.01 mg solids dry wt). • Reactors were sealed with atmospheric-gas headspace, which was recirculated through the solution. • Headspace gas and the solution were sampled twice daily for N20 and N03-, respectively » Dissolved 02 measured by Fiber Optic Oxygen (FOXY) Electrodes l Eutrophication may stimulate harmful i species • The propensity for reduction reactions to proceed is often predicted by the oxidation state of a system. > TEAP - widely accepted model for characterizing oxidation state. - Microbes reduce oxidants sequentially from high to low energy. » Thus, under the TEAP model denitrification does not proceed until all of the oxygen is reduced or removed from the system. > However, with thermodynamic analyses of field data, Washington et al. (2005) found dissolved oxygen and Corg draw other couples toward potentials imposed by them, generating a bimodal clustering. Results Dissolved NO3 & N2O - Control 0.025 - N03sv 2 0.02 - y = 0.1181x -3680.6 R2 = 0.0105 B.*>K -« Low 13-Apr 23-Apr 3-May 13-May Control jar only modest increase in N20 to slightly above atmospheric values and no significant decrease in N03", while the inoculated jar produced N20 almost 3 times greater than atmosphere accompanied by significant reduction in N03- at p = 0.99. Dissolved oxygen was -6.4 mg/L in both jars for the duration of the experiment. * High Dissolved NO3 & N^O - Inoculated 0.025 -| i[ 0.02 - ° 0.015 - £ 0.01 - **.H W S 930 \ 910 o 13-Apr 23-Apr 3-May 13-May Date (Mori'YR) Log [Reactant] (M) • While the redox clustering indicates thermodynamic control, the two potential clusters, instead of one, suggests kinetic constraints as well. > Moreover, the rate of reaction is determined more by concentration than relative potential with reactants < 10_6M being slow to react & divergent from the clusters, whereas reactants >10-6M are near equilibrium & in a cluster These results have been corroborated with aerobic experiments on synthetic solutions we have inoculated and several environmental samples, including: spring, wetland, and river waters; GA wetland sediment, TN soil, SC pond sediment, and SC estuarine sediment suspensions. In every environmental medium we have tested, without exception, we have observed significant N20 ingrowth under aerobic conditions, supporting that aerobic denitrification is a common environmental process. Hypothesis Conclusions • These observations suggest that reactions of higher-potential oxidants with lower-potential reductants commonly proceed simultaneously, regardless of the presence of other potential reactants > Thus, redox reactions can proceed in parallel as opposed to sequentially as often conceived in the TEAP conceptual model. > Conventional thought long has held that nitrate reduction, or denitrification, does not take place when oxygen is present. > However, based on the thermodynamic modeling, dissolved 02 and N03- were reduced to nearly equal redox potentials in every sample we analyzed, suggesting simultaneous reduction of 02 and N03_. We propose that denitrification is NOT a strictly anaerobic process Disclaimer: Although this work was reviewed by EPA and approved for presentation, it may not necessarily reflect official Agency policy. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. • Contrary to TEAP theory, N03" can be reduced in oxic settings (e.g., the Mississippi River) • Aerobic denitrification is expected to have a high impact on N-fate modeling as most existing models call for denitrification to take place in anoxic settings, yet most impacted surface waters are aerobic. - For example, within the denitrification module of the EPA's Water Quality Analysis Simulation Program (WASP), the kinetic expression for denitrification contains a first order rate constant, a temperature correction term, and a DO correction term, which calculates the decline in denitrification rate as DO levels rise above 0 using a user specified half-saturation constant KN0 (KN03 = DO level where denitrification rate is reduced by half). Currently, the default value is effectively zero (10-20), preventing this reaction at all non-zero DO levels. • While our experiments have qualified aerobic denitrification, our future work will be towards quantifying denitrification rates at varying DO concentration and temperature. Plans are to collaborate with WASP developers to incorporate aerobic denitrification into the model. epascienceforum Your Health • Your Environment • Your Future ------- |