Washington,J.W., Thomas,R.C., Endale,D.M., Schroer,K.L. and Samarkina,L.P.(2006): Groundwater N speciation and redox control of organic N mineralization by O2 reduction to H2O2. Geochimica et Cosmochimica Acta, 70, 3533-3548.

『地下水の窒素化学種形成とH2O2へのO2還元による有機窒素の無機化の酸化還元コントロール』

『Abstract
　Excess N from agriculture induces eutrophication in major river systems and hypoxia in coastal waters throughout the world. Much of this N is from headwaters far up the watersheds. In turn, much of the N in these headwaters is from ground-water discharge. Consequently, the concentrations and forms of N in groundwater are important factors affecting major aquatic ecosystems; despite this, few data exist for several species of N in groundwater and controls on speciation are ill-defined. Herein, we report N speciation for a spring and well that were selected to reflect agricultural impacts, and a spring and well that show little to no agricultural-N impact. Samples were characterized for NO3-, NO2-, N2O, NH4+, urea, particulate organic N (Norgp), and dissolved organic N (Norgd). These analytes were monitored in the agricultural spring for up to two years along with other analytes that we reported upon previously. For all samples, when oxidized N was present, the dominant species was NO3- (88-98％ of total fixed N pool) followed by Norgd(＜4-12％) and only trace fractions of the other N analytes. In the non-agriculturally impacted well sample, which had no quantifiable NO3- or dissolved O2, Norgd comprised the dominant fraction (68％) followed by NH4+(32％), with only a trace balance comprised of other N analytes. Water drawn from the well, spring and a wetland situated in the agricultural watersheds also were analyzed for dissolved N2 and found to have a fugacity in excess of that of the atmosphere. H2O2 was analyzed in the agricultural spring to evaluate the O2/H2O2 redox potential and compare it to other calculated potentials. The potential of the O2/H2O2 couple was close in value to the NO3-/NO2- couple suggesting the important role of H2O2 as an O2-reduction intermediate product and that O2 and NO3- are reduced concomitantly. The O2/H2O2 and NO3-/NO2- couples also were close in value to a cluster of other inorganic N and Fe couples indicating near partial equilibrium among these species. Urea mineralization to NO2- was found to approach equilibrium with the reduction of O2 to H2O2. By modeling Norgd as amide functional groups, as justified by recent analytical work, similar thermodynamic calculations support that Norgd mineralization to NO2- proceeds nearly to equilibrium with the reduction of O2 to H2O2 as well. This near equilibration of redox couples for urea- and Norgd-oxidation with O2-reduction places these two couples within the oxidized redox cluster that is shared among several other couples we have reported previously. In the monitored agricultural spring, [NO3-] was lower in the summer than at other times, whereas [N2O] was higher in the summer than at other times, perhaps reflecting a seasonal variation in the degree of denitrification reaction progress. No other N analytes were observed to vary seasonally in our study. In the well having no agricultural-N impact, Corg/Norg = 5.5, close to the typical value for natural aqueous systems of about 6.6. In the agricultural watershed Corg/Norg varied widely, from 〜1.2 to ≧9.』

1. Introduction
2. N speciation and transformations
3. Materials and methods
　3.1. Particulate and dissolved total N
　3.2. Particulate and dissolved organic N
　3.3. Urea N
　3.4. Collection and measurement of dissolved N2
　3.5. Measurement of H2O2
　3.6. Collection and measurement of N2O, NO2-, NH4+, Corg, and O2
　3.7. Collection and measurement of other analytes in 2005 sampling rounds
4. Results
5. Discussion
　5.1. Confirmation that the O2/H2O2 redox couple approaches equilibrium with several N and Fe couples
　5.2. Mineralization of urea to near partial equilibrium with dissolved O2 reduction
　5.3. Mineralization of Norg for energy and near partial equilibrium with dissolved O2 reduction
　5.4. Modes of accumulation of excess N2 and its possible effervescence
　5.5. Denitrification as a possible cause of NO3- and N2O covariation
　5.6. Corg/Norg indicates that the SpW2 aquifer microbial ecosystem is N saturated
6. Conclusion
Acknowledgments
References

Fig. 1. N transformations−emboldened species are the dominant forms for biological assimilation and emboldened processes can be exergonic. Together, ammonification and nitrification often are coined mineralization. Nitrogen species, excluding N2, often are grouped under the heading ‘fixed N’ or ‘reactive N,’ in reference to their common trait of having undergone fixation and being readily available for one or more biological transformations.

Fig. 7. Ratio of N-species concentration to initial [NO3-] vs reaction progress for the denitrification reaction sequence of NO3- → NO2-→ NO → N2O → N2 using the Michaelis-Menten half-saturation constants for Flavobacterium as reported in Betlach and Tiedje (1981). For much of the reaction period from 〜0.05 to 〜0.1, increasing progress diminishes [NO3-] at the same time as [N2O] increases and [N2O-], near its maximum, remains nearly constant−a qualitatively similar pattern as that observed for spring SpW2. Note that [N2O] and [N2] are decreased by half relative to other species to account for reaction stoichiometry. Also note that: (1) the half-saturation constant for NO, which was not reported by Betlach and Tiedje (1981), herein is assumed equal to that of N2O; and 2) Betlach and Tiedje (1981) did not report Michaelis-Menten maximum velocities (v) which vary dramatically with environmental conditions−to model relative concentrations similar to our data we used νNO3- = 1, νNO2- = 6, νNO = 1 and νN2O = 0.2.

〔Washington,J.W., Thomas,R.C., Endale,D.M., Schroer,K.L. and Samarkina,L.P.(2006): Groundwater N speciation and redox control of organic N mineralization by O2 reduction to H2O2. Geochimica et Cosmochimica Acta, 70, 3533-3548.から〕

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