Groundwater N speciation and redox control of organic N mineralization by O2 reduction to H2O2

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 � ,N O 2 � ,N 2O, NH4 + , urea, particulate organic N(N p org), and dissolved organic N(N d org). 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 N d org (<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, N d org 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 watershed 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 N d org as amide functional groups, as justified by recent analytical work, similar thermodynamic calculations support that N d org 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 N d org -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 J 9.