GROUNDWATER N SPECIATION AND REDOX CONTROL OF ORGANIC N MINERALIZATION BY O2 REDUCTION TO H2O2

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(Nporg), and dissolved organic N(Ndorg). 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 Ndorg (<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, Ndorg 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 Ndorg as amide functional groups, as justified by recent analytical work, similar thermodynamic calculations support that Ndorg 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 Ndorg-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.