Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Review Article
Publication Acceptance Date: 11/17/2005
Publication Date: 7/15/2006
Citation: Washington, J.W., Thomas, R.C., Samarkina, L.P., Endale, D.M. 2006. Groundwater N speciation and redox control of organic N mineralization by O2 reduction to H2O2. Geochimica et Cosmochimica Acta. 70(14):3533-3548.
Interpretive Summary: Knowledge of the concentrations and forms of nitrogen (N) in groundwater is crucial to address N-related environmental problems but such data are generally lacking. Groundwater supplies over half of the flow to small streams of watersheds that eventually reach large rivers and coastal waters where excessive quantities of nitrogen can result in problems of hypoxia and red tides. Scientists from the Ecosystem Research Division of the US-EPA in Athens, GA, and the USDA-ARS, J. Phil Campbell Sr. Natural Resource Conservation Center in Watkinsville, GA, investigated forms of N for groundwater from both agriculturally impacted and non-impacted settings in the Piedmont region of southeastern US over a two-year period. They characterized water samples collected from two springs and two wells for reactive forms of nitrogen consisting of nitrate, nitrite, ammonium, nitrous oxide, urea, total organic and dissolved organic N as well as dissolved organic carbon. Nitrate was the dominant form (88-98%) followed by dissolved organic N (< 4-12%) in samples exposed to oxygen in the natural setting. Dissolved organic N was dominant (68%), followed by ammonium (32%), in samples lacking exposure to oxygen. The organic carbon to organic nitrogen ratio, an index often used to gauge N excess, was about 6.6 in a well with little agricultural impact - typical for natural aqueous systems. The ratio was about 1.2 for samples from a spring with a cattle-congregation area close by, and was higher than 10 for a well higher up in the watershed where cattle graze but seldom congregate. The data also support the hypothesis, contrary to generally accepted paradigm, that some organic N might persist in the environment because it approaches partial equilibrium with surroundings rather than because it is difficult to degrade. These sets of information will find wide use in many scientific disciplines including the physical, chemical and biological sciences and environmental engineering.
Technical Abstract: Ground water supplies over 50% of total flow of headwaters and other small streams, which join to make up flows in larger rivers and coastal waters, and where nitrogen (N)-related problems such as hypoxia and red tides are evident. Knowledge of the concentrations and forms of N in groundwater is crucial to address these problems but there is general paucity of such data. Samples were collected from one spring and one well selected to reflect agricultural impacts, and a spring and a well that were considered non-agricultural. These samples were characterized for fixed N species including NO3-, NO2-, N2O, NH4+, urea, particulate organic N (Nporg) and dissolved organic N (Ndorg). For all these samples, when oxidized N was present, the dominant species was NO3- (88-98% of total N pool) followed by Ndorg (<4-12%). In one well sample having no quantifiable NO3- or dissolved O2, Ndorg comprised the dominant fraction (68%) followed by NH4+ (32%). Water drawn from the shallower of one of two wells situated in a pasture for beef cattle also was analyzed for dissolved N2, and found to have a fugacity much in excess of that of the atmosphere indicating high rates of denitrification. In the agriculturally-impacted 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. The Corg/Norg ratio varied greatly with land use. In the spring located closely down gradient of a cattle-congregation area, Corg/Norg was ~1.2, low relative to Corg/Norg values typically observed in natural aqueous systems, i.e. 6.6, probably owing to nearby N contribution from cattle waste. In a well higher up in the watershed, where cattle graze but seldom congregate, Corg/Norg was 9. In the well having little apparent agricultural impact, Corg/Norg =5.5, close to the typical values of about 6.6 for natural aqueous systems. Thermodynamic calculations indicate urea mineralization to NO2- approaches 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 N Ndorg-oxidation with O2-reduction places thse two couples within an oxidized redox cluster that is shared among several other couples we have reported previously.