Searching for the missing nitrogen: biogenic nitrogen gases in groundwater and streams

Biogenic nitrogen (N-2) and nitrous oxide (N2O ) accumulations were measured in groundwater, streams and the vadose zone of small agricultural watersheds in the Mid-Atlantic USA. In general, N-2 and N2O in excess of atmospheric equilibrium were found in groundwater virtually everywhere that was sampled. Excess N-2 in groundwater ranged from undetectable to 616 mu mol N-2-N/l, the latter representing c. 50% of background N-2. The N2O-N concentrations varied from undetectable to 75 mu M, and usually greatly exceeded values at atmospheric equilibrium (25-30 nM); however, N2O was generally 1-10% of excess N-2. Intermediate levels of deficit and excess N-2 in flowing streams (-65 to +250 mu mol N-2-N/L) resulting from both abiotic and biotic processes were also measured. In vadose zone gases, multiple N-2/Ar gas profiles were measured which exhibited seasonal variations with below atmospheric values when the soil was warming in spring/summer and above atmospheric values when groundwater was cooling in fall/winter. Both abiotic and biotic processes contributed to the excess N-2 and N2O that was observed. The current data indicate that large concentrations of excess N gases can accumulate within soil, groundwater, and streams of agriculturally dominated watersheds. When excess N gases are exchanged with the atmosphere, the net fluxes to the atmosphere may represent an important loss term for watershed N budgets.