Modified isotope pairing technique to study n transformations in polluted aquatic systems: Theory

Denitrification of nitrate in sediments of polluted aquatic systems has the potential to release considerable nitrogen amounts into the atmosphere in the form of dinitrogen (N(2)) and/or nitrous oxide (N(2)O). Nitrate may diffuse into the anoxic sediment layer either from the oxic sediment layer (after being produced there by nitrifiers) or directly from the overlying water. Currently, the most common technique that explicitly distinguishes between the two main nitrate sources is the Isotope Pairing Technique (IPT). (15)N-labeled nitrate is added to the water column, and formation of (15)N labeled N(2) molecules is subsequently monitored. The main shortcoming of the IPT is that the formation of N(2)O is ignored, thus resulting in an underestimation of sediment denitrification. Another limitation is the inability to account for a possible influx of nitrate through the hyporheic zone (e.g., nitrate-polluted groundwater) into the anoxic sediment layer. We have further developed and analyzed the theoretical basis of the original IPT. The two important factors that we took into account are the isotopic composition of N(2)O and the input of an external source of nitrate. We also examined the option of adding (15)N-labeled ammonium to the water column to specifically adjust the technique to field studies. The presented modified technique allows us to (i) improve the estimation of sediment denitrification capacity, (ii) gain an insight into the N(2)O formation mechanism(s) and fluxes, and (iii) assess inputs of nitrate-polluted water through the hyporheic zone.