Isotopic biogeochemistry of dissolved organic nitrogen: A new technique and application

We present a new technique for isolating and isotopically characterizing dissolved organic nitrogen (DON) for non-marine waters, δ15N values for DON from lacustrine samples and data suggesting that this technique will be applicable to marine samples. Our technique involves preconcentration of DON via rotary evaporation and removal of dissolved inorganic nitrogen (DIN) via dialysis using a membrane that retains material above 100 Da. Results demonstrate quantitative removal of DIN, complete recovery of DON (95% or greater) and retention of isotopic integrity (isotope effect less than 0.4‰) for a solution containing a DON standard (tripeptide) and DIN in deionized water. Reproducibility of carbon and nitrogen isotope values and elemental concentrations is demonstrated for DOM from Chefswet Basin, Lake Superior and Grand Traverse Bay, Lake Michigan. The applicability of this technique to marine samples is suggested by demonstrating 99% removal of DIN from a sample of Gulf Stream water amended with ammonium and nitrate.