Measuring nitrification in sediments--comparison of two techniques and three 15NO(-)3 measurement methods.

Nitrification is a crucial process in sediment nitrogen cycling. We compared two (15)N tracer-based nitrification measurement techniques (isotope pairing technique (IPT) combined with (15)N nitrate pool dilution and (15)N ammonium oxidation) and three different (15)N analyses from bottom water nitrate (ammonia diffusion, denitrifier and SPINMAS) in a sediment mesocosm. The (15)N nitrate pool dilution technique combined with IPT can be used to quantify the in situ nitrification, but the minimum detection limit for the total nitrification is higher than that in the (15)N ammonium oxidation technique. The (15)N ammonium oxidation technique, however, is not applicable for sediments that have high ammonium content. If nitrate concentration and the amount of (15)N label in the sample are low, the (15)N nitrate analysis should be done with the denitrifier method. In higher (15)N concentrations, the less sensitive SPINMAS method can also be applied. The ammonia diffusion method is not suitable for bottom water (15)N nitrate analyses.