Discrimination of nitrogen isotopes, during absorption of ammonium and nitrate at different nitrogen concentrations by rice (Oryza sativa L.) plants

Abstract

Studies of uptake of ionic sources of N by two hydroponically grown rice (Oryza sativa L.) cultivars (paddyfield-adapted Koshihikari and dryland-adapted Kanto 168) showed that the magnitude of the nitrogen isotope fractionation (ε) for uptake of NH4+ depended on the concentrations of NH4+ and cultivar (averaging -6.1‰ for Koshihikari and -12.0‰ for Kanto 168 at concentrations from 40 to 200 mmol m-3 and, respectively, -13.4 and -28.9‰ for the two cultivars at concentrations from 0.5 to 4 mol m-3). In contrast, the ε for uptake of NO3- in similar experiments was almost insensitive to the N concentration, falling within a much narrower range (+3.2‰ to -0.9‰ for Koshihikari and -0.9‰ to -5.1‰ for Kanto 168 over NO3- concentrations from 0.04 to 2 mol m-3). From longer term experiments in which Norin 8 and its nitrate-reductase deficient mutant M819 were grown with 2 or 8 mol m-3 NO3- for 30 d, it was concluded that the small concentration-independent isotopic fractionation during absorption of this ion was not related to nitrate reductase activity.