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

Studies of uptake of ionic sources of N by two hydroponically grown rice (O ryza sativa L.) cultivars (paddy-field-adapted Koshihikari and dryland adap ted Kanto 168) showed that the magnitude of the nitrogen isotope fractionat ion (epsilon) for uptake of NH4+ depended on the concentrations of NH4+ and cultivar (averaging -6.1 parts per thousand for Koshihikari and -12.0 part s per thousand for Kanto 168 at concentrations from 40 to 200 mmol m(-3) an d, respectively, -13.4 and -28.9 parts per thousand for the two cultivars a t concentrations from 0.5 to 4 mol m(-3)). In contrast, the epsilon for upt ake of NO3- in similar experiments was almost insensitive to the N concentr ation, falling within a much narrower range (+3.2 parts per thousand to -0. 9 parts per thousand for Koshihikari and -0.9 parts per thousand to 5.1 par ts per thousand 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-reduct ase deficient mutant M819 were grown with 2 or 8 mol m(-3) NO3- for 30 d, i f was concluded that the small concentration-independent isotopic fractiona tion during absorption of this ion was not related to nitrate reductase act ivity.