Uptake of stable N isotopes by Myriophyllum spicatum is not selective

Preference for 14N over 15N was assessed for Myriophyllum spicatum L. by exposing shoots to the same initial concentration of N, but different isotopic compositions (100% 14N, 50% 14N and 50% 15N, and 100% 15N) for 6 weeks. Regardless of treatment, all available inorganic N was taken up by M. spicatum at similar rates. Significant differences in tissue isotopic composition were evident across treatments in 2 weeks, and persisted for the remainder of the experiment. At 6 weeks, atom (at) % 15N was twice as high in the 100% 15N treatment as in the 50% 15N treatment, and the at% of the 14N treatment (0% 15N) remained unchanged. This observed absence of preference for 14N in M. spicatum suggests that this species can be useful in determining N sources to freshwater systems, which often differ in 15N/14N ratio.