Seasonal Changes of Shoot Nitrogen Concentrations and 15 N/ 14 N Ratios in Common Reed in a Constructed Wetland.

Nitrogen (N) concentrations and stable N isotope abundances (δ15N) of common reed (Phragmites australis) planted in a constructed wetland were measured periodically between July 2001 and May 2002 to examine their seasonal variations in relation to N uptake and N translocation within common reed. Nitrogen concentrations in P. australis shoots were higher in the growing stage (7.5 to 24.8 g N kg-1)than in the senescence stage (4.2 to 6.8 g N kg-1), indicating N translocation from shoots to rhizomes. Meanwhile, the corresponding δ15N values were higher in the senescence stage (+12.2 to +22.4‰) than in the growing stage (+5.1 to +11.3‰). Coupled with the negative correlation (R² 0.24, P < 0.05, n 18) between N concentrations and δ15N values of shoots in the senescence stage, our results suggested that shoot N became enriched in δ15N due to N isotopic fractionation (with an isotopic fractionation factor, αs/p' of 1.012) during N translocation to rhizomes. However, the positive correlation between N concentrations and δ15N values in the growing stage (R² 0.19, P < 0.001, n 54) suggested that P. australis relies on N re-translocated from rhizome in the early growing stage and on mineral N in the sediment during the active growing stage. Therefore, seasonal δ15N variations provide N-isotopic evidence of N translocation within and N uptake from external N sources by common reed. [ABSTRACT FROM AUTHOR]