Seasonal changes of shoot nitrogen concentrations and 15N/14N ratios in common reed in a constructed wetland

Nitrogen (N) concentrations and stable N isotope abundances (delta N-15) 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 515 N values were higher in the senescence stage (+12.2 to +22.4 parts per thousand) than in the growing stage (+5.1 to +11.3%). Coupled with the negative correlation (R-2 = 0.24, P < 0.05, n = 18) between N concentrations and delta N-15 values of shoots in the senescence stage, our results suggested that shoot N became enriched in 15 N due to N isotopic fractionation (with an isotopic fractionation factor, alpha(s)/(p), of 1.012) during N translocation to rhizomes. However, the positive correlation between N concentrations and 51 5 N values in the growing stage (R-2 = 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 5 15 N variations provide N-isotopic evidence of N translocation within and N uptake from external N sources by common reed.