Temporal variations in the nitrogen content of Phragmites australis (Cav) Trin ex Steud from a shallow fertile lake

Despite large differences in the amounts of nitrogen that accumulated in shoots, the amounts of nitrogen stored over a 4 year period in the rhizomes of Phragmites australis from a fertile English lake did not change. Rhizomes sampled in November contained a 4-year mean (+/- SD) of 22 +/- 3 g of nitrogen in an annual mean dry weight of 2360 +/- 290 g m(-2) Shoots contained between 8 +/- 1 g m(-2) and 20 +/- 10 g m(-2) nitrogen in the midsummer of the different years and from 0.2 +/- 0.4 g m(-2) to 18 +/- 7 g m(-2) in November. In the early winters, the mass of nitrogen still contained in standing dead stems was equivalent to between 1% and 49% of the combined shoot and rhizome-nitrogen. Thus, the sometimes large amounts of nitrogen that were potentially available for downward translocation to rhizomes were not reclaimed. This behaviour is typical of relatively open nutrient cycles where, over the longer-term, net storage of incoming nutrients does not occur.