Nitrogen and phosphorus economy of reed vegetation in the polder Zuidelijk Flevoland (The Netherlands)

N and P shortages might have explained the expected degeneration of closed stands of reed developed a few years after emergence of the polder in 1968. Distribution of N and P among shoots and rhizomes was determined to estimate the extent of internal recirculation of N and P. The nitratereductase activity of the shoots and the numbers of NH4-oxidizers at various water levels gave indications about the availability of nitrate. N losses by denitrification over the 1968-1975 period were assessed from tentative N budgets based on the uptake of N by the reed vegetation - including N losses due to burning in early spring - and the decrease of total soil N in the top 1 m of the soil as well as of mineral N down to the pleistocene sand at about 3 m depth. The points of time, up till when no N and P shortages are expected, were based on the stock of mineral N and citric acid-soluble P in the top 2 m of the soil in 1975, respectively, and the uptake of N and P by the vegetation since 1968.