Nitrogen standing stock in Phragmites australis growing in constructed wetlands—Do we evaluate it correctly?

•Concentration of nitrogen vary widely in leaves, stems and flowers of Phragmites.•Concentration of N also varies in relation to position of the tissue on the shoot.•It is necessary to measure N concentration in leaves, stems and flowers separately.•N standing stock is higher in leaves than in stems.•Flowers should be included in standing stock evaluation.

It is quite obvious that nutrient standing stocks in live biomass depend on both nutrient concentrations in the plant tissue as well as on the amount of live plant biomass. While determination of plant biomass is usually easy through harvesting of defined area of vegetation, determination of nitrogen concentration of the biomass is complicated by the fact that the concentration varies widely in various plant tissues and also in relation to position on the shoot. During 2013, three constructed wetlands and five natural stands were surveyed in order to evaluate biomass of plant tissues (leaves, stems and flowers) and nitrogen concentrations in these tissues in relation to their position on the shoot. Our study clearly indicated that most biomass weight is formed by lower parts of the stems (33%) followed by middle stem parts (19%) and leaves from the upper part of the shoot (18.8%). The nitrogen standing stock exhibited the opposite pattern with the highest values found in the upper leaves (38%) and the lowest in the stem bottom (3.7%). Therefore, in order to correctly evaluate nitrogen standing stock in the aboveground biomass, it is necessary to analyse separately leaves, stems and flowers and in case of stems and leaves it is necessary to analyse all the biomass from the shoot. The use of only certain parts of the shoots may cause substantial over- or under-estimation of nitrogen standing stock.