Concentration is not enough to evaluate accumulation of heavy metals and nutrients in plants

Wetland plants produce high aboveground biomass and possess the ability to accumulate heavy metals and nutrients. This ability is used for phytoremediation purposes including removal of nutrients and heavy metals from polluted waters. The concentrations of heavy metals are usually much higher in the belowground then in aboveground biomass, especially in roots which are primary sites of uptake. This may lead to the conclusion that accumulation of heavy metals is higher in the belowground biomass. However, in case the aboveground is much higher than belowground biomass the accumulation could be higher in the aboveground biomass. Concentration of nitrogen and phosphorus is always higher in leaves than in stems. However, the stem biomass is often much higher in robust emergent species such as Phragmites australis and therefore, more nutrients can be stored in stems. The examples shown in this communication clearly reveal that to evaluate properly the accumulation of heavy metals and nutrients in particular plant compartment biomass amount must be taken into consideration. In the first study, concentrations of Cd, Cr and Hg in Phalaris arundinacea belowground/aboveground biomass were 150/80 μg/kg, 5420/228 μg/kg and 38/18 μg/kg. The high aboveground biomass (1196 g/m(2)) and low belowground biomass (244 g/(2)) resulted in much higher accumulation of Cd and Hg in aboveground biomass (96 μg/m(2) and 21.2 μg/m(2), respectively) than in belowground biomass (36 μg/m(2) and 9.3 μg/m(2), respectively). Only for chromium, belowground accumulation (1312 μg/m(2)) was higher than aboveground accumulation (272 μg/m(2)). In the second study, both nitrogen and phosphorus concentrations were higher (26.7 mg/g and 749 mg/kg, respectively) in leaves than in stems (8.2mg/g and 534 mg/kg, respectively) of P. australis. The higher biomass of stems (1835 g/m(2)) than leaves (967 g/m(2)) resulted in higher accumulation of nitrogen but lower accumulation of phosphorus in leaves as compared to stems.