Precipitated iron and manganese plaques restrict root uptake of phosphorusin Lobelia dortmanna

Plaques of Fe and Mn on root surfaces of submerged aquatic plants are forme d in reduced organic sediments with high concentrations of reduced soluble Fe and Mn. Since oxidized metal compounds have a high capacity for P bindin g, we grew Lobelia dortmanna L. for 6 months on four sediment types, in whi ch the content of organic matter ranged from 0.32 to 1.73%, to test the inf luence of plaques on assimilation of P and biomass development. Concentrati on of oxidized Fe and Mn in plaques on Lobelia roots increased significantl y with the content of organic matter in the sediment. Lobelia with high con centrations of Fe in the plaques showed very low tissue content of P and lo w biomass despite the fact that these plants were from the two organic sedi ments with high concentrations of P Plants from the two sediments with low content of organic matter and P showed, in contrast, low plaque concentrati ons but higher biomass and P content. No significant correlations between t he content of Fe and Mn in root plaques and in stems and leaves of Lobelia were found. The results suggest that the formation of plaques on Lobelia ro ots on sediments rich in reduced Fe and Mn restricts uptake of P and biomas s production because P is adsorbed to oxidized Fe and Mn in the plaques.