Precipitated iron and manganese plaques restrict root uptake of phosphorus in Lobelia dortmanna

Plaques of Fe and Mn on root surfaces of submerged aquatic plants are formed in reduced organic sediments with high concentrations of reduced soluble Fe and Mn. Since oxidized metal compounds have a high capacity for P binding, we grew Lobelia dortmanna L. for 6 months on four sediment types, in which the content of organic matter ranged from 0.32 to 1.73%, to test the influence of plaques on assimilation of P and biomass development. Concentration of oxidized Fe and Mn in plaques on Lobelia roots increased significantly with the content of organic matter in the sediment. Lobelia with high concentrations of Fe in the plaques showed very low tissue content of P and low biomass despite the fact that these plants were from the two organic sediments with high concentrations of P Plants from the two sediments with low content of organic matter and P showed, in contrast, low plaque concentrations but higher biomass and P content. No significant correlations between the 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 roots on sediments rich in reduced Fe and Mn restricts uptake of P and biomass production because P is adsorbed to oxidized Fe and Mn in the plaques.