Immobilisation of phosphorus by iron-coated roots of submerged macrophytes

To observe effects on the phosphorus retention mechanisms of a lake after re-colonisation by macrophytes, Potamogeton crispus L. and Elodea canadensis Michx. were planted in lab aquariums containing an iron and phosphorus rich sediments from the highly eutrophic Lake Muggelsee (Germany). Microsensors were used to analyse oxygen gradients near roots and diurnal changes of oxygen saturation in the rhizosphere. Under light conditions oxygen is detectable in a 0.5-1 mm thick zone around the roots. The detected maximum oxygen saturation at the root surface was approximately 30%. The sharp redox gradient at the root surface led to an oxidation of ferrous iron and a deposition of Fe(III) crusts around the roots. X-ray micro-analyses have shown that those crusts contain iron as well as phosphorus in high quantities. Chemical extraction of roots with iron crusts showed that more than 90% of the phosphorus they contain is reductive soluble phosphorus. Based on mesocosm experiments a phosphorus retention by iron precipitates around the roots of 0.08 g P per m for E. canadensis and 0.48 g P for P. crispus per m 2 sediment surface was determined. These first results have shown, that the root oxygen release of submerged plants can form iron crusts in anaerobic sediment leading to an enhanced sorptive phosphorus fixation.