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 Müggelsee (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 m2 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.

The oxygen dynamics in the rhizosphere of Zostera marina was studied by use of planar optodes. Oxygen leakage to the rhizosphere was restricted to the root tip and extended only up to ∼8 mm up along the root. The oxic sediment volume around the roots increased linearly with irradiance in the interval of 0-250 μmol photons m-2 s-1, but the leakage rate saturated at the maximum irradiance of 500 μmol photons m-2 s -1. Oxygen leakage decreased by ∼60% from light to darkness and Z. marina was able to maintain an oxic zone around the root tip even in darkness as long as oxygen in the overlying water was at 100% air saturation (280 μmol L-1). O2 leakage from the root tips stopped at 25% air saturation (70 μmol L-1) and the oxic microniche rapidly disappeared. Increasing the oxygen concentration above 100% air saturation induced oxygen leakage from zones that otherwise appeared impermeable to oxygen. The roots on average grew by 8.7 mm d-1, and a series of O 2 images documented the high spatial and temporal dynamics of the oxic microniches around the root tips. The estimated total oxygen release to the rhizosphere of Z. marina beds was 2.3 mmol m-2 d-1, which only corresponded to 12% of the diffusive oxygen uptake at the primary sediment-water interface. Rhizospheres of seagrass are thus probably of minor importance for total benthic O2 uptake rates. © 2006, by the American Society of Limnology and Oceanography, Inc.

O2 and pH microsensors were used to analyse the microdistribution of O2 and pH inside and outside roots of lowland rice (Oryza sativa L.). The roots of 3-week-old transplants had O2 concentrations of about 20% air saturation at the surface, but due to a high rate of O2 consumption in the rhizosphere, the oxic region only extended about 0.4 mm into the surrounding soil. Also the fine lateral roots created an oxic zone extending about 0.15 mm into the soil. The O2 concentration within the roots approached air saturation close to the base, but only about 40-60% of air saturation in a region about 8 cm below the base where lateral rootlets were present. A shift from air to N2 around the leaves led to a drop of 50% in the O2 concentration after 12 min at a distance of 8.5 cm from the base. Flowering plants did not export O2 to the soil from the majority of their roots, but high microbial activity was present in a very thin biofilm covering the root surface. A brown colour around the thin lateral roots indicated some O2 export from these also during flowering. No oxidized zone was present around the roots at later stages of crop growth. The roots caused only minor minima in pH (<0.2 pH units) in the rhizosphere as compared to the bulk soil. Illumination of the plants had no effect on rhizosphere pH.