Rhizosphere metabolism and its effect on phosphorus pools in the root zone of a submerged macrophyte, Isoëtes kirkii

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The presence of oxygen in lake sediments reduces sediment oxygen demand, and potentially improves sediment phosphorus retention and coupled nitrification/denitrification. However, the release of oxygen from the roots of macrophytes has not previously been measured in highly reducing sediments. Here, in the highly reducing environments of a commercial garden soil and sediment from a hyper-eutrophic lake, we used nine oxygen optodes, placed onto scintillation vials to detect oxygen in the rhizosphere of Isoetes kirkii referred to as quillworts. We calculated rhizosphere metabolism using “night-time regression” a method designed to estimate stream metabolism at the reach scale. After the incubations, sediment was collected from each vial (with and without macrophytes) and was subjected to sequential phosphorus extractions. A lag period between light availability and increasing oxygen concentration, that varied between individual optodes, was used to improve the accuracy of metabolism estimates as it was postulated to represent the distance between the root and the optode. Higher sediment oxygen demand in the lake sediment caused I. kirkii to have higher root oxygen release than those plants grown in the garden soil and may have pushed plants in lake sediment close to their ability to survive. This was evident as a significant, negative relationship between root oxygen release and increasing sediment oxygen demand, indicating that if photosynthesis decreased or sediment oxygen demand increased, the plants would no longer being able to oxygenate the sediment surrounding their roots, which would likely lead to death. Finally, the presence of quillworts in lake sediments significantly increased stores of metal oxide and recalcitrant phosphorus in the lake sediment but not the garden soil.