Quantifying the underestimated plant potential for phosphorus removal in constructed wetlands: Revealing the mediating mechanism of radial oxygen loss

Abstract

Substrate adsorption is considered the primary phosphorus removal pathway in subsurface flow constructed wetlands (CWs), but the role of plants in this process remains underestimated. To clarify this issue, this study conducted root restriction by decreasing substrate depth from 60cm to 10 and 20cm to intensify the radial oxygen loss (ROL). On this basis, the resultant enhancement to soluble total phosphorus (TP) removal from late spring to early winter was quantified. Further, the underlying enhancing mechanisms were revealed by analyzing the P mass balance and substrate surface interfacial plant-microbial synergy. CWs shallower than 0.2m significantly intensified ROL and root biomass, showing 33.4% more volumetric P removal on average, and the increased rhizosphere P adsorption accounted for all increments. In the rhizosphere, ROL was activated by Fe (II) of Fe₃O₄ on the substrate surface and produced solid-liquid interfacial-bounded ·OH. Aromatic compounds in root exudations, microbial metabolites, and influent organics that occupied P adsorption sites were mainly converted to aliphatic compounds by ·OH and further biodegraded, thereby re-exposing the adsorption sites. This process contributes to 65.1% (9.41mgP/M·OH) of the volumetric efficiency increment, and the rest is owing to the ROL-induced P content increasing in extracellular polymeric substances. Larger root biomass created more rhizosphere substrates (accounting for 75.0%, 56.4%, and 16.1% in 0.1, 0.2, and 0.6m CWs, respectively), amplified the rhizosphere P removal increment, and durably more than halved the effluent TP. Intensifying ROL prolonged the substrate life cycle, and the effective mediation started after 40-day adaptive cultivation in shallow substrates. This study deepened the current theory and inspired CWs’ design and management.