Rethinking phosphorus removal in constructed wetlands: Amino acid exudation-mediated root-surface iron redox cycling drives plant luxury uptake to outperform substrate adsorption.

Plants can exhibit phosphorus luxury uptake, absorbing several-fold more P than required under P-sufficient conditions. Harnessing this underutilized capacity helps to overcome substrate adsorption saturation and improve the phosphorus removal efficiency and sustainability of subsurface flow constructed wetlands (SFCWs). Here, we engineered a dense, fiber filter-like mat-structured root system to replace conventional inert substrates and systematically investigated its P removal performance, mechanisms, and engineering feasibility. From late spring to early winter, the root-mat system more than halved the effluent P compared with a conventional SFCW despite operating at an eight-fold hydraulic loading. Root-exudated aliphatic and carboxylic compounds scavenged the root-surface hydroxyl radicals, protecting the significantly more exudated histidine and lysine from being degraded. During the daytime, these amino acids coordinated with Fe3O4 and FeOOH in root-surface iron plaque via Fe(III)-COOH bonding, and their protonation formed a positively charged coating that increased P accumulation capacity by 77.8% through electrostatic attraction. The coordination enhanced the electron-transfer capacity of Fe by 62.9%, and increased nighttime Fe(III) reduction by 75%. The cleavage of Fe(III)-COOH released coordinated lysine and histidine, and their subsequent degradation at the iron plaque-water interface released electrostatically bound P, substantially elevating root-surface P supply and triggering plant luxury uptake. This pathway accounted for 91.2% of total P removal, with microorganisms assimilating the remainder. The root mat increased auxin and cytokinin synthesis, rebalanced their root-shoot allocation, and facilitated the growth of P storage sites. At the pilot scale, the root-mat system outperformed conventional SFCWs under 67% higher hydraulic loading, and seasonal stability can be improved using cold-hardy plants. Overall, these results advanced the current understanding and provide a scalable substrate-independent strategy that leverages plant luxury uptake to achieve efficient, economical, and sustainable P removal in SFCWs.