Promoting aerobic denitrification in the rhizosphere by vertical spatial stress: A strategy to enhance nitrogen...

To overcome nitrogen removal limitations in constructed wetlands (CWs), this study employed spatial stress (substrate depth restriction) to enhance the rhizosphere effect and strengthen the aerobic denitrification pathway. Compared to conventional subsurface flow CWs (CCWs), shallow-substrate subsurface flow CWs (SCWs) significantly promoted the overgrowth of root, thus improving the radial oxygen loss (ROL) rate and root activity and maintaining dissolved oxygen within the substrate stable at more than 2 mg/L. Correspondingly, the pollutant removal efficiency of SCWs was significantly improved, with the removal rates of NH₄⁺-N, TN, and TP all exceeding 80 %. In addition, SCWs enhanced the rhizosphere effect and promoted nitrogen transformation processes with the cultivation of plants. After 90 days of operation, compared to CCWs, the ROL and root activity of typha in SCWs increased by 24.18 % and 19.12 %, respectively. Similarly, the heterotrophic nitrification rate and aerobic denitrification rate in the rhizosphere of SCWs increased by 4.96 % and 12.98 %, respectively. And the activities of periplasmic nitrate reductase and nitrite reductase, increased by 25.94 % and 30.59 %, respectively. Furthermore, spatial stress also induced elevated concentrations of root exudates (Isobutyl butyrate and Isobutyl isobutyrate), which further stimulated the enrichment of aerobic denitrifying bacterial genera Pseudomonas, Exiguobacterium, and Sphingomonas. The key functional genes (napA/B, narB, ppk, ppx, glk) were effectively enriched in SCWs, thereby promoting the enhancement of nitrogen and phosphorus transformation. Overall, above findings demonstrate that the shallow-substrate configuration serves as a potential and effective strategy for intensifying rhizosphere effects and enhancing the aerobic denitrification process in CWs.