In the subsurface flow of a constructed wetland (CW) used for treating
wastewater, low
oxygen diffusion results in long-term anoxic or anaerobic surroundings, which cannot meet the needs of plant respiration and poses a threat to the survival of macrophytes. Although sweet sedge (Acorus calamus L.) has a significant ability to resist
hypoxia,
membrane lipid oxidation would still occur in the plant due to the long-term
hypoxia in the CW. According to reports in the existing literature, activation of the antioxidative response system could be promoted by adding
biochar, thereby significantly decreasing the malonic dialdehyde in the plants. However, the specific reasons why
biochar alleviates the stress from
anoxia are still not clear. Thus, the responses of macrophyte roots to
biochar application were studied in five different CWs built in a greenhouse, using plant ecology analyses combined with root aerenchyma, root porosity, and radial
oxygen loss (ROL). The results showed that adding
biochar to CW was beneficial for sweet sedge to form root aerenchyma and to increase root porosity. Moreover, there was a significant positive correlation between root porosity and the amount of
biochar applied. Photosynthetic metabolism could be indirectly promoted by
biochar application by increasing
oxygen partial pressure in the blades, helping to transport O2 to underground parts through aerenchyma, and spreading O2 to the rhizosphere in the form of ROL. The reduction environment could be improved by applying
biochar in CWs, which was also beneficial for ROL. Compared with other light conditions, 3000 μmol·(m2·s)-1 was more suitable for the growth of A. calamus in CWs with
biochar, where the ability of the plants to secrete
oxygen would be stimulated and enhanced. However, the effect of the
biochar application ratio on ROL was not significant.