[Influences of Biochar Application on Root Aerenchyma and Radial Oxygen Loss of Acorus calamus in Relation to Subsurface Flow in a Constructed Wetland].

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.

[Influences of Biochar on Pollutant Removal Efficiencies and Nitrous Oxide Emissions in a Subsurface Flow Constructed Wetland].

Biochar, pyrolyzed from agricultural biomass wastes, has been widely used as an improver in wastewater treatment to regulate the oxygen distributions and microbial communities because of its extended surface area and porous structure. In addition, biochar has been shown to play a role in enhancing the porosity, adsorbing ammonium (NH4+-N), and reducing nitrous oxide (N2O) emissions. In this paper, five groups of constructed microcosm wetlands (CW) were built in a greenhouse with different biochar doses of 40%, 30%, 20%, 10%, and 0% (named as BW-40, BW-30, BW-20, BW-10, and CW-K, respectively) to investigate the influences of biochar on pollutant removal efficiencies and N2O emissions. The results showed that the concentration of effluent dissolved oxygen (DO) was less than 0.5 mg·L-1, and the pH was stable at around 7.2 in every CW. Additionally, the effluent oxidation-reduction potential (ORP) was found to have moderately increased with the increases in the quantity of biochar, and the conductivity (Cond) test results showed the opposite trend. However, the effects of biochar on DO, pH, ORP, and Cond were not significant (P>0.05). The chemical oxygen demand (COD) removal rates were up to 90% in all CWs. On the other hand, significantly higher removal efficiencies for NH4+-N and total nitrogen (TN) were found in CWs filled with biochar (P<0.05). The average NH4+-N removal rates were (57.96±10.63)%, (51.12±11.74)%, (48.55±8.75)%, (43.95±9.74)%, and (34.76±14.16)% in BW-40, BW-30, BW-20, BW-10, and CW-K, respectively, while the total nitrogen (TN) average removal rates were (80.21±10.63)%, (78.48±5.73)%, (76.80±4.20)%, (75.88±5.85)%, and (70.92±5.68)%, respectively. Nitrate (NO3--N) was not detected in the CWs for there were sufficient carbon sources and suitable denitrification environments. Moreover, the average fluxes of N2O ranged from 13.53 mg·(m2·d)-1 to 45.30 mg·(m2·d)-1 in the experimental systems. Compared with the control, the reduction rates of N2O in the BW-40, BW30, BW20, and BW10 were 70.13%, 68.26%, 50.83%, and 37.90%, respectively, and the ratios of N2O emissions to the removed nitrogen in CWs with biochar were significantly lower than those in the CW without biochar. Positive correlations were observed between the N2O fluxes and nitrite (NO2--N) concentrations, and the lower N2O emissions could be attributed to the higher oxygen transfer and lower NO2--N accumulation rates in response to the biochar addition. These results demonstrate that biochar could be used as an amendment to strengthen the nitrogen removal and reduce the N2O emissions in CWs.