A comprehensive evaluation of biochar for enhancing nitrogen removal from secondary effluent in constructed wetlands

Biochar has been used to improve nitrogen removal in constructed wetland (CW) treating secondary effluent from wastewater treatment plants (WWTPs). Nevertheless, the mechanism on enhancing nitrogen removal is unclear. Herein, biochar derived from three typical wetland plants, Typha latifolia (TLB), Phragmites australis (PAB), and Cyperus alternifolius (CAB) at 300 degrees C, were used as substrates in CWs to uncover the gap. Results indicated that the total nitrogen removal efficiencies of TLB-CWs (59.84%), PAB-CWs (65.08%), and CAB-CWs (78.61%) were significantly higher than the gravel control (39.20%). Biochar enhanced plant growth for nitrogen uptake and boosted the secretion of root exudates for fueling microbial denitrification. Meanwhile, biochar notably increased the activities of dehydrogenase, electron transport system, and denitrifying enzymes and enriched the corresponding genes. Mass balance calculation showed that microbial denitrification was dominated in nitrogen removal in CWs. Pearson correlation analysis revealed that the proportion of C=O and pyrrolic N in biochar's surface, biochar's electron accepting capacity and electron donating capacity, and the lignin proportion of biochar feedstock, were significantly correlated with the removal efficiency of nitrogen. PLSSEM demonstrated that biochar boosted nitrogen removal in CWs by enhancing plant growth, which promoted microbial activities. Overall, our results showed the enhancing interactive mechanisms among biochar properties, plant growth and microbial enzyme activity.