Calcium regulates nitrogen removal in constructed wetlands: The interactions between dissolved organic matter and microbial response

Limited carbon bioavailability in low C/N tailwater aggravated the difficulty of nitrogen (N) removal. Calcium (Ca) can regulate carbon transformation, but the effects of Ca on N removal in constructed wetlands (CWs) remain elusive. To fill the gap, this study constructed two types of Ca-based CWs, and the results revealed that the removal efficiencies of nitrate (NO3--N) and total nitrogen (TN) in Ca-based CWs increased by 9.83-22.25 % and 8.59-18.72 %, respectively, compared to control group. The proportion of protein-like DOM and P/H index (represent bioavailability of DOC) increased in Ca-based CWs, supporting the growth of heterotrophic denitrifiers and facilitated NO3--N and TN removal. In addition to directly enhancing complete denitrification, Ca-based CWs also optimized the N removal pathway. Moreover, Ca enhanced complexity and robust of the microbial network, increased microbial communication, thereby jointly facilitating N removal. This study highlighted Ca improved N removal by collaborating the interactions of DOM transformation and microbial activity, which provided an abundant and cheap substrate for the treatment of low C/N wastewater by CWs.