Plant species diversity impacts nitrogen removal and nitrous oxide emissions as much as carbon addition in constructed wetland microcosms

•High species richness impacted N removal as much as carbon (C) addition.•The effect of species richness on N2O emissions was equal to that of C addition.•High species richness enhanced N removal by promoting plant C release.•Species richness impacts on N removal stronger than species identity.

With the aim to develop an approach for treating wastewater with low carbon (C) to nitrogen (N) ratio in constructed wetlands (CWs), we compared the effects of C addition and species diversity on N removal and nitrous oxide (N2O) emissions. Compared with the monocultures without C addition, C addition significantly increased N removal efficiency in monocultures (reached 71% on average); increasing species richness impacted N removal as much as C addition, and the N removal efficiency in the four-species mixture reached 75%. Both C addition and increasing species richness level significantly increased N2O emission rates (reached 1.26 and 1.40 mg m−2 d−1, respectively), but had no significant effect on N2O emissions per unit N removal. The effects of species richness on N removal and N2O emissions were stronger than species identity. Compared with C addition, assembling plant communities with high diversity in actual applications could be an effective and more economical approach for treating wastewater with low C/N ratio in CWs.