Previous studies have shown that plant diversity can improve the wastewater purification efficiency of constructed wetlands (CWs), but its effect on the nitrous oxide (N 2 O) emission in CWs has been unknown. To investigate the effect of plant diversity on the N 2 O emission, we established four plant species richness levels (each level containing 1, 2, 3 and 4 species, respectively) by using 96 hydroponic microcosms. Results showed that plant species richness enhanced the N 2 O emission, ranging from 27.1 to 115.4 μg N 2 O m −2 d −1 , and improved nitrate removal (P < 0.001). The presence of Phalaris arundinacea within a given plant community increased the N 2 O emission (P < 0.001). The presence of Rumex japonicas had no influence on the N 2 O emissions (P > 0.05), but improved nitrogen removal (P < 0.001). Hence, our study highlights the importance of both plant species richness and species identity in mediating the N 2 O emission and nitrogen removal in CWs.