Biodiversity has a close relationship with ecosystem functioning. For most biodiversity–ecosystem functioning studies, biodiversity has been linked to a single indicator variable of ecosystem functioning. However, there are generally multiple ecosystem processes contributing to ecosystem functioning and they differ in their dependence on biodiversity. Thus, the relationship between biodiversity and ecosystem functioning can be stronger when multiple rather than single ecosystem processes are considered. Using both mass-balance and stable-isotope approaches, we explored the effects of plant diversity on nitrogen (N) removal sustained by multiple N-cycling processes in experimental microcosms simulating constructed wetlands, an ecosystem treating wastewater with high N loading. Four species were used to assemble different plant communities, ranging in richness from one to four species. The removal of N, indicated by low levels of total inorganic N concentration (TIN) present in the effluent, was considered as an integrated measure of ecosystem functioning, combining three constituent N-cycling processes: plant uptake, denitrification and substrate adsorption. Our results showed that: (1) species richness had a positive effect on N removal, in particular, the four-species mixture reduced effluent TIN to a lower level than any monoculture, however, polycultures (two-, three- and four-species mixtures) did not outperform the most-efficient monoculture when each of the three constituent N-cycling processes was considered by itself; (2) species identity had significant impacts on single processes. Communities with the species Coix lacryma-jobi showed the greatest capacity for N uptake and communities with Phragmites australis had the highest denitrification rates; (3) isotope fractionation in the rhizosphere of Coix lacryma-jobi was primarily due to microbial denitrification while multistep isotope fractionation was detected for Phragmites australis and Acorus calamus (indicating recycling of N), suggesting that species differed in the way they transformed N; (4) the enhanced N removal at high diversity may be due to mutualistic interactions among species belonging to different functional types. Our findings demonstrated that although plant species richness had negligible effects on individual N-cycling processes, it enhanced the overall ecosystem functioning (N removal) when these processes were considered collectively. Our study thus contributes to improve the treatment efficiency of constructed wetlands through proper vegetation management.