Data from: Plant diversity increases N removal in constructed wetlands when multiple rather than single N processes are considered

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.