Effect of woody plant diversity on biomass production and nitrogen and phosphorus uptake in constructed wetland microcosms

Investigations into how plant diversity influences constructed wetland functionality have predominantly focused on herbaceous species has been increasing. More recently, however, the applications of woody species in constructed wetland have been increasing. This study established vertical-flow constructed wetland microcosms to systematically evaluate the effects of woody plant diversity on biomass production and plant uptake of nitrogen (N) and phosphorus (P). Four woody species, Ligustrum obtusifolium , Hibiscus mutabilis , Hibiscus syriacus and Nerium oleander , were assembled into three species richness levels (1, 2 and 4 species) and eleven species compositions (four monocultures, six two-species mixtures and one four-species mixture) with four replicates per treatment. The results showed that biomass production, N and P uptake all increased significantly with increasing woody species richness, but this apparent richness effect was not independent of species composition: once species composition was accounted for, richness had no statistically detectable effect. Communities containing the high-yielding species N. oleander increased 89.86% more biomass production, exhibited 54.50% higher N uptake and 59.58% greater P uptake than communities that did not include this species. Additive partitioning of the net biodiversity effect revealed that selection effects were dominant and accounted for nearly the entire positive biodiversity effect in all mixtures containing N. oleander , while significant complementarity effects were only detected in the three two-species mixtures that excluded this species. The results suggest that vegetation configuration in constructed wetland design should prioritize targeted species selection and optimized species assemblage, rather than simply maximizing plant species richness.