Aridland constructed treatment wetlands I: Macrophyte productivity, community composition, and nitrogen uptake

Urbanized areas increasingly rely on constructed treatment wetlands (CTW) for cost effective and environmentally-based wastewater treatment. Constructed treatment wetlands are particularly attractive treatment options in arid urban environments where water reuse is important for dealing with scarce water resources. Emergent macrophytes play an important role in nutrient removal, particularly nitrogen (N) removal, in CTW. However, the role of plant community composition in nutrient removal is less clear. Numerous studies have shown that macrophyte species differentially affect N uptake processes (e.g., direct plant uptake, coupled nitrification–denitrification, soil accretion). However, many of these studies have been based on small-scale experiments and have been carried out in mesic environments, which means that their findings are difficult to extrapolate to aridland CTW systems. Our study sought to examine the relationships among emergent macrophyte productivity, plant community composition, and N uptake [by both the plants and the entire ecosystem] at a 42 ha CTW in arid Phoenix, Arizona, USA. We quantified above- and belowground biomass bimonthly and foliar N content annually for four species groups (Typha latifolia + Typha domingensis, Schoenoplectus californicus + Schoenoplectus tabernaemontani, Schoenoplectus acutus, and Schoenoplectus americanus) from July 2011 to September 2013. We quantified dissolved inorganic N fluxes into and out of the system and compared plant N removal to total system fluxes. Additionally, we estimated monotypic N content for each to compare the system's current community composition and plant N removal to hypothetical scenarios in which the system was dominated by only one species. Peak aboveground biomass ranged from 1586 ± 179 (SE) to 2666 ± 164 (SE) gdw m⁻² of which Typha spp. accounted for an increasing portion (>66%). We observed widespread ‘thatching’ – the toppling of large stands of macrophytes – that was likely related to a decline in peak biomass from July 2011 to July 2012. The foliar N content was similar among the species groups and N content for all species combined, at peak biomass, was 31 ± 8 N g m⁻². This measured foliar N content was higher than our estimates of the foliar N content in hypothetical monotypic stands, suggesting that the system's actual community composition performed better, in terms of direct plant N uptake, than if the system had been planted with only one species group. Overall, direct plant N uptake accounted for 7% of inorganic N inputs and 19% of whole-system inorganic N removal. Our findings suggest that managers and designers should consider diverse plant communities rather than monotypic stands when designing, constructing, and managing CWT wetland systems. Future research is needed to elucidate those management strategies that might best promote or preserve diverse plant communities in these systems in a cost effective manner.