The effects of macrophyte productivity and community composition on the water and nutrient budgets of an aridland constructed treatment wetland.

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Constructed treatment wetlands (CTW) have been well established as effective and sustainable solutions to the problem of urban water treatment and reuse. However, treatment wetlands located in aridland cities may face unique hydrological and ecological challenges, relative to their more mesic counterparts, that challenge their ability to deliver ecosystem services. In hot, dry climates large water losses via evaporation and plant transpiration may evapoconcentrate solutes in the water column and soils, potentially jeopardizing the ability of these systems to perform intended functions –namely nitrogen (N) retention and processing.  In addition, 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.  These studies have been carried out in mesic environments, which means that their findings are difficult to extrapolate to aridland CTW systems.  Our two primary objectives were to 1) develop robust water and N budgets to evaluate the impact of an arid climate on CTW N removal and 2) quantify macrophyte community composition and develop estimates of species-specific contribution to the N and water budgets at a 42 ha CTW in arid Phoenix, Arizona.

We found that total water losses via evapotranspiration peaked at 300,000 m³ month^-1 in the hot, dry summer months and averaged more than 70% of the whole-system water losses over a 27 month time period, substantially higher than similar mesic systems. These water losses evapoconcentrated solutes in the vegetated marsh, but rates of inorganic N removal remained nearly complete, indicating that evapoconcetration of solutes did not seem to affect ecosystem service performance.

Peak aboveground biomass ranged from 1586±179 to 2666±164 gdw m^-2 of which Typha spp. accounted for two-thirds.  Foliar N content was similar among species 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 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. Direct plant N uptake accounted for 7% of inorganic N inputs and 19% of whole-system inorganic N removal.

Contrary to our expectations, large transpirative water losses and diverse plant communities appeared to enhance N treatment efficacy relative to humid, mesic systems by drawing large volumes of replacement water into the marsh via a “biological tide,” providing more opportunities for vegetation and soil microbes to process N.