Plant community changes and related nutrient retention within an aridland constructed wastewater treatment wetland

Background, questions, methods:  Constructed wetland systems (CWS) are increasingly used for wastewater treatment. These systems partially rely on the ability of emergent macrophytes to take up nutrients from wastewater to improve water quality.  However, due to species specific variation in plant nutrient requirements, community composition changes may influence spatial and temporal patterns of nutrient uptake.  In this study, our goal was to examine how community composition varies over seasons in a CWS and how this variation influenced nitrogen retention.  We examined and quantified intra-annual plant community composition changes within a 21 ha CWS at the largest wastewater treatment facility in Phoenix, AZ. Separate multiple regression allometric biomass models were developed to relate various plant characteristics to dry weight for the 7 species present.  Every 2 months, beginning in July 2011, we measured plants within 5 randomly placed 0.25 m² quadrats along ten 50m transects for characteristics found to be significant in allometric models. Water samples were taken at the inflow and outflow, filtered, and analyzed for total dissolved nitrogen content. Aboveground plant tissue samples were collected, dried, milled, and analyzed for carbon and nitrogen content. 

Results, conclusions:  We observed little change in aboveground peak biomass composition from July 2011 to July 2012.  During these two months Typha domingensis and Typha latifolia composed most aboveground biomass (66%).  Relative abundance of Typha spp. declined through fall and winter of 2011 and spring of 2012 while that of Schoenoplectus acutus, Schoenoplectus tabernaemontani, and Schoenoplectus californicus increased over the same period.  Schoenoplectus americanus showed the highest aboveground nitrogen content levels for above and below ground biomass (2.14% and 1.35% dry weight, respectively).  Typha spp. showed the lowest nitrogen content in aboveground biomass (1.34% dry weight); however, due to its high biomass density, proved the most efficient plant at retaining nitrogen per unit area (average N g/m2 at peak biomass).  From March 2012 to May 2012, Typha spp. accounted for the majority (1502 N kg) of total aboveground macrophyte N retention (2319 N kg).  Overall, macrophyte N retention accounted for 6% of total N input.  Other factors (e.g., denitrification, sedimentation) must account for the majority of N retention within the system.