Pontederia sagittata and Cyperus papyrus contribution to carbon storage in floating treatment wetlands established in subtropical urban ponds

Carbon sequestration is considered an ecosystem service of regulation provided by diverse ecosystems, including wetlands. It has been widely evaluated in the soil of natural wetlands while in constructed wetlands, there is scanty information. In Floating Treatment Wetlands (FTW) there is none. Previously, our research group reported the efficient performance of FTW in an urban polluted pond for two years. As a follow up, the aim of this work was to investigate the contribution of Cyperus papyrus and Pontederia sagittata to carbon storage (CS) in four FTW established in eutrophic urban ponds in a subtropical region. Plant growth, productivity, and CS were assessed in the aboveground biomass of C. papyrus and P. sagittata and the belowground biomass (root mix from C. papyrus and P. sagittata), throughout 26 months in 2 FTW with an area of 17.5 m2 (FTW1) and 33 m2 (FTW2) and throughout 19 months in 2 FTW with an area of 25 m2 (FTW3) and 33 m2 (FTW4), respectively. The macrophyte growth depended on various factors, such as the season, the plant species, and the location of the FTW. High relative growth rate values were found for both species (0.125 and 0.142 d−1 for P. sagittata and C. papyrus, respectively), especially during summer and early autumn. The highest values of productivity were 337 ± 125 gdw m−2d−1 for the aboveground biomass of C. papyrus in FTW2, 311 ± 96.90 gdwm−2d−1 for the aboveground of P. sagittata in FTW1, and 270 ± 107 gdw m−2d−1 for the belowground biomass in FTW2. The mean values of CS for P. sagittata found in FTW1 were 1.90 ± 0.94 kg m−2, while for C. papyrus in FTW2 they were 4.09 ± 0.73 kg m−2. The contribution of the belowground biomass to CS was also significant in FTW2 (4.58 ± 0.59 kg m−2).