Plant species effect on CO2 and CH4 emissions from pilot constructed wetlands in Mediterranean area

The aim of this research was to evaluate the role of constructed wetland (CW) horizontal sub-surface flow pilot-plant beds vegetation, comparing four perennial herbaceous plant species with an unvegetated bed, on carbon dioxide (CO2) and methane (CH4) emissions, and CO2(eq) budgets. The research was conducted from April 1st to November 30th in 2012 and 2013 in a pilot plant located in San Michele di Ganzaria (Sicily, Italy) that treated urban wastewaters, studying Phragmites australis (Cav.) Trin. ex Steud. (common reed), Arundo donax L. (giant reed), Chrysopogon zizanioides (L.) Roberty (vetiver) and Miscanthus x giganteus Greef et Deu. (miscanthus). Results showed a greater aboveground biomass yield in the second experimental year than the first one for all species except vetiver, which showed a 10.5% reduction. Considering CWs gases emission, a significantly higher CO2 emission (+52.5%) was monitored in 2013 than 2012 whereas CH4 had the opposite trend (-97.0%). Seasons and plant species influenced gases emission. The lower CO2 emission (median value 5.2 gm(-2) d(-1)) was monitored during the spring seasons when instead was monitored the highest CH4 emission (median value 0.232 gm(-2) d(-1)). Opposite CO2 and CH4 emissions were found in fall. A. donax, M. giganteus and P. australis determined significantly higher (2.9 times) CO2 emission than C. zizanioides and unvegetated bed. Vegetated beds showed a positive CO2(eq) total balance with the best results calculated for A. donax whereas, as expected, it was negative for the unvegetated bed, with a cumulative CO2(eq) emission of 6.68 kgm(-2). Obtained results confirm the active and key role of plant species used in the CW systems and indicate A. donax as the most environmentally friendly species to use under Mediterranean climate conditions, followed by P. australis.