Role of C3 plant species on carbon dioxide and methane emissions in Mediterranean constructed wetland

C3 plant species are widely used to vegetate constructed wetlands (CW), but so far no information is available on their effect on CW CO_2(eq) balance in the Mediterranean climate. The aim of this research was to study carbon dioxide (CO₂) and methane (CH₄) emissions and CO_2(eq) budgets of CW horizontal sub-surface flow pilot-plant beds vegetated with Arundo donax L. and Phragmites australis (Cav.) Trin. ex Steud. compared with an unvegetated bed in Sicily. The highest total plant biomass production was measured in the bed vegetated with A. donax (17.0 kg m^–2), whereas P. australis produced 7.6 kg m^–2. CO₂ and CH₄ emissions and showed significant correlation with average air temperature and solar radiation for each bed. The CO₂ emission values ranged from 0.8±0.1 g m^–2 d^–1, for the unvegetated bed in April, to 24.9±0.6 g m^–2 d^–1 for the bed with P. australis in August. The average CO₂ emissions of the whole monitored period were 15.5±7.2, 15.1±7.1 and 3.6±2.4 g m^–2 d^–1 for A. donax, P. australis and unvegetated beds respectively. The CH₄ fluxes differed significantly over the monitored seasons, with the highest median value being measured during spring (0.963 g m^–2 d^–1). No statistical differences were found for CH₄ flux among the studied beds. Cumulative estimated CH₄ emissions during the study period (from April to December) were 159.5, 134.1 and 114.7 g m^–2 for A. donax, P. australis and unvegetated beds respectively. CO_2(eq) balance showed that the two vegetated beds act as CO_2(eq) sinks, while the unvegetated bed, as expected, acts as a CO_2(eq) source. Considering only the above-ground plant biomass in the CO_2(eq) budgets, P. australis and A. donax determined uptakes of 1.30 and 8.35 kg CO_2(eq) m^–2 respectively.