Mitigation of methane emissions from constructed farm wetlands

Constructed wetlands are increasingly used for water pollution treatment but may also be sources of the greenhouse gas CH4. The effect of addition of two potential inhibitors of methanogenesis – iron ochre and gypsum – on net CH4 emissions was investigated in a constructed wetland treating farm runoff in Scotland, UK. CH4 fluxes from three 15-m2 wetland plots were measured between January and July 2008 in large static chambers incorporating a tunable diode laser, with application of 5 ton ha−1 ochre and gypsum in May. CH4 fluxes were also measured from control and ochre- and gypsum-treated wetland sediment cores incubated at constant and varying temperature in the laboratory. Ochre addition suppressed CH4 emissions by 64 ± 13% in the field plot and >90% in laboratory incubations compared to controls. Gypsum application of 5 ton ha−1 in the field and laboratory experiments had no effect on CH4 emissions, but application of 10 ton ha−1 to a sediment core reduced CH4 emissions by 28%. Suppression of CH4 emissions by ochre application to sediment cores also increased with temperature; the reduction relative to the control increased from 50% at 17.5 °C to >90% at 27.5 °C. No significant changes in N removal or pH and potentially-toxic metal content of sediments as the result of inhibitor application were detected in the wetland during the study.