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-m(2) 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 degrees C to >90% at 27.5 degrees 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. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.