Methane emissions respond slowly to changes in soil water table depth in contrast to carbon dioxide emissions

Abstract We present results from two sets of soil mesocosm experiments designed to explore the impact of changes in water table depth on methane (CH 4 ) and carbon dioxide (CO 2 ) emissions. In all experiments, soils were initially drained, as they are in situ . The first set of experiments (‘short-term experiments’) investigates how CO 2 and CH 4 emissions respond to transient changes in water table depth; the second set (‘long-term experiments’) investigates the response of CO 2 and CH 4 emissions to sustained high water table. In the short-term experiments, the water table was raised then gradually lowered over a month. In the long-term experiments, the water table was raised and maintained for several months. Our short-term experiments show that CO 2 emissions respond rapidly to changes in water table depth (with lower CO 2 emissions when the water table is raised and vice versa). Conversely, short-term changes in water table depth do not significantly impact CH 4 emissions. In the long-term experiments, CO 2 emission was consistently supressed by the high water table. Soil CH 4 production begins shortly after the water table is raised, but CH 4 emissions from the soil remain low until the onset of ebullition (bubble escape). Raising the water table led to a 30-fold reduction in total CO 2 -equivalent emissions which subsequently increased 6-fold after CH 4 ebullition started. Our key findings are that (1) the response time of CO 2 emissions to raising the water table is much shorter (