Restored and reference coastal wetlands emit elevated levels of methane after an extreme rainfall event, but continue to provide climate benefits

Abstract Restoration of coastal wetlands provides climate adaptation and mitigation benefits. However, there is still limited information on the effects of climate change on restoration projects. We assessed the changes in soil greenhouse gas fluxes (GHG; methane, CH 4 , carbon dioxide, CO 2 , and nitrous oxide, N 2 O) on a site previously used for sugarcane production, and currently undergoing tidal reinstatement in subtropical Australia. Simultaneously, we sampled two natural reference mangrove sites. Sampling was conducted over three years, encompassing summer and winter seasons, before and after tidal reinstatement, and after an extreme rainfall event. After cessation of agriculture and before tidal reinstatement, GHG emissions at the restoration site were low and similar to those at the reference sites. Nine months after tidal reinstatement, soil conductivity increased from below detection limits to 5.9 ± 2.3 dS m −1 , and surface soil organic carbon increased by 38%, while GHG emissions remained low. After the cyclone and the extreme rainfall event, a large peak in CH 4 emissions was measured at the restoration site (3,661 ± 1,719 µ g m −2 h −1 ) and at one reference site (7,588 ± 2,193 µ g m −2 h −1 ); small N 2 O uptakes were also recorded in the restoration (−2.2 ± 0.5 µ g m −2 h −1 ) and reference sites (−0.7 ± 0.1 µ g m −2 h −1 ). The fluxes were associated with prolonged freshwater flooding and low soil oxygen. After the waters receded, the emissions returned to baseline levels. Despite the elevated emissions from years with extreme rainfall (1.5 Mg CO 2eq ha −1 yr −1 ), these are still much lower than those from alternative agricultural land uses. Thus, despite the extreme rainfall event, this restoration project continues to provide climate benefits.