Restored coastal wetlands emit high levels of methane after a cyclone, but remain carbon sinks

Restoration of coastal wetlands provides climate adaptation and mitigation benefits.  However, there is still limited information on the effects of climate change-driven events on restoration projects. We assessed the changes in soil greenhouse gas fluxes (GHG; methane, CH₄, carbon dioxide, CO₂, and nitrous oxide, N₂O) on a site previously used for sugarcane production 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 the landfall of a cyclone. Before tidal reinstatement, GHG emissions at the restoration site were low and similar to those from the reference sites.  After tidal reinstatement, soil conductivity increased from zero to 5.9 ± 2.3 dS m^-1, and the soil organic carbon increased by 38%, while GHG emissions remained low. After the tropical storm, a large peak in CH₄ was measured at the restoration site (3,661 ± 1,719 µg m^-2 hr^-1) and at one reference site (7,588 ± 2,193 µg m^-2 hr^-1); small  N₂O uptakes were also recorded in the restoration (-2.2 ± 0.5 µg m^-2 hr^-1) and reference sites ( -0.7 ± 0.1 µg m^-2 hr^-1).   The fluxes were associated with prolonged freshwater flooding and reduced soil conditions (-0.3 ± 12 mV and -151 ± 96 mV, respectively) caused by extreme rainfall. Nevertheless, the emissions from this event did not undermine the carbon sink potential of the restoration project, whose annual emissions (0.8 Mg CO₂eq ha^-1 yr^-1), even for years with cyclones (1.5 Mg CO_2eq ha^-1 yr^-1), remained lower than those from the former agricultural land use (2.6 Mg CO₂eq ha^-1 yr^-1).  Climate change will increase the likelihood of extreme rainfall events; however, mangrove restoration projects are likely to remain carbon sinks.