Greenhouse gas emissions from tropical coastal wetlands and their alternative agricultural lands: Where significant mitigation gains lie

Abstract

Abstract. Tidal coastal wetlands are significant to the global carbon budgets through carbon sequestration and greenhouse gas (GHG; CO2, CH4 and N2O) emissions. The conversion of tidal coastal wetlands to agriculture land alters soil processes changing GHG emissions. The GHG emissions associated with land-use change are important for restoration strategies that rely upon financial incentives such as carbon credits. We measured GHG fluxes from mangroves, saltmarsh and freshwater tidal forest and their alternative agricultural lands including sugarcane and ponded pastures. We investigated seasonal variations between June 2018 and February 2020 in tropical. Australia. The wet ponded pasture had by far the highest CH4 emissions with 1,231 ± 386 mg m−2 d−1, which were 200-fold higher than any other land use. Agricultural lands were the most significant sources of N2O emissions with 55 ± 9 mg m−2 d−1 from dry ponded pasture (wet-hot period) and 11 ± 3 mg m−2 d−1 from sugar cane (hot-dry period), coinciding with fertilisation. The N2O fluxes from the tidal coastal wetlands ranged between −0.55 ± 0.23 and 2.76 ± 0.45 mg m−2 d−1 throughout the study period. The highest CO2 fluxes of 20 ± 1 g m−2 d−1 were from the dry ponded pasture during the wet-hot period, while the saltmarsh had the lowest CO2 fluxes having an uptake of −1.19 ± 0.08 g m−2 d−1 in the dry-hot period. Overall, agricultural lands had significantly higher total cumulative GHG emissions (CH4 + N2O) of 7142 to 56,124 CO2-eq kg ha−1 y−1 compared to those of any type of tidal coastal wetlands, which ranged between 144 and 884 CO2-eq kg ha−1 y−1. Converting agricultural land, particularly wet ponded pasture, to tidal coastal wetlands could provide large GHG mitigation gains and potential financial incentives.