Global greenhouse gas (GHG) emissions have increased alarmingly fuelling climate change. Fossil fuel burning and land-use change are the main contributors to global GHG emissions, and their reductions are required to reduce global GHGs. Protection and conservation of coastal wetlands and agriculture management are the two main sectors that can provide the land-use based climate mitigation, with the potential to be reduced by 35-50%. Therefore, projects and policies involving coastal wetland protection and restoration are being encouraged globally important GHG mitigation approaches. However, the success of all mitigation approaches largely depends upon evidence-based knowledge about the types of mitigation actions, their feasibility, and potential co-benefits. This dissertation consists of six chapters that centre on exploring the magnitude of the GHG emissions due to coastal wetland loss and a potential solution through wetlands restoration on unused agricultural lands. Chapter 2 is a literature review to identify knowledge gaps regarding GHG fluxes from wetlands and other land uses. I synthetised 153 articles and found that GHG fluxes were mainly reported from temperate marshes while GHG fluxes from sub/tropical wetlands were underrepresented. Overall, 48% of the studied wetlands were marshes converted for cropping and grazing, while mangroves were mainly converted to aquaculture. I found that sugarcane fields in tropical Australia, had very high N2O emissions, suggesting that their conversion to wetlands may present a GHG mitigation opportunity. Finally, I found GHG fluxes in tropical ponded pastures, which were potentially very high, were not reported yet, calling to fill in this critical knowledge gap in Australia. In Chapter 3, I explored the soil greenhouse gas (GHG) fluxes from tropical coastal wetlands and their converted agricultural lands. I measured gaseous fluxes throughout two years between 2018 and 2020, considering seasonal differences. The GHG emissions were lowest from coastal wetlands and highest from sugarcane. I found that converted sugarcane and ponded pasture had an order of magnitude higher cumulative GHG (CH4, N2O) fluxes than coastal wetlands, including mangroves, saltmarshes, and supratidal forests (Melaleuca spp.). Therefore, I concluded that wetlands restoration in the tropical region would provide significant GHG mitigation benefits. However, long-term studies on GHG emissions from wetlands and alternative agricultural land use types, including poorly studied systems such as ponded pastures, are required to improve accuracy on the mitigation of GHG through wetland restoration. The Department ii of Industry, Science, Energy and Resources, Queensland used these findings to develop the Australian blue carbon method, which was recently approved. In Chapter 4, a case study was investigated in subtropical Australia, where an unused sugarcane land was successfully restored to coastal wetlands through tidal reinstallation. I measured carbon (C) sequestration and GHG fluxes to estimate abatement. I also measured N soil sequestration and removal through denitrification as indicators of water quality improvement. I compared an active sugarcane farm as a reference site with the restored mangroves, saltmarsh, and supratidal forests (Melaleuca spp.) using a Before-After-Control-Impact design. I found that restoration of unused sugarcane land to supratidal forest provided the highest carbon abatement of ~18.5 Mg CO2-eq ha-1 y-1 followed by restoration to mangroves and saltmarsh at ~11 Mg CO2-eq ha-1 y-1 and ~6.2 Mg CO2-eq ha-1 y-1 respectively. Tree growth, soil accumulation and reduced N2O fluxes due to cessation of fertilisation were the main contributors to the carbon abatement. Carbon abatement benefits overrode the enhanced CH4 emissions due to flooding. Additionally, the restored coastal wetlands provided water quality improvement service through N removal of up to 50.4 mg m-2 d-1. These findings provide baseline data for countries that are seeking to meet their GHG mitigation targets and other SDGs through land management. Chapter 5 of this thesis explored the effects of litter composition on GHG fluxes from the supratidal forest (Melaleuca spp.). In a laboratory incubation experiment, I quantified the GHG fluxes through the addition of 1) a litter fall mixture of Melaleuca quinquenervia bark and leaf litter of other dominant plants found on-site, 2) leaf litter of M. quinquenervia, and 3) oil of M. alternifolia, which is a potent natural antimicrobial. I found that cumulative GHG emissions (CH4 + N2O) were increased in leaf litter and oil treatments, indicating some changes in the microbial communities involved in CH4 and N2O regulation. These studies would add to emerging knowledge of C dynamics in supratidal forest restoration. In summary, this thesis has showed that Australia could have carbon mitigation benefits of up to 63 Mg CO2-eq ha-1 y-1 by restoring tropical wet pastures, and up to 7.5 Mg CO2-eq ha-1 y-1 from restoring unused or marginal sugarcane. The present study presented regional baseline data based on a realistic assessment of GHG mitigation and associated ecosystem services. This research will contribute to national policies targeting GHG reduction targets.