CO2 and CH4 effluxes across six land uses in coastal wetlands of North Sumatra

Mangrove and other coastal wetlands such as saltmarsh and seagrass are termed ‘blue carbon’ ecosystems due to their substantial capacity for carbon storage and sequestration over a long-term time scale. Policymakers and stakeholders are currently promoting mangroves into national carbon management as part of nature-based climate change mitigation and adaptation strategy. Unfortunately, global mangroves area with particularly in the tropics is decreasing at a rapid rate due to land-use and land-cover change (LULCC). Yet, there has been limited study of carbon emissions impacted by multiple mangrove conversions at the landscape scale. Here we assessed spatio-temporal patterns of soil CO₂ and CH₄ effluxes across six land uses, namely mangroves converted to 15 yrs oil palm, 20 yrs coconut, and 20 yrs aquaculture (pond wall and water surface), as well as newly logged mangrove, 10 yrs planted mangrove, and undisturbed mangrove forests reference in North Sumatra, Indonesia. Direct measurement of soil CO₂ and CH₄ effluxes were performed by using an ultra-portable LGR gas analyser during low tide condition between 08.00 and 16.00, with triplicated PVC 10-inch diameter and 25 cm height opaque static chambers (closed system) were installed at each land use in September-October 2021 -- representing wet season in the study site. The soil CO₂ and CH₄ effluxes were collected three times for each chamber and 3 days of measurement during this field campaign with a total of 193 measurements were performed. We observed that the top three highest soil CO₂ and CH₄ effluxes were among aquaculture pond wall soils (591±104 mgCO₂ m² h^-1 and 0.40±0.17 mgCH₄ m² h^-1), logged mangroves (480±104 mgCO₂ m² h^-1 and 3.21±1.34 mgCH₄ m² h^-1), and natural mangroves (274±71 mgCO₂ m² h^-1 and 0.58±0.28 mgCH₄ m² h^-1). By contrast, relatively low effluxes (< 200 mgCO₂ m² h^-1 and < 0.1 mgCH₄ m² h^-1) were observed across other land-use types. Our preliminary results suggest that the variation of soil CO₂ and CH₄ in our study sites may be controlled by the duration of the disturbances, particularly we observed the highest CO₂ and CH₄ effluxes at newly (occurred at the same year with our measurement) constructed pond wall and logged mangrove locations. On the other hand, low CO₂ and CH₄ effluxes were observed at both oil palm and coconut plantations. These new land uses were constructed more than 10 years ago with the application of drainage and tidal blocking. Our current limited data constraint further essential factors that commonly control CO₂ and CH₄ in the coastal wetlands, such as tidal elevation, bioturbation, seasonal variation, and soil properties. Overall, our dataset will be essential to guide policymakers in related to the improvement of land-based low carbon development and climate change mitigation strategies for Indonesia to meet the targeted 29% of unconditional carbon emissions reduction by 2030 as outlined in the Nationally Determined Contributions (NDCs) as part of the Paris Agreement.