Soil greenhouse gas fluxes from tropical coastal wetlands and alternative agricultural land uses

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Abstract

Coastal wetlands are essential for regulating the global carbon budget through soil carbon sequestration and greenhouse gas (GHG-CO2, CH4, and N2O) fluxes. The conversion of coastal wetlands to agricultural land alters these fluxes' magnitude and direction (uptake/release). However, the extent and drivers of change of GHG fluxes are still unknown for many tropical regions. We measured soil GHG fluxes from three natural coastal wetlands-mangroves, salt marsh, and freshwater tidal forests- A nd two alternative agricultural land uses-sugarcane farming and pastures for cattle grazing (ponded and dry conditions). We assessed variations throughout different climatic conditions (dry-cool, dry-hot, and wet-hot) within 2 years of measurements (2018-2020) in tropical Australia. The wet pasture had by far the highest CH4 emissions with 1231±386mgm-2d-1, which were 200-fold higher than any other site. Dry pastures and sugarcane were the highest emitters of N2O with 55±9mgm-2d-1 (wet-hot period) and 11±3mgm-2d-1 (hot-dry period, coinciding with fertilisation), respectively. Dry pastures were also the highest emitters of CO2 with 20±1gm-2d-1 (wet-hot period). The three coastal wetlands measured had lower emissions, with salt marsh uptake of-0.55±0.23 and-1.19±0.08gm-2d-1 of N2O and CO2, respectively, during the dry-hot period. During the sampled period, sugarcane and pastures had higher total cumulative soil GHG emissions (CH4+N2O) of 7142 and 56124CO2-eqkgha-1yr-1 compared to coastal wetlands with 144 to 884CO2-eqkgha-1yr-1 (where CO2-eq is CO2 equivalent). Restoring unproductive sugarcane land or pastures (especially ponded ones) to coastal wetlands could provide significant GHG mitigation.

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Iram, N., Kavehei, E., Maher, D. T., Bunn, S. E., Rezaei Rashti, M., Farahani, B. S., & Adame, M. F. (2021). Soil greenhouse gas fluxes from tropical coastal wetlands and alternative agricultural land uses. Biogeosciences, 18(18), 5085–5096. https://doi.org/10.5194/bg-18-5085-2021

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