Soil greenhouse gases emissions reduce the benefit of mangrove plant to mitigating atmospheric warming effect

Abstract. Mangrove soils have been recognized as sources of atmospheric greenhouse gases but the atmospheric fluxes are poorly characterized, and their adverse warming effect has scarcely been considered with respect to the role of mangrove wetlands in mitigating global warming. The present study balanced the warming effect of soil greenhouse gas emissions with plant carbon dioxide (CO₂) sequestration rate in a highly productive mangrove wetland in South China to assess the role of mangrove wetland in mitigating atmospheric warming. The results showed that mangrove soils were significant sources of greenhouse gases, and the fluxes were significantly higher in summer and also different among mangrove sites. Gases fluxes were positively correlated with the soil organic carbon, total nitrogen, and NH₄⁺-N contents. The mangrove plant was able to sequester a considerable amount of atmospheric CO₂ at 5930 g CO₂ m⁻² yr⁻¹ in the present study, and the ecosystem was source of methane (CH4) and nitrous oxide (N₂O) gases but more intense CO₂ sink. However, the warming effect of soil gas emissions, equivalent to 1222 g CO₂ m⁻² yr⁻¹, was able to offset a large proportion (~22 %) of plant CO₂ sequestration, and the two trace gases comprised ~24 % of the total warming effect. We therefore propose the assessment of the direct mitigation of atmospheric warming by mangrove ecosystem that should take into account both soil greenhouse gases emissions and plant CO₂ sequestration.