Experimental evidence of warming-driven changes in CO2 and CH4 emissions from mangrove soils

Mangrove forests are key coastal ecosystems for carbon sequestration and climate change mitigation. However, their soil functioning under warming scenarios remains poorly understood. This study evaluated how increased temperature affects soil biogeochemical functioning and CO₂ and CH₄ emissions using an experimental approach. Experimental units were established in Cananéia, on the southern coast of São Paulo State, a region with well-preserved mangroves. Soil pH remained stable throughout the 90-day experiment, averaging 5.9 ± 0.4. In contrast, redox potential (Eh) declined markedly, from +56 ± 28 mV at the beginning to −5 ± 7 mV at the end of the experiment. CO₂ and CH₄ fluxes exhibited an inverse temporal pattern: CO₂ emissions decreased from 1,986 ± 749 to 493 ± 129 mg m⁻² h⁻¹, whereas CH₄ emissions increased from 3 ± 2 to 41 ± 47 mg m⁻² h⁻¹, representing an increase of approximately 1,200%. The decline in Eh indicates a progressive shift in soil biogeochemical functioning, consistent with enhanced organic matter decomposition and establishment of reductive metabolic pathways. This shift was evidenced by high initial CO₂ emissions followed by reduced fluxes and a substantial increase in CH₄ fluxes favorable to methanogenesis. Although CO₂ emissions declined over time, the pronounced CH₄ increase represents a climatic concern due to its higher global warming potential. These findings demonstrate that warming can alter not only the magnitude but also the composition of greenhouse gas emissions from mangrove soils, underscoring the need for long-term studies to better constrain biogeochemical responses to climate change in coastal wetlands.