Seasonal Controls of CO₂ and CH₄ Dynamics in a Temporarily Flooded Subtropical Wetland

Subtropical and tropical wetlands play a prominent role in the global carbon (C) cycle; yet factors that influence their C fluxes remain uncertain. We collected measurements from a temporarily flooded subtropical wetland over 3 years to investigate environmental drivers impacting CO₂ and CH₄ fluxes. The wetland was a sink of CO₂ (−469 to −380 g C-CO₂ · m⁻² · year⁻¹) and a source of CH₄ (25.1 to 32.1 g C-CH₄ · m⁻² · year⁻¹) to the atmosphere. Dry season CH₄ emissions represented 41 to 49% of the annual budget, reflecting the importance of continuous CH₄ flux measurements. Gross primary productivity (GPP) increased with temperature and radiation, and the influence of VPD on GPP varied with soil inundation. Higher water tables decreased R_eco and increased GPP, and a higher GPP in turn lead to enhanced R_eco likely through enhancements of GPP on autotrophic respiration. This suggests that the impact of the water table on R_eco depends on the cancelling effects of hydrology and GPP. Emissions of CH₄ increased with soil temperature, water table, and GPP until soils were inundated at which point temperature and GPP became the main drivers. Water table and temperature influenced GPP and CH₄ fluxes, and increases in GPP directly enhanced CH₄ emissions. In addition to impacting C fluxes directly through water table depth, hydrology also determined the hierarchy of the dominance of factors controlling C fluxes and their response. The positive climate forcing of subtropical wetlands may be dictated by plant-mediated and climate interactions, with hydrological factors playing a major role in determining the greenhouse gas sink or source strength of subtropical wetlands.