Patterns of methane flux on different temporal scales and its environmental controls over a subtropical “floating blanket” wetland in southwest China

Abstract

Wetlands are the primary natural source of methane (CH₄) emissions to the atmosphere, however, the quantification of wetland CH₄ flux and net carbon budgets remains uncertain due to limited measurements and large variability across different types of wetlands. Based on continuous measurements using the eddy covariance (EC) technique over a “floating blanket” wetland in southwest China for a period of over three years (2018–2021), we examined the temporal variations of CH₄ flux and its environmental controls on different temporal scales. The results showed that this wetland acted as a sink for carbon dioxide (CO₂) while simultaneously serving as a source for CH₄. The annual CO₂ uptake and CH₄ emission was 155.9–221.2 g C-CO₂ m⁻² yr⁻¹ and 44.6–56.9 g C-CH₄ m⁻² yr⁻¹ during 2019–2021, respectively. The annual total carbon budget varied from -176.6 g C m⁻² yr⁻¹ to -99.0 g C m⁻² yr⁻¹. From half-hourly to monthly timescales, CH₄ flux was mainly controlled by water temperature with a positive correlation. There was a negative correlation between CH₄ flux and CO₂ flux, and the importance of CO₂ flux to CH₄ flux increased as the timescale extended. CH₄ flux exhibited a significant positive correlation with the ecosystem gross primary production (GPP) and respiration (Re) on daily and monthly timescales. In addition, the fraction of vegetation cover had a positive impact on CH₄ emissions. There was a significant variation in the fraction of vegetation cover with wind direction. The southwestern wind sector had the highest fraction of vegetation cover compared to other sectors, leading to the formation of the CH₄ emission hotspot. The cumulative CH₄ emissions in the southwestern wind sector were also higher than those in other sectors, accounting for around 58.13% of the annual total CH₄ emissions, while the cumulative CH₄ emissions in other wind sectors accounted for 5.21%–21.50%.