Tidal influence on carbon dioxide and methane fluxes from tree stems and soils in mangrove forests

Abstract. Mangroves are critical blue carbon ecosystems. Measurements of methane (CH4) emissions from mangrove tree stems have the potential to reduce uncertainty in the capacity of carbon sequestration. This study is the first to simultaneously measure CH4 fluxes from both stems and soils throughout tidal cycles. We quantified carbon dioxide (CO2) and CH4 fluxes from mangrove tree stems of Avicennia marina and Kandelia obovata, which have distinct root structures, during tidal cycles. Tree stems of both species served as net CO2 and CH4 sources. Compared to fluxes in the soils, the mangrove tree stems exhibited remarkably lower CH4 fluxes but no difference in CO2 fluxes. The stems of A. marina exhibited an increasing trend in CO2 flux from low to high tides. However, CH4 fluxes showed high temporal variability, with the stems of A. marina functioning as a CH4 sink before tidal inundation and becoming a source after ebbing. In contrast, the stems of K. obovata showed no consistent pattern in the CO2 or CH4 fluxes. Based on our findings, the stem CH4 fluxes in A. marina may vary by up to 1200 % when considering tidal influence, compared to when tidal influence is ignored. Therefore, sampling only during low tides might underestimate stem CO2 and CH4 fluxes on a diurnal scale. This study highlights the necessity of considering tidal influence and species when quantifying greenhouse gas (GHG) fluxes from mangrove tree stems. Further study is needed to explore the underlying mechanisms driving the observed flux variations and improve the understanding of GHG dynamics in mangrove ecosystems.