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

Mangroves are critical blue carbon ecosystems. Measurements of methane (CH4) emissions from mangrove tree stems have the potential to reduce the uncertainty in the capacity of carbon sequestration. This study is the first to simultaneously measure the 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 during tidal cycles, which have distinct root structures. The mangrove tree stems served as both net CO2 and CH4 sources. Compared to those of the soils, the mangrove tree stems exhibited markedly lower CH4 fluxes, but no difference in CO2 fluxes. A. marina (with pneumatophores) exhibited significantly higher CO2 and CH4 fluxes than K. obovata. The stems of A. marina exhibited an increasing trend in the CO2 flux from low to high tides, while the CH4 flux showed high temporal variability, with this species functioning as a sink before tidal inundation and becoming a source during low tides after ebbing. In contrast, the stems of K. obovata showed no consistent pattern of the CO2 or CH4 flux. Based on our findings, sampling only during low tides might overestimate the stem CO2 and CH4 fluxes on a diurnal scale. The stem CO2 and CH4 fluxes of A. marina could be 55 % and 194 % less when considering tidal influence, as opposed to ignoring tidal influence. This study highlights species distinctness in the greenhouse gas (GHG) fluxes and the necessity of considering tidal influence when quantifying GHG fluxes from mangrove tree stems.