Multi-scale temporal variation of methane flux and its controls in a subtropical tidal salt marsh in eastern China

Abstract

CH4 emissions could vary with biotic and abiotic factors at different time scales. However, little is known about temporal dynamics of CH4 flux and its controls in coastal marshes. In this study, CH4 flux was continuously measured with the eddy covariance technique for 2 years in a subtropical salt marsh in eastern China. Wavelet analysis was applied to explore the multi-scale variations of CH4 flux and its controls. Additionally, partial wavelet coherence was used to disentangle confounding effects of measured variables. No consistent diurnal pattern was found in CH4 fluxes. However, the hot-moments of CH4 flux were observed after nighttime high tide on days near the spring tide. Periodic dynamics were also observed at multi-day, semilunar and seasonal scales. Tide height in summer had a negative effect on CH4 flux at the semilunar scale. Air temperature explained most variations in CH4 fluxes at the multi-day scale but CH4 flux was mainly controlled by PAR and GEP at the seasonal scale. Air temperature explained 48% and 56% of annual variations in CH4 fluxes in 2011 and 2012, respectively. In total, the salt marsh acted as a CH4 source (17.6 ± 3.0 g C–CH4 m−2 year−1), which was higher than most studies report for inland wetlands. Our results show that CH4 fluxes exhibit multiple periodicities and its controls vary with time scale; moreover, CH4 flux is strongly modified by tide. This study emphasizes the importance of ecosystem-specific measurements of CH4 fluxes, and more work is needed to estimate regional CH4 budgets.