Abstract
Mangrove wetland soils have been considered as important sources for atmospheric CH4, but the magnitude of CH4 efflux in mangrove wetlands and its relative contribution to climate warming compared to CO2 efflux remains controversial. In this study, we measured both CH4 and CO2 effluxes from mangrove soils during low or no tide periods at three tidal zones of two mangrove ecosystems in Southeastern China and collected CH4 efflux data from literature for 24 sites of mangrove wetlands worldwide. The CH4 efflux was highly variable among our field sites due to the heterogeneity of mangrove soil environments. On average, undisturbed mangrove sites have very low CH4 efflux rates (ranging from 0.65 to 14.18 μmol m−2 h−1; median 2.57 μmol m−2 h−1), often less than 10% of the global warming potentials (GWP) caused by the soil CO2 efflux from the same sites (ranging from 0.94 to 9.50 mmol m−2 h−1; median 3.67 mmol m−2 h−1), even after considering that CH4 has 28 times more GWP over CO2. Plant species, study site, tidal position, sampling time, and soil characteristics all had no significant effect on mangrove soil CH4 efflux. Combining our field measurement results and literature data, we demonstrated that the CH4 efflux from undisturbed mangrove soils was marginal in comparison with the CO2 efflux in most cases, but nutrient inputs from anthropogenic activities including nutrient run-off and aquaculture activities significantly increased CH4 efflux from mangrove soils. Therefore, CH4 efflux from mangrove wetlands is strongly influenced by anthropogenic activities, and future inventories of CH4 efflux from mangrove wetlands on a regional or global scale should consider this phenomenon.
Highlights
Global wetlands are considered as important carbon sinks for sequestering high amounts of carbon dioxide (CO2 ) from the atmosphere and contain more than 30% of the world’s organic carbon in the soils, despite accounting for only 5%–8% of the global terrestrial surface [1,2,3]
Organic-rich soils dominate in mangrove carbon storage, accounting for 49%–98% of carbon stocks in mangrove wetlands [9,10]
CH4 efflux was highly variable among the sampling sites, for most sampling sites; CH4 efflux was small and almost negligible, ranging from 0.65 ± 0.91 to 14.18 ± 6.35 μmol m−2 h−1, while at the landward zone in Zhangjiang Estuary site during the wet season, CH4 efflux was about 10 times higher than the highest value found at other sites (123.59 ± 41.79 μmol m−2 h−1 ) (Figure 2a,b and Table S1)
Summary
Global wetlands are considered as important carbon sinks for sequestering high amounts of carbon dioxide (CO2 ) from the atmosphere and contain more than 30% of the world’s organic carbon in the soils, despite accounting for only 5%–8% of the global terrestrial surface [1,2,3]. Mangrove wetlands could be key ecosystems in addressing climate regulation through their high productivity and effective carbon (C) sequestration rates [4,5,6,7]. Organic-rich soils dominate in mangrove carbon storage, accounting for 49%–98% of carbon stocks in mangrove wetlands [9,10]. Wetlands are identified as major CH4 sources for the atmosphere, emitting 177 to 284 Tg CH4 year−1 , corresponding to approximately 40% of the total global CH4 emission [11].
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