Patterns and magnitudes of offsetting ecosystem CO<sub>2</sub> uptake by CH<sub>4</sub> emissions in coastal salt marshes

<p indent="0mm">Coastal salt marsh is an important blue carbon ecosystem that efficiently removes carbon dioxide (CO₂) from the atmosphere. However, the coastal salt marsh is also a significant source of methane (CH₄), which has a higher heat-trapping capacity than CO₂ over 20-year and 100-year scales, and may partially offset the net ecosystem CO₂ uptake. Using static flux chambers of different opacities, we measured the net ecosystem exchange of CO₂ (NEE) and ecosystem respiration (ER) under four different light levels (100, 80, 50, 30 and 0% ambient light) with a Licor-7810 CO₂/CH₄ analyzer, monthly at midday in a <italic>Scirpus mariqueter</italic> salt marsh and a <italic>Phragmites australis</italic> salt marsh within the Minjiang River Estuary National Nature Reserve, China. Based on the results, we established empirical models between gross primary productivity (GPP) and photosynthetic active radiation (PAR), and between ER and air temperature. Finally, using PAR and air temperature data from a meteorological station within the reserve, we estimated the GPP, ER and NEE at <sc>15 min</sc> intervals and extrapolated the results to monthly- and annually-integrated NEE and ER. We also measured CH₄ fluxes monthly at midday with the static flux chambers in the two salt marshes, and estimated the daily CH₄ fluxes based on a day-to-night ratio of 1.62 for CH₄ flux in the Minjiang River estuary salt marsh. Both <italic>S</italic>. <italic>mariqueter</italic> and <italic>P</italic>. <italic>australis</italic> salt marshes exhibited distinct diurnal and seasonal variations in NEE and ER. Both salt marshes acted as net CO₂ sinks, with annual NEE of −1673.7±<sc>122.7 g</sc> CO<sc>₂ m⁻² a⁻¹</sc> and −5550.0±<sc>130.1 g</sc> CO<sc>₂ m⁻² a⁻¹,</sc> respectively. <italic>P</italic>. <italic>australis</italic> salt marsh acted as net CO₂ sinks in all four seasons; however, <italic>S</italic>. <italic>mariqueter</italic> salt marsh acted as net CO₂ sources in winter. CH₄ flux at midday was <sc>0.01–1.7 mg m⁻² h⁻¹</sc> in <italic>S</italic>. <italic>mariqueter</italic> marsh and <sc>2.4–9.4 mg m⁻² h⁻¹</sc> in <italic>P</italic>. <italic>australis</italic> marsh, and the annually-integrated flux was <sc>5.3±0.1 g</sc> CH<sc>₄ m⁻² a⁻¹</sc> and <sc>39.5±0.3 g</sc> CH<sc>₄ m⁻² a⁻¹,</sc> respectively. Based on the global warming potential (GWP) of CH₄, the annual CH₄ emission from <italic>S</italic>. <italic>mariqueter</italic> marsh would offset 8.6% (over a 20-year time horizon) to 25.2% (over a 100-year time horizon) of its net CO₂ uptake, with the highest offset occurring in autumn (22.4%–66.1%) and the lowest offset occurring in spring (3.2%–9.4%). For <italic>P</italic>. <italic>australis</italic> marsh, CH₄ emission would offset 19.2% to 56.7% of its net CO₂ uptake, with the highest offset occurring in winter (54.9%–161.9%) and the lowest offset occurring in autumn (23.6%–69.7%). Based on GWP₁₀₀ for CH₄ emission, the carbon sink rate for <italic>S</italic>. <italic>mariqueter</italic> marsh was <?A3B2 pi6?><sc>1530.6 g</sc> CO₂-eq m⁻² a⁻¹, with a net carbon release in winter. Likewise, <italic>P</italic>. <italic>australis</italic> marsh carbon sink rate was <sc>4483.5 g</sc> CO₂-eq m⁻² a⁻¹, with a net carbon uptake in all four seasons. This study showed that, although CH₄ emission from coastal salt marshes would partially offset the ecosystem’s radiative cooling effect, coastal salt marsh ecosystem still plays a significant role as carbon sinks and contributes to mitigating global climate change.