Patterns and magnitudes of offsetting ecosystem CO2 uptake by CH4 emissions in coastal salt marshes
Year: 2025
Type: article
Abstract: 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 Scirpus mariqueter salt marsh and a Phragmites australis 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 15 min 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 S. mariqueter and P. australis 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±122.7 g CO2 m−2 a−1 and −5550.0±130.1 g CO2 m−2 a−1, respectively. P. australis salt marsh acted as net CO2 sinks in all four seasons; however, S. mariqueter salt marsh acted as net CO2 sources in winter. CH4 flux at midday was 0.01–1.7 mg m−2 h−1 in S. mariqueter marsh and 2.4–9.4 mg m−2 h−1 in P. australis marsh, and the annually-integrated flux was 5.3±0.1 g CH4 m−2 a−1 and 39.5±0.3 g CH4 m−2 a−1, respectively. Based on the global warming potential (GWP) of CH4, the annual CH4 emission from S. mariqueter 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 P. australis 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 S. mariqueter marsh was <?A3B2 pi6?>1530.6 g CO2-eq m−2 a−1, with a net carbon release in winter. Likewise, P. australis marsh carbon sink rate was 4483.5 g 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.
DOI: 10.1360/tb-2024-1345
Authors: Yixuan Yu, Jiafang Huang, Lin Li, Shuyuan Fang, Wenbo Lin, Jujuan Gao, Chuan Tong
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W4408637769