Abstract
Considerable variability in methane production and emissions has been reported in mangroves, explained by methane inhibition and oxidation. In this study, soil pore waters were collected from mangrove forests located in the Gulf of California (Mexico) exposed to shrimp farm disturbance. The δ13C of dissolved inorganic carbon (DIC) and CH4 were analyzed along with the δ13C of the soil organic matter to assess the proportion of CO2 derived from methanogenesis, its main pathway, and the fraction of methane oxidized. We performed slurry incubation experiments to fit the isotope–mass balance approach. Very low stoichiometric ratios of CH4/CO2 were measured in pore waters, but isotope mass balances revealed that 30–70% of the total CO2 measured was produced by methanogenesis. Mangrove soils receiving effluent discharges shifted the main methanogenesis pathway to CO2 reduction because of an increase in refractory organic matter. Isotope–mass balances of incubations indicated that methane was mainly oxidized by anaerobic oxidation of methane (AOM) coupled to sulfate reduction, and the increase in recalcitrant organic matter should fuel AOM as humus serves as a terminal electron acceptor. Since methanogenesis in mangrove soils is strongly controlled by the oxygen supply provided by mangrove roots, conservation of the forest plays a crucial role in mitigating greenhouse gas emissions.
Highlights
Wetlands are the primary source of non-anthropogenic methane to the atmosphere [1]
Whereas high methane fluxes are explained by the use of non-competitive substrates by methanogens [14,15], low methane fluxes are mainly explained by methanogenesis inhibition and, to a lesser extent, by anaerobic methane oxidation (AOM) linked to sulfate reduction, which might consume a significant fraction of CH4 [16,17]
Our isotope–mass balances indicated that the low concentration of CH4 measured in mangrove soil pore waters was due to CH4 oxidation under anaerobic conditions (AOM) instead of methane inhibition by sulfate reducers, which outcompete methanogens for common substrates [55]
Summary
Wetlands are the primary source of non-anthropogenic methane to the atmosphere [1]. Methanogenesis is considered the dominant pathway for the decomposition of organic matter in freshwater wetlands [2], whereas sulfate-reducing bacteria outcompete methanogens for common substrates in coastal areas and saline wetlands [3]. Tidal seawater is thought to provide enough sulfate-to-sulfate-reducing bacteria with much higher affinity and competition for H2 and acetate than methanogens [4,5]. It would explain the relatively low methane emission rates measured in many coastal wetlands, including mangrove swamps, compared to freshwater wetlands [3,6,7,8]. Whereas high methane fluxes are explained by the use of non-competitive substrates by methanogens [14,15], low methane fluxes are mainly explained by methanogenesis inhibition and, to a lesser extent, by anaerobic methane oxidation (AOM) linked to sulfate reduction, which might consume a significant fraction of CH4 [16,17]. Because methanogenesis and AOM zones overlap in the soil profile, it is impossible to spatially segregate each chemical involved to assess the balance between methane production and consumption in these coastal wetlands
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