Seasonal methane and nitrous oxide exchange in a sandy coastal eelgrass meadow

Abstract. Seagrass meadows are considered as potential blue carbon ecosystems through burial and long-term storage of organic carbon in their sediments. However, this climate benefit may be reduced or even negated by emissions of greenhouse gases (GHG) such as methane (CH₄) and nitrous oxide (N₂O). Here, we quantified seasonal CH₄ and N₂O fluxes and identified controlling biogeochemical factors in a shallow, sandy Zostera marina meadow (Z. marina) and an adjacent unvegetated site (Sand) in Denmark. We used in situ benthic chamber incubations under dark and light conditions, combined with measurements of plant biomass, porewater chemistry, solid-phase sediment variables, and ²¹⁰Pb concentration profiles. Fluxes of CH₄ were low overall, ranging from -0.8 to 15.5 µmol m^-2 h^-1, with the largest emissions occurring in autumn. Across sites and seasons, CH₄ fluxes increased with Z. marina belowground biomass and decreased with the porewater NO_x (= NO₂^- + NO₃^-) inventories, indicating that root/rhizome-derived organic matter fueled methanogenesis, while more oxidizing conditions constrained net CH₄ release. In contrast, N₂O fluxes were small and variable (-0.44 to 0.32 µmol m^-2 h^-1), without a clear seasonal pattern. The most important controllers of N₂O fluxes were light conditions and porewater NH₄⁺, suggesting a role for coupled or light-driven NO₂^- reduction and nitrification-denitrification processes. Sediment OC concentrations were uniformly very low (< 0.4 %), and excess ²¹⁰Pb was restricted to the upper few centimeters without a consistent downcore decline, reflecting sediment mixing and suggesting low sedimentation rates, although neither these nor the OC sequestration rates could be quantified. Together with the negligible fine-sediment content, these results point to very limited long-term OC burial at this site. We therefore conclude that, in shallow organic-poor sandy sediments of Z. marina such as those studied here, even modest CH₄ emissions are sufficient to offset any likely climate-mitigation benefit from sedimentary carbon sequestration.