| 18 Aug 2026
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 NOx (= 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.
Review of „Seasonal methane and nitrous oxide exchange in a sandy coastal eelgrass meadow“
Here Frederiksber et al. studied the CH4 and N2O dynamics in the interface sediment/soil-water in a sandy coastal eelgrass meadow. They carried out a particular on-site incubation experiment to determine potential flux exchange across soil-water interface with and without eelgrass. I clearly understand the justification of the study and the main objective, however, I believe the manuscript requires several restructurings in the approach and methodology used to support the conclusion. I hope my comments will help the authors improve the manuscript in terms of presentation and interpretation of the results.
CH4 and CO2 fluxes
The first topic that comes to me is the method used to estimate the soil-water CH4 and N2O flux. All fluxes come only from an initial and a final syringe sample over 3 or more hours (If I understand). Without intermediated samples, the rate cannot detect leaks, CH4 ebullition events, or even saturation events. There is no information about the detection limit flux for both gases, and for N2O I think is extremely relevant according with the data reported in Figure 1. For example, the N2O data shows a rate of 0.3 micromol m-2 h-1 over 4 hours is perhaps very small and are very close to the analytical noise, so please show this information in the manuscript, and I recommend you check if your values are not in that range and flag fluxes below the range.
The second issue concerns the conceptual approach used to determine the contribution these fluxes to atmospheric fluxes (air-water interface). The chambers measure CH4 and N2O across the soil-water interface, but authors subsequently convert them directly to CO2-eq, apparently assuming that the CH4 and N2O emitted from soils to water column are emitted to the atmosphere. I know, at 1 m, a substantial fraction of the gases may scape to the atmosphere. However, this should not be assumed without considering methanotrophy in shallow waters (manuscript itself discussed it), accumulation in the water column (gas storage), therefore some important amounts of CH4 can be consumed/stored before reaching the air-water interface, similarly, for N2O. I suggest to toning down the interpretation of your measurements as direct emissions to the atmosphere and instead work on the dynamic of soil-water interface measured. Also, I recommend to include values of dissolved CH4 and N2O concentrations to the manuscript elsewhere, I think, those values are very important for the scientific community.
The third issue is related to the incubation approach. As was planned, it does not account for permeability of sandy soils or for advection transport processes associated with the constants influence of waves. So, wave pressure changes can drive advective porewater exchange through permeable sand, and it may dominate the CH4 and N2O transport across the soil-water interface. Therefore, the incubations performed in closed chambers with the stirring at 60rpm may not correctly reproduce the in-situ processes. Moreover, with the limited sampling during the incubations, assuming a linear increase over time maybe is problematic.
Other issues related to the design: (i) please explain the reason to use two different chamber approaches, 30 and 18 cm heights differs importantly, so water volume and stirring hydrodynamic differ; (ii) light incubation is the strongest predictor for N2O model but the light incubations had only one measurement in January and June, it is very risky to use this data; (iii) why did you change sample containers (Exetainers and Hungate)? (iv) the annual budgets (which I recommend above to toning down) are estimated from day time samplings with one to 3 samplings.
Carbon Balance
First, there is no CO2 taking into account in the balance. You ran light/dark incubations but you did not measure CO2 and/or other inorganic carbon in water and soils. Therefore, your carbon balance is only including CH4 and N2O not the benthic metabolism (line Net Ecosystem Production).
Second, the organic carbon was measured only in 25cm and the 210Pb alone cannot detect burial rates, because there is no a decreasing trend with depth. It implies that surface soil layer is being homogenized (e.g. due to waves, sandy setting, bioturbation, root activity, etc.). So, the mixing soils erase the age vs depth relationship. Therefore, the actual comparison that supports your conclusion, emission flux vs. burial flux, is never made.
Finally, in the balance of net emission for Z. marina is 79 kg CO2-eq ha-1 yr-1, which equals only about 2.2 g C m-2 yr-1 burial. Your own comparison shows the lowest Danish eelgrass accumulation rate (220 kg CO₂-eq ha-1 yr-1, is ca. 6 g C m-2 yr-1) is about three times larger than their emissions (Line 375). So, to claim that CH4 and N2O emissions “offset any likely climate-mitigation benefit” is not completely supported. Moreover, you must show that the burial rate is below to 2 gC m-2 yr-1 and I think your data cannot show it.
Also, it is also a problem with the stock difference between meadow and sand, which it is about 0.18 kg C m-2 and not significant (p = 0.099, with a n = 5 vs 4, so statistical power is low). Still, if it is because the meadow and the meadow is a few decades old, I think the accumulation are in several order of magnitude of g C m-2 yr-1 (what do you estimate?), exceeding the CH4 and N2O emission offset. I would recommend to report the meadow age somehow, for example, from historical information.