| 23 Oct 2025
Carbon dioxide release driven by organic carbon in minerogenic salt marshes
Abstract. Coastal wetlands play an important role in the global carbon cycle by sequestering carbon (referred to as “blue carbon”). At the same time, organic carbon (OC) in the subsurface is decomposed, releasing greenhouse gases (GHGs) such as carbon dioxide (CO2) and methane (CH4). To predict how this carbon balance in salt marshes will change under future climate scenarios (e.g., higher temperatures, sea level rise), it is essential to understand the controls on OC decomposition in these systems. Here, we investigated OC turnover and CO2 release in a minerogenic salt marsh at the Wadden Sea, Germany. We first characterized the porewater and sediment of a pioneer marsh and adjoining intertidal flat to identify key biogeochemical processes. We then performed an in situ experiment by injecting two OC sources (labile (acetate)/complex (humic acid)) and subsequently monitored GHG release over four injection cycles along with subsurface geochemistry. Overall, we found that the microbially mediated CO2 release was limited by OC availability and composition, and not by electron acceptor availability, as evidenced by the presence of aqueous sulfate (SO42-) at all depths and the lack of CH4. Following the addition of labile OC, CO2 release in the pioneer marsh increased by up to 47.4 ± 36.4 % compared to the control, with a generally similar trend in the intertidal flat. The CO2 release from the complex OC treatment was similar to the control. The results of our work improve understanding of minerogenic salt marsh OC dynamics in temperate zones and enable better prediction of future changes.
This manuscript presents a well-designed and timely study investigating controls on organic carbon (OC) decomposition and CO₂ release in minerogenic salt marsh sediments of the Wadden Sea. The work addresses an important gap in the blue carbon literature, which has historically focused on peat-dominated North American marshes. The combination of detailed geochemical characterisation with an in situ organic carbon manipulation experiment is innovative and provides new insight into the relative roles of electron acceptors and OC quality in regulating greenhouse gas dynamics.
The manuscript is generally well written, logically structured, and well-grounded in relevant biogeochemical literature. The conclusions are consequential and should interest the coastal biogeochemistry, blue carbon, and wetland modelling communities.
However, several aspects of the experimental design, interpretation, statistics, and framing require clarification or strengthening before publication. Some assumptions remain insufficiently justified, and a few conclusions appear stronger than the presented evidence supports.
Major Comments:
Specific comments:
Lines 40-42: This statement is very general. Authors should mention which climate-driven processes, such as temperature, sea-level rise, vegetation shifts, storm frequency, are most relevant to OC turnover in minerogenic marshes. This would help frame the specific hypothesis tested later in the manuscript.
Lines 64-68: This is a key knowledge gap the study is addressing. I would suggest explaining in more detail examples of the work in European Saltmarshes with respect to TEA turnover and not GHG emissions.
Lines 56 – 59: I suggest clarifying why minerogenic sediments might differ from organogenic ones here. For example, in TOC content, mineral surface area, or porewater exchange, would better justify the hypothesis. This distinction is central to the paper, but it’s currently presented it in broad terms.
Lines 59-61: I suggest rewording this text to focus on how previous work has focused on locations that are biogeochemically different to the current study and mention the geographic locations as an aside. Currently this text could be misinterpreted as implying that previous work is geographically narrow or uninformative. Explaining how the U.S. sites differ biogeochemically (peat-dominated, high TOC, strongly reducing conditions) would prevent oversimplification and more clearly position the present study as filling a genuine gap.
Lines 91-95: I suggest including quantitative information here. The current phrasing provides qualitative differences but lacks quantitative information (inundation duration, frequency, elevation relative to MHW). Hydrology strongly influences redox conditions and solute transport and providing explicit values or ranges would help readers assess how representative and comparable the two zones are.
Lines 111-115: Were actions taken to reduce compaction? Push-core sampling can cause compression or smearing, especially in fine-grained sediments. This text would benefit from a short note on steps taken to minimise disturbance.
Lines 175-177: It would be useful for the readers to know the chamber volume here.
Line 195: Should be ‘inner diameter of 2.5 cm, and a length of 10 cm’
Lines 246-248: This section lacks detail for the reader to appreciate what numerical analysis was carried out. I appreciate all the details of statistical analysis is provided in the supplement, but some of the important information should be provided within the main body of the text. As a minimum, a summary of the statistical workflow should be included in the main text.
Line 315: ‘for the’ is repeated
Line 341: ‘Complimentarily’ is not an appropriate word here. Something like ‘Similarly’ or ‘In comparison’ should be used
Line 496: Typo ‘decreasing to 0mM below.’
Line 503: I suggest rewording ‘We speculate’. Speculation is not something that’s encouraged in scientific work.
Line 623: should be ‘led’ instead of ‘lead’
Line 627: ‘was much higher’ should be ‘were much higher’
Figures 4-8: Statistically significant differences should be denoted on the figures. It is common practice to place italics letters above bars in such comparison to show statistical groupings. This makes it easier for the reader to very quickly determine the statistical differences, if any are present.