General Comments

This manuscript investigates the deep-soil biogeochemical responses to Spartina alterniflora eradication, comparing plastic mulching (PM) and deep tillage (DT) over an 18-month period. The study's evaluation of the 0-100 cm soil profile addresses a relevant gap in coastal wetland restoration assessments, which often restrict their focus to the top 30 cm. The finding that PM-induced desalinization increases deep-soil microbial biomass and destabilizes iron-bound organic carbon (Fe-OC) provides useful mechanistic insights into subsoil carbon dynamics. Overall, the manuscript presents valuable data. However, several areas require further clarification and deeper discussion before it can be recommended for publication.

Major Comments

1. Coupling of Carbon and Sediment Nitrogen TransformationsThe manuscript details the depletion of soil carbon pools, noting that PM reduced total carbon (TC) by 19-35% and soil organic carbon (SOC) by 34-65%. Concurrently, the results indicate a significant restructuring of the nitrogen pools, with PM causing a 51%-74% loss in total nitrogen (TN) alongside altered distributions of ammonium and nitrate (Figure 2e-g). Given the stoichiometric coupling of carbon and nitrogen cycles in coastal wetland sediments, the impact of Spartina alternifloramanagement on sediment nitrogen transformation warrants more detailed discussion. Elaborating on how these shifts in inorganic nitrogen availability might regulate or interact with the deep microbial reactivation would strengthen the mechanistic interpretation of the observed decomposition dynamics.
2. The Fate of Destabilized CarbonThe study reports a net soil C decline of 65 ± 12 Mg C ha⁻¹over 18 months under the PM treatment. As acknowledged in Section 4.2, the absence of pore water chemistry data makes it difficult to definitively partition the fate of the lost Fe-OC between complete microbial mineralization to greenhouse gases ($CO_2$ / $CH_4$) and the lateral export of dissolved Fe(II)-organic complexes via leaching. Consequently, references to "climate impacts" or "carbon emissions" in the introduction and abstract should be carefully moderated. Framing the results strictly around carbon "destabilization and potential loss" will align the conclusions more closely with the measured variables and avoid overextrapolating the findings.
3. Temporal LimitationsThe 18-month timeframe captures the initial perturbation responses to the restoration interventions. Ecological restoration is a long-term process, and it remains uncertain whether these soil carbon losses will stabilize, decelerate, or continue over time. This temporal limitation should be explicitly stated earlier in the abstract and introduction to establish appropriate scientific expectations for the temporal scope of the findings.

Minor Comments

• Abstract Clarification:The abstract states that "Amino-sugar biomarkers indicated coherent shifts in microbial necromass C with Fe-OC dynamics". However, the methodology and results for these amino-sugar biomarkers are absent from the main text and methods section. Please include the relevant analytical protocols and data, or remove this statement if it falls outside the scope of the current manuscript.
• Carbon Stock Calculations:Section 2.5 mentions using both fixed-depth (FD) and equivalent soil mass (ESM) approaches to calculate SOC stocks. While the FD formula is provided (Equation 2), the results section must explicitly specify which method is being utilized when discussing the carbon stock depletions. Bulk density changes resulting from treatments like deep tillage (DT) can significantly skew interpretations if only fixed-depth calculations are applied without clear context.
• Figure Clarity:Ensure the resolution of Figure 1 is sufficient in the final production stage so that the text detailing the core schematic and measurement purposes is clearly legible.