Soil water content and salinity regulate the temperature sensitivity of CO<sub>2</sub> and CH<sub>4</sub> emissions in a coastal salt-affected land

Diao, Mengmeng; Sun, Wenyao; Zhang, Zhaosong; Chen, Yitong; Zhang, Qian; Ma, Xiang; Sun, Juan; Yang, Chao

doi:10.5194/egusphere-2026-217

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https://doi.org/10.5194/egusphere-2026-217

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05 Mar 2026

| 05 Mar 2026

Status: this preprint is open for discussion and under review for SOIL (SOIL).

Soil water content and salinity regulate the temperature sensitivity of CO₂ and CH₄ emissions in a coastal salt-affected land

Mengmeng Diao, Wenyao Sun, Zhaosong Zhang, Yitong Chen, Qian Zhang, Xiang Ma, Juan Sun, and Chao Yang

Abstract. Soil carbon emissions from coastal saline-alkaline ecosystems significantly influence the global carbon cycle, yet their responses to key environmental drivers such as soil water content and salinity remain insufficiently understood. This study employed controlled incubation experiments using soils collected from the Yellow River Delta, China, to systematically investigate the effects of varying soil water content (5 %, 15 %, 30 %, 45 %, and 60 %) and salinity levels (S1: EC=1.9 dS/m; S2: 10.8 dS/m; S3: 58.8 dS/m; S4: 66.3 dS/m; S5: 96.0 dS/m) on CO₂ and CH₄ emissions and their temperature sensitivity (Q₁₀). The results demonstrated that under constant temperature conditions, CO₂ emission flux followed a unimodal pattern in response to increasing soil water content, peaking at 45 % water content, with CH₄ flux exhibiting a similar trend. Soil salinity significantly suppressed the fluxes of both greenhouse gases, with reductions observed across all temperature levels as salinity increased. Both soil water content and salinity played substantial regulatory roles in modulating the Q₁₀ of gas emissions. Specifically, Q₁₀ values for CO₂ and CH₄ initially decreased and then increased with rising soil water content. Along the salinity gradient, the Q₁₀ of CO₂ decreased from S2 to S4, whereas the Q₁₀ of CH₄ increased progressively from S2 to S5. These findings reveal the complex and interactive effects of soil water content and salinity on carbon cycling processes in coastal saline-alkaline lands. The study provides crucial theoretical insights for improving the prediction of carbon cycle dynamics under climate change and offers a scientific basis for the adaptive management and conservation of these vulnerable ecosystems.

Received: 15 Jan 2026 – Discussion started: 05 Mar 2026

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Mengmeng Diao, Wenyao Sun, Zhaosong Zhang, Yitong Chen, Qian Zhang, Xiang Ma, Juan Sun, and Chao Yang

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CC1:
'Comment on egusphere-2026-217', Jinsheng Li, 03 Apr 2026 reply
General assessment:
This study presents a well-designed incubation experiment investigating the effects of soil water content and salinity on CO2 and CH4 fluxes and their temperature sensitivity (Q10) in coastal saline-alkaline soils from the Yellow River Delta. The results are novel and provide valuable insights for carbon-climate feedback modeling in vulnerable ecosystems. The manuscript is clearly written and generally sound. I recommend acceptance after the following revisions.
Lines 12–13 (Abstract): “CO2 emission flux followed a unimodal pattern in response to increasing soil water content” – Consider adding “clearly” or rephrasing to “CO2 emission flux showed a unimodal response to increasing soil water content” for better readability.

Lines 70–71 (Introduction, Hypothesis 2): “Elevated salinity will inhibit the emission potential…” – Change to “Elevated salinity is hypothesized to inhibit the emission potential…” to maintain consistency with the hypothetical framing used in Hypothesis 1 and 3.

Lines 90–91 (Methods): “Soils from points with identical vegetation types were composited” – Please clarify whether soils from different points with the same vegetation were thoroughly mixed before subsampling, to avoid potential misinterpretation of spatial heterogeneity.

Lines 134–135 (Results): “CO2 flux exhibited a distinct unimodal relationship” – For consistency, use the same wording as in the abstract (“unimodal pattern” or “unimodal response”).

Lines 232–234 (Discussion): “substrate availability replaces the activation energy” – This phrasing is somewhat strong. Suggest revising to “substrate availability may become more rate-limiting than the activation energy required for reaction kinetics”.

Figures 1 & 2: The lowercase/uppercase letters indicating significant differences are densely placed. Please clarify in the caption that lowercase letters compare water content (or salinity) treatments at the same temperature, and uppercase letters compare temperature treatments at the same water content (or salinity). This is already described but could be made more explicit.

Figures 3–8: The R2 values shown in each panel have inconsistent decimal places (e.g., 0.99 vs. 0.95). Please unify to two decimal places throughout.

Tables 2 & 3: Add a brief footnote explaining the meaning of parameter “b” and how Q10 was derived from b (although described in the main text, tables should be self-explanatory). Also, consider highlighting the minimum Q10 values (e.g., at 45% water content) in the text or table caption for better emphasis.

Lines 255–258 (Discussion): When stating that both mcrA and pmoA may decline with increasing salinity, please add one sentence clarifying that net CH4 emission is suppressed only if methanogenesis (mcrA) is reduced more strongly than oxidation (pmoA), or if substrate availability becomes more limiting. This will prevent potential misinterpretation.

Lines 263–265 (Discussion): The cited Q10 range (2.6 to 5.2) for CO2 under salinity stress appears much higher than the values reported in this study (1.44–1.75). Please comment briefly on this discrepancy (e.g., differences in ecosystem type, incubation conditions, or substrate availability) to improve contextualization.

Line 452 (Figure 1 caption): “Significant differences (P < 0.05) among water content treatments at the same temperature are indicated by lowercase letters” – This is clear, but please ensure that in Figure 1 itself, the letters are legible and correctly placed (some appear slightly crowded).

The manuscript is scientifically sound and well written; the above suggestions aim to improve clarity, consistency, and interpretability.

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Citation: https://doi.org/10.5194/egusphere-2026-217-CC1
RC1:
'Comment on egusphere-2026-217', Anonymous Referee #1, 12 Apr 2026 reply
The authors provide a straightforward analysis of soil CO₂ and CH₄ emissions under different soil water content and salinity levels. The results suggest that both greenhouse gases peak at ~45% water content and increase as salinity levels increase. The results also show that soil water content and salinity affect the temperature sensitivity (Q₁₀) of CO₂ and CH₄ differently, indicating predictions of carbon cycling in coastal regions should include soil water content and salinity under climate change.
The experimental design appears sound, and the approach is appropriate for addressing the research question. However, I recommend providing a more detailed description of the methods to ensure readers can fully follow the experimental setup.
Comments:
Line 12 (Abstract): “Soil carbon emissions” – change to “Soil CO₂ and CH₄ emissions”
Line 80 – 85 (Methods): For better understanding of the results and discussion, it would be helpful to provide more detail on the study site conditions and the ecological meaning of the manipulated soil water content. For example, does this represent tidal ecosystems, wetlands, or systems influenced by freshwater-saltwater mixing?
Line 87 – 97 (Methods): “In May 2022” – can add a brief seasonal description for May. Clarify the sample characteristics: “Line 87: soil sampling was conducted across the main salinity gradients of the study area” vs. “Line 91: Soils from points with identical vegetation types were composited” vs. “Line 96: soils were categorized into five salinity gradients” – do the composited samples represent salinity gradients or vegetation type?
Line 99 – 108 (Methods): Please provide more detail on the incubation setup. For example:
were soils from 0–20 cm mixed and then packed into PVC columns?

what chemicals were used to establish different salinity and soil water content treatments?

what was the initial water content of the soil samples?

how was soil water content set and monitored during incubation?

was soil water content measured on a gravimetric or volumetric basis?

may need to consider the effect of drying and rewetting on microbial activity (e.g., Beem et al., 2021).

Line 107, 113 (Methods): “Flux measurements were conducted over a 10-day period…” – clarify measurement frequency (e.g., once after stabilization?). Measurement time (9–11 am) may not be relevant in a controlled incubation where temperature is maintained continuously.
Line 116 – 118 (Methods): Please clarify the dependent variables. Why was one- or two-way ANOVA used instead of a three-way ANOVA?
Figures: Figures 1 and 2 appear to contain similar information as Figures 3 and 4, with only different visualization styles (bar vs. scatter). Figures 5–6 also seem similar to Figures 7–8. Where do the scatter points come from? For example, in Figure 3 at 5% water content and 5°C, Methods mention “five replicates,” but more than five points are shown – likely related to measurement frequency? Please clarify.

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Citation: https://doi.org/10.5194/egusphere-2026-217-RC1

Mengmeng Diao, Wenyao Sun, Zhaosong Zhang, Yitong Chen, Qian Zhang, Xiang Ma, Juan Sun, and Chao Yang

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Mengmeng Diao, Wenyao Sun, Zhaosong Zhang, Yitong Chen, Qian Zhang, Xiang Ma, Juan Sun, and Chao Yang

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Short summary

Salt-affected coastal lands play a key but poorly understood role in the global climate system. Through controlled laboratory experiments with soils from China's Yellow River Delta, we discovered that both gases respond strongly to changes in moisture and salinity. The emissions peak at a specific moisture level, while higher salt content generally suppresses them. These findings are crucial for creating more accurate climate models and for managing these vulnerable coastal areas effectively.


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Soil water content and salinity regulate the temperature sensitivity of CO2 and CH4 emissions in a coastal salt-affected land

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Soil water content and salinity regulate the temperature sensitivity of CO₂ and CH₄ emissions in a coastal salt-affected land