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

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.