Seawater intrusion causes substantial release of dissolved organic carbon in coastal paddy soils

Paddy soils located in coastal areas are readily exposed to seawater intrusion; however, the impact of salt ions on the adsorption of dissolved organic carbon (OCDissolved) at the soil–water interface and the transformation of solid-phase soil organic carbon (SOC) fractions remains quantitatively underexplored. Therefore, we investigated the dynamics of SOC, greenhouse gases (CH4 and CO2), and soil chemical properties under anoxic (40 d) and oxic (55 d) conditions at different salinities. A significant release of soil OCDissolved was observed in response to high salinity, which were mainly attributed to the competitive processes as the presence of large amounts of ions (e.g., Na+, Cl– and so on). High salinity inhibited anaerobic Fe(III) reduction by affecting the activity of iron-reducing bacteria (FeRB), leading to a decrease in the decomposition of Fe/Al mineral-bound organic carbon (OCFe/Al mineral-bound). Moreover, high salinity notably inhibited the expression and activity of soil microbial carbon cycling functional genes, thereby reducing the production of CH4 and CO2. Further kinetic modeling showed that under high salinity, OCDissolved contributed 2.54 % of the SOC after 40 days of anoxic incubation, which was significantly higher than that of the control (1.35 %) and low salinity treatments (1.42 %). After 15 days of oxic incubation, the OCDissolved content also showed relatively higher contribution in the high salinity treatment (1.91 %) compared to the control (0.75 %) and low (0.72 %) treatments. These results found that the release of labile organic carbon increased with increasing salinity levels, indicating a potential risk of organic carbon loss. These findings provide a fundamental understanding of SOC dynamics in coastal paddy fields.