Optimization of nitrogen, water and salinity for maximizing soil organic carbon in coastal wetlands

Maintaining carbon sequestration is becoming increasingly significant and challenging during coastal wetland carbon restoration. To clear the influence of water, nitrogen and salinity on soil organic carbon (SOC), indoor incubation experiments were carried out, where soil from wetlands with dominant vegetation of Phragmites australis and Suaeda salsa were sampled and Box-Behnken central composite design was used to maximize SOC. The results indicated that:1) water, nitrogen and salinity altered soil chemical and physical properties, then significantly affect SOC in Phragmites australis wetland, but possessed little impact on Suaeda salsa wetland. 2) Optimal condition was determined with water of − 5.49 cm, salinity of 3.52 ds/m and nitrogen of 13.40 g/m2 for the biggest SOC of 8.6 g/kg in Phragmites australis wetland, whereas in Suaeda salsa wetland, SOC reached out to 8.7 g/kg with water of − 4.09 cm, salinity of 2.51 ds/m and nitrogen of 16.61 g/m2. 3) Nitrogen and water were determining factors on SOC. In Phragmites australis wetland, SOC was substantially influenced by water, nitrogen and its interaction, while in Suaeda salsa wetland, water and nitrogen displayed no direct influence, but its interactive effect worked. 4) Quadratic models were found the most desirable to reveal connections between SOC and three factors (p < 0.05). Our finding could provide support for maintaining soil carbon sink in coastal wetland restoration management.