Salinity Mediates Tamarix chinensis Litter Decomposition to Enhance Soil Enzyme Activity in Coastal Saline–Alkali Soils

The aim of this study was to explore the decomposition characteristics of <i>Tamarix chinensis</i> litter and its soil-improving capacity under different salinities. Four treatments were designed: a control (CK) treatment without saline water injection and three treatments encompassing slightly (SS, 0.4% soil salinity), moderately (SM, 0.8%), and highly saline (SH, 1.2%) conditions. <i>T. chinensis</i> litter at three degrees of decomposition (undecomposed, semidecomposed, and already decomposed) was studied. After 180 days, the litter substrate quality, 0-10 cm soil physicochemical properties, and enzyme activities were measured. Correlation analysis and structural equation modeling were employed to elucidate the interactions and response patterns among soil salinity, the decomposition characteristics of <i>T. chinensis</i> litter, and the physicochemical properties and enzyme activities of surface soil. The results revealed the following: (1) With increasing soil salinity, the contents of litter lignin, cellulose, total carbon and nitrogen residues first decreased but then increased, reaching minima under SM, whereas the content of hemicellulose residue exhibited the opposite trend. With increasing degree of litter decomposition, the contents of lignin and cellulose residues decreased, whereas the contents of hemicellulose, total nitrogen and phosphorus residues increased. (2) With increasing soil salinity, the soil water content, organic matter content, total nitrogen content, and activity of several enzymes increased, peaking under SH. The pH performance followed the order of SS > SM > CK > SH. The total carbon and phosphorus contents first increased but then decreased, with a maximum under SS. The activity of N-acetylamino glucosidase first decreased but then increased and was greatest at moderate and high salinities. (3) The soil water content and level of enzyme activity were significantly correlated with the litter substrate quality. Salinity negatively affected litter substrate residues but positively affected soil physicochemical properties. Litter decomposition under different soil salinities indirectly influenced soil enzymes by affecting soil properties, whereas salinity modulated soil properties directly or through litter decomposition. <i>T. chinensis</i> litter decomposition notably increased enzyme activity in moderate- to high-salinity alkali coastal soils, offering insights for low-efficiency <i>T. chinensis</i> forest management and saline-alkali soil remediation in the Yellow River Delta.