Effects of Different Salinity Conditions on the Stoichiometric Characteristics of the Tamarix Litter–Soil System in the Yellow River Delta

To investigate the total carbon (TC), total nitrogen (TN), and total phosphorus (TP) contents and ecological stoichiometric characteristics of the litter–soil system under different soil salinity conditions, the litter–soil system of Tamarix chinensis in coastal saline-alkali soils of the Yellow River Delta was selected as the study system. Slight, moderate, and severe salinity treatments were established under simulated conditions, with nonsaline irrigated soil used as the control treatment. Tamarix chinensis litter was placed under different salinity conditions. This study aimed to investigate the changes in the TC, TN, and TP contents and stoichiometric characteristics in the litter after 90 days of decomposition; analyze the soil TC, TN, and TP contents and their stoichiometric ratios; determine the soil NH4+, NO3−, and AP contents; and conduct correlation analyses among the main monitoring variables in the litter–soil system. (1) Compared with the initial nutrient contents of Tamarix chinensis litter, the TC, TN, and TP contents tended to decrease under all soil salinity conditions. With increasing soil salinity, the litter TC and TN contents exhibited a decreasing–increasing–decreasing trend, with the lowest values observed under slight salinity, followed by those in the severe salinity treatment. The litter TP content decreased and then increased with increasing salinity, reaching a minimum value of 0.70 g/kg under moderate salinity, which was significantly reduced by 12.80% compared with that in the CK treatment. The C/N, C/P, and N/P ratios of the litter initially increased but then decreased, reaching their maximum values under moderate salinity conditions. (2) With increasing soil salinity, the soil TC and TP contents generally decreased with increasing salinity, reaching minimum values of 14.48 g/kg and 0.55 g/kg under severe salinity, respectively, which were 9.52% and 1.96% lower than those in the CK treatment. The soil TN content increased continuously and reached its highest value under severe salinity conditions, the soil C/N ratio gradually decreased, and the soil C/P and N/P ratios tended to decrease and then increase with increasing salinity. (3) The soil NH4+ and NO3− contents exhibited decreasing–increasing and increasing–decreasing trends, respectively, with increasing salinity, reaching their minimum values under slight and severe salinity conditions. Compared with those in the CK treatment, these values decreased by 15.71% and 9.31%, respectively. The soil AP content exhibited no significant trend with increasing salinity but reached its maximum value under severe salinity conditions, increasing by 45.54% compared with that in the CK treatment. This study revealed that soil salinity indirectly regulates soil C, N, and P cycling by affecting Tamarix chinensis litter decomposition and microbially mediated nutrient transformation processes, providing a scientific basis for the management of low-productivity Tamarix plantations and the ecological restoration of saline-alkali soils in the Yellow River Delta.