Salinity Gradient Modulates the Effects of Tamarix chinensis Litter Decomposition on Soil Microbial Community Structure in the Yellow River Delta

This study aimed to investigate the influence of Tamarix chinensis litter decomposition on soil physicochemical properties and soil microbial community structure under different saline conditions. The experimental design included four salinity treatments: control (CK), slight (SS, 0.4%), moderate (SM, 0.8%), and high (SH, 1.2%). Litter from three decomposition degrees (undecomposed, semidecomposed, and already decomposed) was examined. The litter decomposition rate, 0–10 cm soil physicochemical properties, and soil microbial community structure were measured. The results revealed that as soil salinity increased, a rise was observed in both litter decomposition rate and soil bulk density, whereas soil saturated water content exhibited a declining trend. The soil available nutrients tended to increase overall and were greater under the SM and SH conditions. Compared with CK condition, under SM and SH conditions, the litter decomposition rate increased by 5.05% and 21.01%, respectively, while the soil saturated water content decreased by 6.02% and 5.34%, respectively. The soil NO3−-N content was 17.54 and 16.45 times that of CK, respectively, whereas the AK content increased by 28.02% and 29.91%, respectively. The soil microbial diversity decreased significantly. Compared with CK, under SM and SH conditions, the bacterial Chao1 index decreased by 14.42% and 17.37%, respectively, and the bacterial Shannon index decreased by 4.46% and 3.82%, respectively. The fungal Chao1 index decreased by 62.52% and 59.99%, respectively, and the fungal Shannon index decreased by 9.57% and 12.79%, respectively. The microbial community structure was similar under the SM and SH conditions, and the relative abundance and number of genera of the dominant phylum were greater. Soil salt content and electrical conductivity (EC) were significantly (p < 0.05) or extremely significantly (p < 0.01) positively correlated with soil available nutrient content. Soil salinity directly or indirectly affected soil physicochemical properties by regulating the litter decomposition rate, subsequently affecting the soil microbial community structure. The T. chinensis litter decomposition enhanced soil available nutrients in moderate and high coastal saline-alkaline soil and the screening ability of the salt-tolerant soil microbial community. These findings offer a scientific reference for the management of low-productivity T. chinensis forests and the ecological restoration of saline–alkaline soil in the Yellow River Delta.