Afforestation is critical for enhancing soil quality and optimizing land use in coastal saline–alkali regions. However, the dynamics of soil quality and ecosystem multifunctionality (EMF) during afforestation, as well as their coupling relationships, remain poorly understood. In this study, the effects of four stand ages (6, 12, 22, and 36 years) of a leguminous tree species (Robinia pseudoacacia) on the soil quality index (SQI), enzymatic stoichiometry, and soil ecosystem multifunctionality (EMF) were investigated in coastal saline–alkaline lands of the Yellow River Delta, China. Results showed that compared to non-afforestation, afforestation increased the SQI by 81%, 74%, 146%, and 184% in the 6-, 12-, 22-, and 36-year-old stands, respectively. Concurrently, afforestation increased the activities of the carbon acquisition (C_acq), nitrogen acquisition (N_acq), and phosphorus acquisition (P_acq) enzymes and alleviated microbial N limitation. Afforestation increased the soil EMF by 182%, 243%, 263%, and 295% in the 6-, 12-, 22-, and 36-year-old stands, respectively. The SQI was significantly positively correlated with soil EMF, regardless of afforestation age. Soil salinity, SOC, TN, and TP were the best predictors of soil EMF. The afforestation of Robinia pseudoacacia improved the soil quality and stimulated enzyme activity, thereby increasing the soil EMF. Our findings provide a theoretical foundation for the sustainable management of stands in coastal saline–alkali lands.

Coastal saline–alkaline land has considerable potential for soil carbon sequestration, but how different land-use types affect microbial resource limitation and carbon-use efficiency (CUE) in coastal saline–alkaline soils remains unclear. Four representative land-use types, namely bare land (BL), wetland (WL), grassland (GL), and forest land (FL), were investigated in the coastal saline–alkaline soils of the Yellow River Delta. Soil physicochemical properties, microbial biomass, and extracellular enzyme activities were measured, and ecoenzymatic stoichiometry, microbial resource limitation, and CUE were subsequently calculated. Compared with BL, vegetated land-use types decreased electrical conductivity by 52.1–95.8%, while soil water content, soil nutrient indicators, and microbial biomass indicators increased by 47.1–77.5%, 2.6–136.8%, and 2.2–274.4%, respectively. WL was mainly phosphorus-limited, whereas BL, GL, and FL were primarily nitrogen-limited. Despite relatively high soil organic carbon and nutrient availability, GL showed the strongest N limitation and was the only land-use type showing C limitation. Model-estimated CUE ranged from 0.544 to 0.579 and followed the order FL > BL > WL > GL. Random forest analysis showed that soil physicochemical properties contributed most to CUE variation (42.9%). Structural equation modeling further indicated that soil physicochemical properties were indirectly associated with CUE, mainly through stoichiometric characteristics and microbial resource limitation, showing positive and negative associations, respectively. These findings provide microbial evidence for optimizing land-use patterns, vegetation restoration, and carbon-oriented ecological restoration in coastal saline–alkaline land.

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.