Afforestation Enhances Soil Ecosystem Multifunctionality by Improving Soil Quality and Enzyme Activities in Coastal Saline–Alkali Land

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 (<i>Robinia pseudoacacia</i>) 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 <i>Robinia pseudoacacia</i> 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.