Forest and grass composite patterns improve the soil quality in the coastal saline-alkali land of the Yellow River Delta, China

The Yellow River Delta (YRD) in China has a high concentration of coastal saline-alkali land, and biological measures are important techniques for the ecological restoration of this land. This study aimed to explore the soil-improving effects of various vegetation restoration patterns and identify the best vegetation pattern for improving the soil quality in this coastal saline-alkali land. Six vegetation patterns consisting of mixed forest-grass, forest-crop intercropping, and pure forest were selected from the saline-alkali land of the YRD, with bare land as the control. A total of 18 indicators were measured and analyzed, including the soil water physical properties, salt and alkali contents, nutrients, and microbial number. The soil-improving effects of the various vegetation patterns were evaluated using statistical methods such as principal component analysis, cluster analysis, and membership functions in fuzzy mathematics. The results showed that the forest-grass composite patterns significantly improved the soil physicochemical properties, reduced the soil density, increased the soil porosity and water storage, and elevated the levels of soil organic matter and available nutrient contents, as well as the soil microbial number. However, the soil-improving effects under the various vegetation patterns were significantly different. A mixed tree-grass pattern consisting of Salix matsudana + Distichlis spicata was found to be best for improving the soil water physical properties, while the mixed tree-shrub-grass pattern of Fraxinus chinensis + Tamarix chinensis + Medicago sativa showed the best effects for preventing alkalization and inhibiting salinization, as well as increasing the soil nutrients and microbial number. The mixed forest-grass patterns showed the highest effects in increasing the soil available K and available N, while these patterns had little effect on the available P. The mixed forest-grass patterns significantly increased the number of bacteria in the saline-alkali soil; the number of actinomycetes ranked second, and the number of fungi was the lowest. In the saline-alkali habitat, the mixed tree-shrub-grass pattern of F. chinensis + T. chinensis + M. sativa demonstrated the best soil-improving effects among the various vegetation restoration patterns. The mixed tree-grass pattern consisting of S. matsudana + D. spicata ranked second, followed by the forest-grass patterns of S. americana + D. spicata and T. chinensis + M. sativa. The forest-crop pattern of S. americana + Glycine max and the pure forest of S. americana showed low soil-improving effects. When biological measures are used to improve the saline-alkali soil of the YRD, it is recommended to prioritize the mixed tree-shrub-grass pattern, followed by the mixed tree-grass and shrub-grass patterns; the forest-crop intercropping or pure forest patterns should be avoided as much as possible.