Effects of In Situ Tomato Straw and Green Manure Returning on Greenhouse Soil Properties and Production in Coastal Saline–Alkali Areas

To clarify the effects of in situ tomato straw and green manure returning on soil quality and vegetable production in coastal saline–alkali greenhouse soils, this study employed a split-plot design to evaluate three green manure treatments (sweet corn, sorghum–sudangrass, and sesban) under two main treatments (tomato straw return or no straw return). The impacts on tomato and celery yield and quality, as well as soil physicochemical and biological properties, were assessed over a two-year rotation cycle. The results showed that: (1) compared to the control, green manure returning could significantly reduce soil bulk density, salinity, and fungal abundance; (2) different green manures specifically enriched functional microbes: Sesban enriched Nitrospira and Gemmatimonas; Sorghum–sudangrass enriched Streptomyces and Acidibacter; sweet corn enriched Pseudomonas; green manure reduced the relative abundance of Fusarium, whereas tomato straw showed the opposite trend; and (3) green manure, especially sorghum–sudangrass, significantly increased yields of both tomato and celery, while reducing celery cellulose content. Therefore, in situ sorghum–sudangrass returning is recommended as an effective strategy for maintaining soil health and achieving sustainable production in greenhouse systems within coastal saline–alkali regions.