Soil-Improving Effect of Sesbania–Sorghum Rotation in a Heavily Saline–Alkaline Coastal Region

Planting salt-tolerant plants is an efficient method of biological improvement for saline–alkali land. However, few studies have examined the soil improvement effects of the rotation of the green manure plant sesbania and the grain crop sorghum. Thus, we planted sesbania in native soil on heavily saline–alkaline coastal land and subsequently planted sorghum after returning the sesbania straw to the soil. The effect of this sesbania–sorghum rotation on soil improvement was clarified by comparing indicators of soil quality before and after sesbania and sorghum were planted, such as the soil structure, water infiltration, soil salt content, and soil microbial changes. The results showed that the soil bulk density of the plow layer (0–40 cm) after crop harvest decreased by 9.63% compared with that of bare land, and the soil porosity increased by 5.67%. The cumulative infiltration, initial infiltration rate, and stable infiltration rate of saline soil were 3.6 times, 2.8 times, and 3.3 times higher than those of bare land, respectively. With the growth of sesbania and sorghum, the soil salt content in the plow layer of the cultivated land decreased by 37.73%, while that of bare land decreased by 9.1%. A further analysis of desalination showed that the total desalination amount in the plow layer was 15.58 t/ha, of which 5% was due to plant absorption, and the rest was from salt leaching. Moreover, sesbania–sorghum rotation increased the soil organic matter content in the plow layer from 69.1 t/ha to 73.8 t/ha. The quantities of some microorganisms that are mainly found in coastal saline soil decreased, while those of some common soil microorganisms increased, reflecting an improvement in the soil quality. The above results prove that sesbania–sorghum rotation had a significant effect on soil improvement and salt reduction, which is of great significance for the further utilization of saline–alkali land to enhance crop productivity.