Multi-Stress Alleviation and Rapeseed Yield Enhancement by Combined Application of Organic–Inorganic Fertilizers with Super Absorbent Polymer in Arid Sandy Soils

To achieve sustainable agriculture development in arid regions, it is imperative to improve the soil quality of arid sandy soils. This study explored the effects of the combined application of organic–inorganic fertilizers with soil conditioners on the physiological characteristics, yield, and quality of rapeseed in arid sandy lands. The aim was to provide a technical reference for improving sandy soil and increasing rapeseed yield in arid regions. This field study designed six treatments (control group: organic fertilizer + chemical fertilizer (CK); T1: organic fertilizer + chemical fertilizer + super absorbent polymer (SAP); T2: organic fertilizer + chemical fertilizer + humic acid (PI); T3: organic fertilizer + chemical fertilizer + attapulgite (PII); T4: organic fertilizer + chemical fertilizer + PI + PII; HF: chemical fertilizer) to evaluate their effects on the nutrient absorption, physiological characteristics, yield, and quality of rapeseed. The results showed that the combination of organic–inorganic fertilizers with SAP, PI, PII, or PI + PII could significantly reduce the salinity of sandy soil while increasing the nutrient content in various parts of rapeseed. Among the combinations, the SAP treatment (T1) had the most significant effect, with the following specific impacts: (1) Alleviation of salt stress: The SAP treatment increased the root potassium ion content by 63.09% and reduced sodium ion content by 60.16% compared with CK, significantly increasing the potassium/sodium ratio. (2) Physiological improvement: The SAP treatment increased the total chlorophyll content (TCC), superoxide dismutase/catalase activity, and dry matter accumulation by 86.85%, 161.58%, and 376.8%, respectively, compared with CK. (3) Yield and quality enhancement: The SAP treatment increased rapeseed yield and the crude protein content in stems and leaves by 148.32% and 86.05%, respectively, but decreased crude fiber content by 43.59% compared with CK. (4) Economic benefits: The net revenue (NR) of the SAP treatment reached 197.62 USD per hectare, which was significantly higher than that of other treatments. A comprehensive evaluation showed that the combined application of organic–inorganic fertilizers with SAP enhanced plant antioxidant enzyme activity and photosynthetic efficiency, synergistically enhancing the yield and quality of rapeseed in sandy areas. This study provides an economically efficient solution for sustainable agricultural development in arid regions.