Sustainable Reuse of Aquaculture Wastewater in Saline–Alkali Paddy Fields: Interactive Effects of Irrigation and Microalgae on Water Nutrient Removal and Rice Yield

To identify an optimized management strategy for the safe reuse of aquaculture wastewater in saline–alkali paddy fields, a pot experiment was conducted to investigate the interactive effects of irrigation modes (flooded and shallow–wet) and Chlorella application on wastewater purification, nitrogen and phosphorus transport, and rice yield. The results showed that Chlorella effectively improved the removal rates of nitrogen and phosphorus in field surface water, but its efficacy depended on the irrigation mode. The purification efficiency of shallow–wet combined with Chlorella (ISCW) was highest, and the removal rate of total phosphorus at the heading stage was 88.67%. The leaching risk of deep nitrate nitrogen (NO3−-N) was the lowest, but the rice yield was significantly reduced. In contrast, flooded irrigation combined with Chlorella (IFCW) produced the highest rice yield, whereas its water purification effect was moderate. The entropy-weighted TOPSIS evaluation further indicated a clear trade-off. ISCW improved phosphorus removal in surface water, but reduced grain yield by 60.7% compared with IFCW. These findings demonstrate that irrigation mode is a key factor in regulating the purification effect of Chlorella and its trade-off with rice yield. These findings provide theoretical support for wastewater resource utilization in saline–alkali regions and contribute to the sustainable development of coastal agriculture.