Trade-Off Between Nutrient Enhancement and Salt Input from Combined Organic Amendments in Coastal Saline–Alkali Soil: Implications for Rice Yield in a Pot Experiment

Coastal saline–alkali soils are valuable yet constrained by salinity and nutrient deficiency. Organic amendments are widely used, but their combined effects and trade-offs on soil properties, microbes, and rice production remain unclear. This study applied biochar, peat, and straw, alone and in all combinations, to coastal saline–alkali soil. Straw alone significantly increased rice yield, most combined treatments containing biochar or peat increased yields, but the effect was not significant. Combined treatments elevated rice N and P contents, as well as soil available K, Na, EC, SOM, and SOC within 0.053–0.25 mm aggregates, but promoted rice Na+ accumulation while suppressing K+ uptake. Soil urease and alkaline phosphatase activities were significantly enhanced by SP2 and SB3P1. Combined treatments also improved microbial richness, increasing the relative abundances of dominant chemoheterotrophy and aerobic chemoheterotrophy. Mantel analysis indicated that soil K, P, SOM, EC, urease, and catalase activity were positively correlated with rice yield, and rice P content in roots and stems also mattered. Overall, organic amendments improve soil fertility and microbial diversity but may introduce salt-related trade-offs, highlighting the need for balanced combination strategies in saline–alkali soil management.