Improving Soil Health and Rice Yields with the Application of Soil Amendments in Acidic Paddy Soils

The over-application of nitrogen fertilizers has expedited soil acidification, resulting in the deterioration of agricultural soil quality and a decline in rice yields. This study evaluated the performance of seven soil amendments, including lime (L), biochar (BC), composted manure (CM), and alkaline inorganic material (AM), and their combinations, such as L with BC, L with CM, and BC with AM, in regulating soil pH, nutrient levels, heavy metal bioaccumulation, and rice yields at two field sites. The results demonstrated that soil pH increased by 0.33–1.57 units after amendment application. Compared with the control, the amendments reduced the concentrations of available cadmium in soils by 7–57%, available copper by 32–91%, available nickel by 12–88%, and available zinc by 18–99%. Moreover, they induced a reduction in exchangeable H+ and Al3+ levels, improving various properties and soil health. Furthermore, these amendments caused an increase in rice yields and a decrease in Cd and Ni accumulation in rice grains by 5–30% and 11–40%, respectively. Structural equation modeling indicated that the accumulation of heavy metals in rice is mainly mediated by soil pH via its impact on exchangeable acidity. This impact subsequently modifies soil nutrient availability, thereby influencing metal bioaccumulation. Overall, the application of these amendments presents promising strategies for mitigating soil acidification and improving agricultural productivity.