Nitrogen Reduction Combined with Calcium-Based Amendments Regulates Soil Acidification Buffering and Maize Productivity in Acidified Croplands of Northeast China

Excessive nitrogen fertilization accelerates soil acidification and threatens soil fertility and crop productivity, highlighting the need for management strategies that reduce nitrogen inputs while maintaining soil function and crop production. This study investigated the effects of nitrogen reduction combined with calcium-based amendments on soil acidity, nutrient availability, and maize productivity in an acidified aquic brown soil. A field experiment was conducted with three nitrogen-reduction levels (10%, 20%, and 30%) and four amendment treatments (no amendment, dolomite, quicklime, and fused calcium–magnesium phosphate fertilizer). The results showed that, under non-amended conditions, greater nitrogen reduction resulted in higher soil pH and measured total exchangeable base cations (TEBC), whereas nitrogen-reduction level had no significant effect on soil pH buffering capacity (pHBC). Among the calcium-based amendments, dolomite and quicklime increased pHBC by 25.3% and 21.4%, respectively, relative to the non-amended treatment, while dolomite generally showed higher measured concentrations of exchangeable Ca2+ and Mg2+ and higher TEBC. Amendment effects on nutrient availability differed among treatments: fused calcium–magnesium phosphate fertilizer produced the highest available phosphorus, whereas dolomite increased alkali-hydrolyzable nitrogen and generally resulted in higher NH4+-N and available potassium. Maize grain yield was maintained when nitrogen reduction increased from 10% to 20% but declined significantly under 30% nitrogen reduction. Across nitrogen-reduction levels, dolomite was associated with the highest grain yield, exceeding the non-amended and quicklime treatments by 4.1% and 5.4%, respectively. Overall, moderate nitrogen reduction maintained maize productivity, while dolomite showed favorable responses across multiple indicators of soil acidity, exchangeable base-cation status, nutrient availability, and grain yield. These findings highlight the importance of coordinating nitrogen-reduction intensity with amendment-specific effects to balance soil amelioration and maize productivity in acidified croplands.