Evaluating the effects of partial replacement of chemical fertilizers by organic fertilizers to achieve fertilizer reduction based on integrated quality indicator method

The irrational application of chemical fertilizers leads to soil quality degradation and ecological problems, making the partial replacement of chemical fertilizers with organic fertilizer as a key approach for reducing inputs while increasing efficiency. This study, based on the Integrated Quality Index (IQI) method combined with a linear scoring approach, the effects of substituting 15% and 30% of nitrogen fertilizer with organic fertilizer on soil quality and maize yield in Jiuli Village (Chernozem soil) and Jieji Village (meadow soil), located in Longjiang County, Qiqihar City, Heilongjiang Province, were systematically evaluated. The results show that, compared to optimized chemical fertilization, 30%N substitution with organic fertilizer in the Jiuli Village area increases soil pH by 6.2%, decreases electrical conductivity and bulk density by 7.6% and 3.8% respectively, and improves IQI by 32.7%. While 15% substitution treatment performs better in the Jieji Village area, which increases the IQI by 20.2% and significantly decreases soil electrical conductivity by 10.9%. IQI is significantly and positively correlated with maize yield (P<0.001). Linear regression reveals a high goodness-of-fit between yield and IQI in the Chernozem soil (R²=0.8449), whereas the model for the meadow soil (R²=0.5908) requires targeted optimization of weight allocation to further improve its fit. This study demonstrates that the partial replacement of chemical fertilizer with organic fertilizer can improve soil quality and maize yield, achieving the goal of reducing inputs while increasing efficiency. The IQI can accurately evaluate soil quality after such fertilizer reduction. It is recommended to promote the 30% organic fertilizer substitution rate in the Chernozem soil area of Jiuli Village, while a 15% substitution rate is advised for the meadow soil area of Jieji Village to improve soil quality and yield stability.