Objective The study aimed to analyze the differences in soil carbon (C) and nitrogen (N) contents, and enzyme activities under different rates and types of organic fertilizer substitution for chemical fertilizer. It also sought to explore the effects of changes in soil nutrient contents and soil enzyme activities on corn yield, in order to provide a scientific basis for achieving green agricultural development through organic fertilizer substitution.

Method Fluvo-aquic soil was used as the research subject. Seven treatments were established: no fertilization (CK), mineral NPK fertilizer application alone (NPK), pig manure organic fertilizer replacing 30% of chemical fertilizer (OF₃₀), pig manure organic fertilizer replacing 60% of chemical fertilizer (OF₆₀), full pig manure organic fertilizer (OF₁₀₀), rapeseed cake fertilizer replacing 30% of chemical fertilizer (COF₃₀), and rapeseed cake fertilizer replacing 60% of chemical fertilizer (COF₆₀). A long-term localized experiment was conducted. After corn harvest, yield was measured, soil C and N contents, and enzyme activity were analyzed as well.

Result When the organic fertilizer substitution ratio was 30% and 60%, rapeseed cake fertilizer was found to be more effective than pig manure organic fertilizer in enhancing the content of active organic C components such as soil organic matter (SOM), dissolved organic C (DOC), readily oxidizable organic C (ROC), as well as ammonium N (NH₄ ⁺ -N) and nitrate N (NO₃⁻-N). The SOM content in the COF₆₀ treatment was measured at 30.45 g kg⁻¹, which was significantly higher than that in the OF₆₀ treatment. The substitution of organic fertilizer for chemical fertilizer was observed to significantly increase the activity of key enzymes in the C and N cycles. At both 30% and 60% substitution ratios, rapeseed cake fertilizer treatments were found to have more pronounced effects on acid phosphatase (ACP) and urease enzyme activities. Variance partitioning analysis (VPA) revealed significant interactions between soil nutrients and enzyme activities, which were found to have an important impact on corn yield. Aggregated Boosted Tree (ABT) analysis further revealed that ACP and urease had the greatest impact on corn yield at 14.77% and 13.94%, followed by active organic C components such as DOC and ROC.

Conclusion The study concluded that rapeseed cake fertilizer replacing 60% of chemical fertilizer (COF₆₀) was the most beneficial for improving soil nutrient supply and promoting crop growth. It was recommended for widespread application in farmlands, potentially contributing to more sustainable agricultural practices.