Objective  Soil fixed ammonium (NH₄ ⁺ ), as a temporary pool of nitrogen (N), the change of its content can affect the function of soil N retention and supply. The response mechanisms of soil fixed NH₄ ⁺ and crop residue return to N fertilizer reduction should be investigated, in order to provide theoretical basis for optimizing N fertilizer management in agroecosystems.

  Method  Based on the experimental platform of conservation tillage corn planting system with full crop residue return (9 years) in the black soil region of Northeast China, a fertilizer reduction experiments were set up with: 3 crop residue return treatments, 0 (S0), 33% (S33) and 100% (S100); 4 N fertilizer application rates, 240 kg hm⁻² (N240), 190 kg hm⁻² (N190), 135 kg hm⁻² (N135), and 0 kg hm⁻² (N0).

  Result  When the crop residue removal, the total amount of fixed NH₄ ⁺ gradually decreased with the decrease of N application and was significantly lower than that of the fertilization treatment in the N0 treatment. N deficiency appeared in the soil-crop system at S33N190, and the fixed NH₄ ⁺ decreased with the decrease of N application. There was a significant positive correlation between fixed NH₄ ⁺ reduction and N deficiency. At all N fertilizer levels, crop residue return, especially full crop residue return, was beneficial to alleviating the release of fixed NH₄ ⁺ , but the crop residue effect was weaker under N135 treatment. Overall, the S100N190 treatment showed the smallest reduction in fixed NH₄ ⁺ compared to the control. In addition, crop residue return reduced the nitrate N content in the fertilization treatment.

  Conclusion  Under the condition of reducing N fertilizer application, fixed NH₄ ⁺ can release and maintain N supply, reducing N fertilizer application by 20% with no-tillage and full crop residue return can maintain the N supply and balance in the soil-crop system.