Long-term Chemical and Organic Fertilization with Nitrification Inhibition Enhances Ammonium Fixation by Clay Minerals and Nitrogen Retention in Thin-Layer Black Soils

Abiotic clay mineral fixation and release of ammonium (NH₄⁺) is a key process regulating nitrogen (N) retention and supply in Northeast China's black soil region. However, the long-term effects of chemical fertilizer, manure, and nitrification inhibitors on NH₄⁺ fixation capacity, as well as their relationship with organic matter and yield,remain unclear. This study evaluated 42-year fertilization practices (CK, NPK, NPKM1 with low-level manure, NPKM2 with high-level manure) and the nitrification inhibitor DMPP on fixed NH₄⁺ and maximum fixation capacity in thin-layer black soil (0-100 cm). Results showed that fixed NH₄⁺ ranged from 134.9–225.4 mg kg⁻¹ after 42 years. All fertilization treatments increased fixed NH₄⁺ in the 0-20 cm layer, but only NPKM2 increased it in 20-100 cm depth. Fixed NH₄⁺ accounted for 9.9-32.9% of total N across the 100 cm profile. Maximum fixation capacity (511.0-810.4 mg kg⁻¹) was much higher than actual fixed NH₄⁺, indicating high fixation potential. Manure application (NPKM1 and NPKM2) significantly increased soil organic matter (SOM) in 0-20 cm and promoted maize yield. However, although NPKM2 had higher SOM than NPKM1, yields did not differ between the two treatments. DMPP increased fixed NH₄⁺ by 6.6-15.5% in the early stage (3-7 days) and consistently enhanced fertilizer-derived fixed NH₄⁺, though this effect was weaker under NPKM treatments. In conclusion, integrated fertilizer-manure application (especially with nitrification inhibitors) can increase the fixed NH₄⁺ pool, thereby enhancing N retention and supply, SOM, and crop yield in Northeast China's thin-layer black soil.