Residual nitrogen exhibits lower stability and greater influence on wheat yield formation compared to phosphorus and potassium in drylands of the Loess Plateau

Following the implementation of China’s “Zero-Growth Action Plan on Fertilizers” in 2015, research has predominantly focused on replacing synthetic fertilizers with organic amendments to address over-fertilization concerns. However, insufficient attention has been given to the sustainable supply capacity of soil residual nutrients accumulated from previous over-fertilization. To investigate the transformation dynamics and supply capacity of residual nutrients during crop production, a six-year field experiment was conducted in the dryland wheat growing region of China’s Loess Plateau. Five treatments were established: farmer’s fertilization (FF), regulated fertilization (RF), regulated fertilization without N (RF-N), regulated fertilization without P (RF-P), and regulated fertilization without K (RF-K). The study examined wheat yield formation, variations and stability of soil N, P, and K fractions, and their correlations with yield. Results indicated that wheat yield sensitivity to nutrient deficiency followed the sequence N>P>K. During the six-year period, the average yield under RF-N decreased by 22.0% compared to RF, showing statistical significance (P<0.05). Mineral N, light fraction organic N (LFON), and heavy fraction organic N (HFON) in RF-N showed progressive decline relative to RF and initial 2018 levels. Dissolved organic N (DON) and easily oxidizable organic N (EON) in RF-N initially decreased but subsequently increased due to N fraction transformations. Under RF-P, H₂O-P, NaHCO₃-P, and NaOH-P levels decreased by 40.0, 51.5, and 10.3% respectively (P<0.05) compared to the RF treatment, while HCl-P, residual P, and total P (TP) remained stable. The absence of K application (RF-K) reduced water-soluble K (WSK) by 10.9% (P<0.05), whereas exchangeable K (EK), non-exchangeable K (NEK), mineral K (MK), and total K (TK) showed no significant changes compared to the RF treatment. These findings demonstrated that the soil nitrogen pool exhibits lower stability compared to phosphorus and potassium pools during continuous residual nutrient supply. Notably, NO₃-N and LFON significantly influenced spike number and kernels per spike, driving yield formation. This research advances our understanding of sustained residual nutrient supply capacity in soil and provides theoretical foundations for optimizing fertilization strategies in dryland agroecosystems.