Excessive phosphorus (P) application can alter soil P availability and limit plant growth by compacting soil and fixation of P into different organic and inorganic forms. However, it remains uncertain whether these changes happen after limited fertilization or an excessive rate applied under the winter wheat cropping system. The current study aimed to identify the transformation of P into different organic (P_o) and inorganic (P_i) fractions, and their role in the plant P uptake and winter wheat (Triticum aestivum L.) production. A long-term study (12 years) was conducted to assess the changes in soil P_i and P_o fractions in response to different P rates (0, 50, 100, 150, and 200 kg P₂O₅ ha⁻¹) applied to winter wheat. Phosphorus fractions were determined using the Hedley modified Tiessen and Moir fractionation scheme. Our findings demonstrated that different P rates significantly increased the available P, particularly NaHCO₃-P_i, in the inorganic P fractions compared to P0 treatment. NaHCO₃-P_i showed a strong relationship with grain yield (R² = 0.91) and P uptake (R² = 0.80). Grain yield was significantly higher in the P100 treatment, but no significant difference was observed between P100 and P200 treatments. The P200 treatment had a maximum grain P content and plant P uptake. Compared with the P0 treatment, all organic fractions yielded the highest P_o with the P rate increase, ranging from 27.3 to 75.6 mg kg⁻¹, 27.2 to 35.6 mg kg⁻¹, and 58.8 to 124 mg kg⁻¹ for NaHCO₃-P_o, NaOH-P_o, and HCl-P_o, respectively. Among all P_i fractions, the maximum fraction, known as apatite (HCl_D-P_i), was found in the P200 treatment with the range of 165 to 245.9 mg kg⁻¹. HCl_D-P_i accounted for 32% of total P, which can be transformed into the available P form with the passage of time. An increase of 78% in residual P was found under the treatment of P200. The residual P fraction was positively correlated with grain yield, P uptake, and other inorganic fractions. It can be concluded that application of P increases P availability and grain yield with an increase in its application rate, but too much use of P can cause soil pollution and higher fixation of P. Consequently, a balanced application of fertilizer is recommended to reduce its fixation and increase its availability for higher crop yield.

Long-term cropping systems require balanced phosphorus (P) management for better yield and environmental sustainability. However, the soil P transformations under fallow rotations with and without long-term nitrogen (N) and P fertilization largely remained unknown. This study evaluated the status of P forms in loess soils in response to varied combined rates of N and P fertilizers, tillage management practices, fallow land systems (natural fallow (NF), and bare fallow (BF)). Four NP treatments (N0P0, control; N0P100, 100 kg P ha⁻¹; N160P0, 160 kg N ha⁻¹; and N160P100), and two treatments with no fertilizer application and crops (NF and BF) were conducted. The treatments N0P100 and N160P100 significantly increased soil total P, inorganic P (P_i), organic P (P_o), and Olsen P concentrations compared to the control, NF, and BF treatments. Labile P fractions (NaHCO₃-P_o and NaHCO₃-P_i) were 7.30% and 11.8–12.4% higher in fertilized treatments than in control, NF, and BF treatments. The moderately labile NaOH-P_i was stable in all treatments, but NaOH-P_o significantly decreased in the NF (2.60%) and BF (1.40%) treatments compared to the control and fertilized treatments; however, HCl_D-Pi was 59.1–66.0% higher in NF and BF compared to the control and fertilized treatments. Non-labile P (HCl_C-P_i and HCl_c-P_o) fractions showed no significant difference between the fertilized and unfertilized treatments. Residual P levels were substantially greater in the P fertilized (N0P100) treatment than in the fallow treatments. The conceptual framework and redundancy (RDA) analysis revealed that the labile (NaHCO₃-P_i and NaHCO₃-P_o) and moderately labile P fractions (NaOH-P_o, NaOH-P_i, and HCl_D-P_i) were substantially associated with Olsen P contents, grain yield, and P uptake. Higher moderate fraction concentrations in fallows and their positive correlation with yield, P uptake, and Olsen P predict the importance of reserved P in these soils upon long-term fertilization, suggesting the utilization of P legacy and optimizing fertilizer applications.