Changes in Soil Phosphorus Fractions and their Relationships with Selected Soil Properties After 14 Years of Combined Fertilization and Cultivation Practices in a Sloping Cropland with Entisols

Understanding the soil phosphorus (P) pool fraction subjected to fertilization and cultivation practices was conducive to improving the effectiveness of P and revealing the changes in and storage of soil organic and inorganic P. However, the changes in soil P fractions caused by long-term fertilization and cultivation remain largely elusive. This study investigated the various soil P fractions and their relationships with selected soil properties in a representative purple soil sloping cropland experiencing long-term fertilization and cultivation. The experiments comprised five treatments: no fertilizer and downslope cultivation (CK); chemical fertilizers and downslope cultivation (T1); 1.5-fold chemical fertilizers and downslope cultivation (T2); manure plus chemical fertilizers and downslope cultivation (T3); and chemical fertilizers and contour cultivation (T4). The soil P fractions were determined at 0–10 and 10–20 cm soil depths using a modified Hedley sequential method. The concentration of soil H₂O-Pi and NaHCO₃-Pi in T1 significantly reduced by 49.5–55.0% and 68.0-85.2% than in other treatments (T2 and T3) at the 0–10 and 10–20 cm soil depths, respectively. The P fractions showed nonsignificant differences between T1 and T4 at the 0–10 cm soil depth, while the H₂O-Pi concentration was 253.9% greater in T4 than in T1 at the 10–20 cm depth. The random forest (RF) model indicated that SOC and TN were the key factors for predicting soil P fractions. Our results show that manure plus chemical fertilizer and contour cultivation can be the recommendable agricultural practices for increasing the labile P fractions (H₂O-Pi and NaHCO₃-P) in purple soil sloping croplands.