Olsen phosphorus (Olsen P) is an important indicator of soil labile phosphorus. Determining the effect of fertilization on Olsen P and P fractions (labile phosphorus, medium labile phosphorus and stable phosphorus) can guide the application of phosphate fertilizer. Therefore, it is of great significance to clarify the changes of Olsen P and P fractions and the influencing factors under long-term fertilization. This study investigated changes in Olsen P and P fractions in haplic phaeozems soils (0–20 cm) in two 30-year fertilization sites (Harbin, Gongzhuling) of northern China. Four treatments were examined: treatment with no fertilizer or manure (CK), nitrogen and potassium (NK), nitrogen, phosphorus and potassium (NPK), and manure, nitrogen, phosphorus and potassium (MNPK). The results showed that after NK application, Olsen P decreased by an average of 1.5 mg kg−1 for every 100 kg ha−2 of soil phosphorus lost due to continuous phosphorus uptake by the crop; with NPK, Olsen P increased by an average of 17.6 mg kg−1 for every 100 kg ha−2 of P surplus; with MNPK, the increase curve of Olsen P was similar to an S-curve with periods of rapid growth and periods of equilibrium. In the equilibrium period of Olsen P, the equilibrium values were 52.0 and 156.2 mg kg−1 in Harbin and Gongzhuling. After 20 years of long-term different fertilization, labile P (LP), medium labile P (MLP) and stable P (SP) decreased by 21.1, 16.6 and 15.1 mg kg−1 on average for the treatment without P application (NK), and the percentage decreases were 2.8, 5.7 and 2.1%, respectively. With the treatment of NPK, LP and MLP increased by 25.5 and 79.2 mg kg−1, and the percentage increases were 1.8 and 16.1%. With the treatment of MNPK, the increase in LP and MLP was significantly higher than that with NPK. Soil organic carbon (SOC), total nitrogen (TN) and carbon/nitrogen (C/N) had the greatest effects on Olsen P and P fractions and the total contribution rate was >40%. In summary, fertilization system caused significant changes in Olsen P and P fractions. After about 20 years of long-term combined application of MNPK, the growth of Olsen P can be kept constant. SOC and TN had important effects on Olsen P and P fractions in black soil. Therefore, the application of phosphorus fertilizer should be adjusted according to the type and time of fertilization in black soil in order to avoid waste of phosphorus fertilizer.

The application of manure (M) and straw (S) will increase the Olsen P and phosphorus activation coefficient (PAC) in soil. Clarifying the increasing trend of Olsen P and PAC is crucial for rational fertilization. This study fitted the equation between the accumulated P surplus, Olsen P, and PAC in four treatments for 28 years and analyzed the changes and rates of P fractions. The results showed Olsen P and PAC increase linearly with NPK and NPKS treatments; for every 100 kg ha−1 of P surplus, Olsen P increased by 5.9 and 6.7 mg kg−1, and PAC increased by 0.52% and 0.50%. With M and MNPK treatments, the sigmoid curve equation was the best fitting method. The equilibrium values were 167 and 164 mg kg−1 for Olsen P, and 10.4 and 10.2 mg kg−1 for PAC. There was a correlation between Al-P, Ca2-P, Resin-P, NaOH-Pi, C/N, SOC, and pH, which had the highest interpretation rates for Olsen P and PAC. Manure is significantly better than straw in improving Olsen P in red soil. It is recommended to reduce the amount of manure applied for a long time to avoid a zero increase in Olsen P.