Objective The study was carried out to reveal the impact of long-term fertilization on the capacities of soil nitrogen (N) and phosphorus (P) activation, and their correlations with pH and soil organic carbon (SOC) in purple paddy soil with different fertility, with a view to providing references for soil fertilization.

Method Based on a 36-year different fertilization experiment of purple soil, different fertilization treatments including no fertilization (LF), balanced chemical nitrogen (N), phosphorus (P) and potassium (K) fertilizer (MF), and chemical N, P and K fertilizer plus organic fertilizer (HF) were set. Dynamics of soil total and available N and P contents, and nutrient activation rates under different fertility were measured, and relationships between soil pH, SOC and nutrient activation rates were analyzed.

Result Crop yields under MF and HF were significantly higher than those under LF. The contents of total N and P had no significant changes, available N content efficiently improved, while available P slightly decreased under LF treatment. However, total and available N and P contents significantly increased under MF and HF treatments. There were variation in promotion of N and P content under different fertility paddy soil. For example Under LF, MF, and HF treatments, the annual increase rates of total N contents were 5.5, 14.2, and 17.0 mg/kg, 0.40, 1.05, and 1.32 mg/kg for available N, 0.8, 17.1, and 18.0 mg/kg for total P, and -0.03, 2.05, and 1.85 mg/kg for available P, respectively. Compared with the LF treatment, N activation rates under MF and HF treatments increased from 8.15% to 428.77%. There was no significant linear relationship between N activation rate and pH or SOC. A significant negative correlation was observed between P activation rate and pH, while there was a significant positive linear correlation between P activation rate and SOC. If the annual application amount of N was 240 kg/hm² and amount of P₂O₅ was 120 kg/hm², the purple soil was N-saturated after 13 years and it was P-saturated after 9 years.

Conclusion In the purple paddy soil regions of Southwest China, balanced fertilization with N, P and K or application of chemical fertilizer with organic fertilizer could promote soil N and P contents and activation rates.