Integrated soil fertility and yield response to long-term different fertilization in purple soil

Abstract: The Nemero index is widely used to assess integrated pollution or fertility index. This index is becoming one of the most common methods because it has higher accuracy and shows better correlation with crops yield than other evaluating methods. However, the Nemero index is less used to evaluate the trend of integrated soil fertility under long-term varied fertilization especially on purple soils. Based on a long-term field experiment in the national monitoring station of soil fertility and fertilizer efficiency on purple soils, evolution trends of integrated soil fertility under different fertilization treatments for 20 years was therefore evaluated by the Nemoro index. By doing so, the authors expected to find out a sustainable fertilization management for high crop yield and soil fertility on purple soils. Results showed that different fertilization treatments had led to different trends of soil fertility after 20 years. Integrated fertility index (IFI) of soil with CK, N and NK treatments showed a declining trend over time. Whereas IFI of soil with NP, NPK, NPKM, 1.5NPKS and NPKS treatments were increased to a platform in 5 years and then kept stabilized after that. Balanced fertilization such as NPK, NPKM, NPKS, 1.5NPKS treatments resulted in high grain yield and their IFI had a small change with time. Furthermore, the average of IFI values for balanced fertilization treatments were significantly higher than that of CK and other partial fertilization treatments. Even so, the increased extents by these treatments were different. Averaged IFI values of soil with balanced fertilization treatments (NPK, NPKM, 1.5NPKS, NPKS) were 36.8%, 34.5%, 52.3%, and 50.1% higher than that of CK treatment, respectively. Correlations between IFI index and soil fertility parameters showed that soil organic matter, total phosphorus and available phosphorus content had more contribution to soil fertility of purple soil. In practice, result showed that fertilization could significantly improve the yield of crops compared to un-fertilization treatment (CK). Furthermore, grain yield with balanced fertilization was significantly higher than that with unbalanced fertilization treatments. The 20-year averaged grain yield of the fertilization treatment (NPKS, 1.5NPKS, NPKM, NPK, NK, NP, N) is 110.8%, 99.2%, 100.4%, 102.4%, 54.4%, 78.6%, and 36.5% higher than that of treatment without fertilization, respectively. Crop yield with NP treatment was higher than that with other partial fertilization treatments (NK, N) because purple soil can supply sufficient amount of K by continuous weathering of K-enriched mineral. Similarly, the integrated fertility index of soil under NP treatment was also higher than the other partial fertilization treatments. Although the IFI of soil with high rate of NPK fertilizer (1.5NPKS treatment) was the highest within all treatments, its averaged annual yield was even lower than that with less fertilizer treatments (NPKS). This indicated that excess fertilization might result in too high nutrient accumulation to cause yield reduction and reduce efficiency of fertilizer utilization. Based on 20-year data, NPK fertilization together with rice straw-return (NPKS) showed the best in term of crops yield and their stability. Regression between yield and IFI can be fitted significantly by a quadratic equation with a coefficient (R2) of 0.9758 (P<0.01). This showed that relative yield could reach the top when the integrated fertility index of purple soil was 1.81. This regression also indicated that IFI of purple soil should remain at a range of 1.63 to 2.01 for maximizing crop yield. In conclusion, long-term balanced fertilization can increase both crop yield and integrated soil fertility on purple soils and such trends could be matched well by the Nemoro index. Whereas, long-term unbalanced fertilization especially only N or NK application resulted in soil P deficiency, resulting in lower grain yield and a declining IFI. Furthermore, balanced NPK fertilization together with rice straw-return (NPKS) is recommended to maintain high soil fertility as same as rice yield on purple soils.