| In South China, acidification of upland soil is more serious due to no reasonable agricultural managements after the second national general survey of soil. Soil acidification associates with a decline in soil pH, loss of base cations and nutrient, increase in exchangeable acidity, aluminum and heavy metals, which limits the growth of crops and restricts the sustainable development of agriculture in red soil region. To reveal the best fertilization model to provide theoretical basis for reducing soil acidification, the effects of long-term (18 years) fertilization on the characteristics of soil acidification were investigated in a wheat-maize experiment situated at Qiyang, Hunan province. We also investigated the role of pH on wheat and maize in pot experiments and analyzed the factors for soil nitrification and soil acidification by incubations. The main results and conclusions are below:(1) Our study suggested that long-term application of chemical fertilizer, especially nitrogen fertilizer was responsible for soil acidification in red soil. After 18-year fertilization, soil pH decreased by 1.11 ~ 1.56 units compared with the initial pH value in the application of chemical fertilizers and chemical fertilizer plus straw treatments. The greatest decrease of 1.56 units of soil pH in the single inorganic nitrogen fertilizer treatment was observed. However, application of manure or manure combined with inorganic fertilizers could maintain or increase soil pH compared with the initial pH value of the experiment, in which the highest increase value of 0.92 units was found in the manure treatment.(2) In this study, the soil pH decreased in the chemical fertilizers treatments (except PK) markedly and then became stable after 8~14 years when soil pH≤4.6. And in the NP, NPK and NPKS treatments, the soil pH achieved stability after 9, 8 and 8 years fertilization and the stable soil pH of them were 4.5, 4.5 and 4.6, respectively. However, the stable soil pH for N and NK treatments was 4.2 after 12 and 14 years fertilization respectively. In application of chemical fertilizers (except PK) and chemical fertilizer plus straw treatments, the actual rates of acidification were 4.0~6.7 kmol (H+)·ha-1·a-1, with 371~614 kg·ha-1·a-1lime to neutralize acid. And the order of actual rate of soil acidification is NPK>NP, NPKS>N, NK. This study also showed that the actual acidification rate was higher than the average acidification rate for 18 years, which revealed that actual rate of soil acidification, was more effectiveness for applying lime to neutralize acid.(3) After 18-year fertilization, in the chemical treatments (except PK) the exchangeable aluminum of soil increased to 5.51~6.74 cmol·kg-1, and total base and base saturation of soil respectively decreased by 3.83~6.52 cmol·kg-1 and 50.8% to 73.4%, respectively, which limited the growth of crops. The critical pH value of wheat and maize was 6.03 and 5.92, respectively, and wheat was more sensitive to soil acidity than maize. When soil pH is below the critical pH, the biomass of wheat and maize decreases by 40.0% and 25.7%, respectively, as the pH of soil decreases one unit.(4) However, application of manure (M) or manure combined with chemical fertilizers (NPKM) could ameliorate soil acidity, including increasing total base and decreasing the exchangeable aluminum, and maintained uptake of nutrients and crop yields. Especially, in the NPKM treatment crop yields for wheat and maize were highest in all of treatments. And it was the best fertilization model for improving soil fertility and gaining sustainable agriculture.(5) Addition of chemical nitrogen fertilizer could significantly stimulate nitrification. When application of nitrogen at the rate of 200 and 400 kgN·ha-1, the pH of soil decreased 0.3 and 0.69 units significantly after 35-day incubation experiment. However, the application of chemical nitrogen fertilizer bellow this rate, soil pH did not change markedly after 35 days. So the rate of 200 kgN·ha-1 was the minimum amount of chemical fertilizer for decreasing soil pH by nitrification. The soil nitrification potential decreased with the rising proportion of organic fertilizer, and reduced the risk of soil acidification.In conclusion, long-term application of chemical fertilizers (except PK), especially chemical nitrogen fertilizer, was the main reason for soil acidification. However, application of manure (M) or manure combined with chemical fertilizers (NPKM) could ameliorate soil acidity. Especially, in the NPKM treatment crop yields for wheat and maize were both highest as compared to other treatments. And it was the best fertilization model for improving soil fertility and gaining sustainable agriculture.
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