| Red paddy soil is one of the leading agricultural soils in southern part of China, especially inJiangxi Province. In recent years, excessive land utilization and unreasonable application ofchemical fertilizers to achieve high yield have caused a series of production and environmentalecological issues. Therefore, to clarify the dynamic changes of red paddy soil and characteristicsof absorption and utilization of nutrient resources by crops is an important issue in the agriculturalscience in this area, and has important theoretical meaning and practical value for both theprotection of ecological environment and sustainable development of agriculture. With thelong-term positioned experiment (1991-2007) of red paddy soil in Jinxian County, JiangxiProvince as a platform, this paper analyzes the fertility evolution and sustainability of productivityof red paddy soil under no fertilization, unbalanced fertilization (N, P, K, NP, NK, PK), balancedfertilization (NPK and2NPK) and mixed application of chemical fertilizers and organic fertilizers(NPKM). Main results and conclusions are as follows:1. Long-term application of organic fertilizers could improve the content of organic substancein the plough-layer red paddy soil. Since the application of fertilizers, the content of organicsubstance increased significantly under the treatment of mixed application of chemical fertilizersand organic fertilizers. The content of organic substance in soil grew32%in2002as comparedwith the level before experiment in1981. The several-year average content of organic substance inthe plough-layer soil was20.55g/kg. With long-term application of chemical fertilizers, thecontent of organic substance in the plough-layer red paddy soil kept stable, with the several-yearaverage content maintaining within16-18g/kg.2. Long-term application of N could improve the content of N in the soil. The content of N inthe soil increased significantly under the treatment of mixed application of chemical fertilizers andorganic fertilizers, and did not increased significantly under other treatments. Long-termapplication of P-contained fertilizer may increase the content of total P and available P inplough-layer soil to different extents. For26years since the application of fertilizers, the contentof total P in the soil grew significantly under the treatments of NPKM and high-volume NPK, atthe greatest growth rate; also grew significantly under the NP and N treatments, at smaller growthrates; and remained flat under the treatments of PK and NPK. As compared with the beginning ofthe experiment, the several-year average content of available P in the soil grew at the greatest rateof672.3%under the treatment of mixed application of chemical and organic fertilizers; grew at409.9%under the treatment of2NPK; and declined by17.47%and21.25%under the N and CKtreatments respectively. The content of available K under the treatment of K application wassignificantly greater than that under the treatment of no K application, indicating that thelong-term application of K-containing mixed fertilizers could significantly improve the content ofavailable K in the plough-layer soil. In terms of the years of changing trend, the content of available K under all treatments changed sharply in different years. But in terms of the treatments,the content of available K was greatest under the treatment of mixed application of chemical andorganic fertilizers.3．The rate of N utilization by chemical fertilizers was significantly different under differentfertilizations. The26-year average N utilization rate was9.4%-11.6%under the N treatment,13.0%-18.5%under the NP treatment,19.8%-26.1%under the NPK treatment, and29.9%-40.1%under the NPKM treatment. P, K and organic fertilizers could improve the N recovery rate in ricesignificantly, and contributed more to early rice than to late rice; the N recovery rate in early riceunder the NPK and NPKM treatments was6.3and10.2percentages points greater than in late ricerespectively. The N recovery rate under the N and NK treatments declined significantly on yearlybasis, with an annual average decline of about0.6percentage points. And the N recovery rateunder the NPK and NPKM treatments remained stable basically. Different P treatments hadgreater effect on the P utilization rate in early and late rice. The greatest annual average Putilization rate in early rice occurred under the NPK treatment, as33.02%；The greatest annualaverage P utilization rate in late rice occurred under the NPK treatment, as20.22%. Under thesame treatment, the P utilization rate in late rice was less than that in early rice. There was a sharpdifference between early rice and late rice under different treatments of K application. The greatestK utilization rate in early rice occurred under the high-volume NPK treatment, as60.18%. Thegreatest K utilization rate in early rice occurred under the ordinary-volume NPK treatment, as57.40%.4．In the continuous farming of early and late rice, the fertility contribution of basic soilfertility to the rice yield was65%and69%respectively. The contribution of NPK to the yieldincrease of early and late rice was35%and31%respectively；In the15-year positionedexperiment, the rate of contribution of red paddy soil to the soil fertility of early rice appeareddeclining in several years. The rate of contribution of red paddy soil to the soil fertility of earlyrice appeared rising in several years. Early rice relied more on fertilizers than late rice. The grainyield of early rice was significantly greater under the NPK treatment and the treatment of mixedapplication of chemical and organic fertilizers than other treatments, and remained stable inseveral years. The grain yield of early rice declined by73.2kg ha-1,61.7and90.6kg ha-1per annumunder the N, K and NK treatments respectively. The grain yield of late rice was significantlygreater under the treatment of organic fertilizer application than under other unbalanced treatmentsand declined significantly by29.8kg ha-1and44.3kg ha-1per annum under the N and NKtreatments respectively. As for early rice, the sustainability index of yield was the greatest underthe treatments of balanced fertilization and mixed application of organic fertilizes; as for late rice,the sustainability index of yield was less different between treatments, and the application of Khad no effect on the sustainability of yield of late rice.In a word, unbalanced fertilization is neither helpful for high yield, nor helpful for cropabsorption and nutrient accumulation, with poor effect of fertility fostering; balanced application of chemical fertilizers could maintain higher yield of crops, and the content of nutrients in the soilwas not significantly different from that at the beginning of experiment, indicating that, both interms of nutrient utilization increase and high yield maintaining, mixed application of chemicaland organic fertilizers is a model of sustainable fertilization for red paddy soil. In addition, resultsof this study clarify that early rice relies more on fertilizers than late rice, and needs more P than K,providing theoretical basis for fertilization depending upon crops in the production in order toincrease the grain yield and maintain the sustainable development of agriculture in the red paddysoil area.
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