| Based on National Monitoring Station of Soil Fertility and Fertilizer Efficiency on Purple Soils, this study was addressed effects of 22-year long-term fertilization on soil tatal potassium (K), exchangeable K, non-exchangeable K, K content in different adsorbed sites, rice and wheat yield, and K utilization by crop. Using simulated experiment, the characteristics of K fixation and release, quantity-intensity and clay mineral under different long-term fertilization were studied to reveal the reasons of impact of long-term fertilization on K pool and supply capacity in purple soil. The objectives of these studies were for rational application of K fertilizer to improve the use efficiency of K fertilizer and maintain K fertility for purple soil.Results showed that rice and wheat yield without K fertilizer (NP) during 22 years were 16.2 and 10.7% lower than that with K fertilizer (NPK) while there was no significant increase in rice and wheat yield with fertilizer NPK and organic fertilizer (rice straw or manure). K absorption, transport and utilization efficiency in rice and wheat were influenced by long-term fertilization. K content of wheat and rice straw in NP treatment were 18.7% and 23.0% lower than NPK while no significant difference for K content in grain was found. K transport was enhanced by increased physiological efficiency and harvest index when it was serious deficient. Fertilizers containing Chloride could significantly promoted K uptake of wheat,64.6% higher than that with potassium sulfate fertilizer. However, luxury absorption of K was observed because it was mainly kept in straw with lower transport and physiological efficiency. Combination application of chemical fertilizer and straw could improve the K absorption by rice, but not by wheat. There was logarithmic curve between rice or wheat yield and K accumulation. K accumulation in crop was direct indicator of soil K supply capacity. Purple soil showed strong capacity of K supply of 150.6 kg/hm2 per year in average. However, soil K supply decreased along with experimental year. Capacity of K supply of flooded season (101.1 kg/hm2) was higher than dry season (50.5 kg/hm2).K application of annually 120 kg/hm2 in rice-wheat system was not enough to keep the balance in purple soil. Averagely, K deficiency in NPK treatment was 110.9 kg/hm2 while NPK with straw could maintain soil K balance. K input-output apparent balance had significant effect on various kinds of soil potassium content. There were significantly positive correlation between K apparent balance and soil exchangeable, non-exchangeable, p site and i site K content. The content of exchangeable, non-exchangeable and all adsorption site K of purple soil decreased significantly along with long-term no fertilization application. Exchangeable potassium and non-exchangeable K in the soil layer of NP treatment decreased at the rate of 1.21 and, 4.03 mg/kg per year while the decreased rate in NPK treatment were 0.85 and 2.62 mg /kg per year. Application organic fertilizer could alleviate the declined rate of K content. Combined application of chemical fertilizer and straw (NPKS) could improve soil available K content, alleviated declined rate of soil slowly available K content. Except for treatments with straw alone (S) or PK fertilizer, exchangeable and non-exchangeable potassium in the soil profile for other treatment showed depletion. Long-term different fertilization significant influenced on K content in different adsorption sites in the purple soil, with great effect on soil p site K of the surface of clay and i site K content between mineral layers, but little effect on e site K content in the edge mineral. Except of S treatment, the content of p site, e site and i site K with decreased in other treatments. In treatment without K application, the content of p site, e site, and i site K were 4.27 mg/kg、37.78 mg/kg and 359.0 mg/kg, declined by 81.4%,12.4% and 7.2% than soil with K application. K contents with different forms were significant correlated with K balance, so soil K balance was an important factor to influence K content with different forms. Combined application of chemical fertilizer and straw is an suitable measure to maintain soil K pool.The simulation experiment showed that release of nonexchangeable K during early period (1-168 h) accounted for 70% release with high rate while release during later period was slower in purple soil. Elovich models were the best to simulate all release process, and the release of non-exchangeable K was the process of diffusion-control. The ability of soil to fix K was strong. The saturation was achieved with 4.0-6.0 g/kg of K addition, and the saturation lever of K fixation was 624.4-1195.9 mg/kg. For neutral purple soil, the mainly mineral clay was montmorillonite, vermiculite, illite, kaolinite and quartz. Along with experiment year, mineral weathered with accompanying formation of montmorillonite in all treatments; Soil clay mineral composition affected the release, fixation and buffer capacity of soil K. there were greatly differences K release amount, release rate, fixation ability, Q/I characteristics parameters (-△ Ko, AReK, Kx, PBCK) between treatments. Here, soil potassium apparent deficiency caused transformation of montmorillonite to illite, with large amounts of K release to supplement soil K non-exchangeable pool, which resulted that G value in purple soils (15.34-20.22 kJ/mol) was higher than K deficiency threshold (14.64 kJ/mol), and that the release amount and rate of nonexchangeable K, the instantaneous intensity of release were lower while the soil K fixation ability was enhanced. Long-term no fertilization caused change of clay minerals form illite content to montmorillonite, decrease of K content in mineral layer and some lacking of interlayer potassium, which led to release amount and rate of purple soil with no fertilizer K apilication were 21.31% and 20.4% lower than the soil with application of fertilizer. The highest soil saturation of fixation was 1159.9 mg/kg in soil without K application,51.0% higher than soil with fertilizer K addition. Chemical fertilizer application could slow the decline of soil active and easy release K while straw returned could increase them. Therefore, combined application fertilizer K and straw returned was recommended to maintain soil K pool, improve soil K supply and buffer capacity, and achieve high crop productivity. |
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