Studies On Evolution Of Potassium In Reddish Paddy Soil Under Long-term Fertilizer And Rice Straw Application

A long-term field experiment was conducted to study the effect of long-term application of K fertilizer and rice straw on the change of form, adsorption and desorption characteristics in soil K, the composition change and transformation process of K-bearing minerals based on double rice and winter fallow cropping system from the Key Field Monitoring Experimental Station for Reddish Paddy Soil Eco-environment in Wangcheng, Ministry of Agriculture, China. The relationship between the change characteristics of soil K form, external source K, composition of soil clay mineral and crop K uptake under long-term application of K fertilizer and rice straw were discussed. In order to reveal the significance of long-term application of K fertilizer and rice straw on K supply capacity and crop growth characteristics, the change of K supplying-capacity, the characteristics of K fixation and release, and the relationship between quantity and intensity potassium in soil were elucidated in reddish paddy soil. Conducting scientific evaluation of soil potassium fertility based on agricultural managment practies and K characteristics of reddish paddy soil, it can be provide the theoretical basis for reasonable and effective applying potassium fertilizer in the reddish paddy soil. The main results were as follows:Contents of total K, slowly available K and available K in the different soil layers under the NPK and NPK+RS treatments are higher than those of counterpart soil layers under the NP and NP+RS treatments. After 54 season rice cropping in 27 years, the soil exchangeable K content in 0-45cm layer was increased with the long-term K fertilization, whereas total K is in deficit state.199.2 K kg/hm2 of chemical potassium fertilizer and 4.2 t/hm2 rice straw (contain 109.2 K kg) was applied into reddish paddy soil with K deficiency under long-term double-crop rice cropping systems, but insufficient to sustain total K pool of the soil; long-term application of 4.2t/hm2 rice straw without applying chemical potassium fertilizer couldn't maintain the soil K level.There are two different phases in change of nonexchangeable K release of each treatment, i. e., the linear increase in the initial and slow increase in late phase. The accumulations of nonecchangeable K release in the long-term K fertilization (NPK, NP+RS and NPK+RS) treatment were higher than those in treatment with no K fertilizer (CK and NP). The accumulations of nonexchangeable K release were 62.22 mg/kg and 56.12 mg/kg in the treatment with no K fertilizer (CK and NP), while 67.52mg/kg, 64.41mg/kg and 75.33mg/kg with K fertilizer (NPK,NP+RS and NPK+RS) treatments, respectively. The accumulations of nonecchangeable K release in treatments with application of rice straw application (NP+RS and NPK+RS) treatments were significantly higher than those of the treatment with no application of rice straw (NP and NPK), which may promote the change from the part of soil mineral potassium to nonexchangeable K under long-term application of rice straw. The four kinds of kinetic equations (parabolic diffusion equation, zero-order equation, first-order equation and Elovich equation) were tested for their suitability to describe the release of nonexchangeable K from the five different treatments. The kinetics of nonexchangeable K release for five treatments could better be simulated by Elovich equation. Considering release rate and effective of nonexchangeable to plants, the segmented straight line equation are more reliable and more accurate than those of the above four equations.Decomposition of X-ray diffraction pattern showed that clay mineralogy altered with the changes of different fertilization treatments. The illite content in soil was increased by application of K fertilizer and rice straw application on the basis of NP application, while the content of illite in soil was depleted due to long-term no application of K fertilizer. From the change of soil composition, the whole-clay fraction (<5μm) more corresponded more to fertilization treatments than did fine-clay fraction (<1μm) in term of peak area pencentage. Chlorite of mixed-layer illite-chlorite mineral was transformed into free illite and layer illite by long-term fertilization. Applying Potassium and rice straw treatments increased the proportion of poorly crystallized illite (PCI) in free illite. Changes in clay mineralogy were reflected in aspects of K+adsorption caused by long-term K fertilization and rice straw application. Removed soil organic matter [+H2O2] experiment clearly showed that there was a positive effect between soil organic C and the adsorption of K+, especially under low K+concentration. Soil organic C could reduce the K+adsorption of clay minerals under the high K+concentration. The soil minerals were more important than soil organic C in K+absorption. K+adsorption potential was influenced by the composition change of mineral in the long-term fertilization treatment. The significantly correlation between peak area percentage of poorly crystallized illite and K+adsorption potential (M) or adsorption coefficient (K) was observed in the long-term fertilization treatments.After 54 season rice cropping over 27 years period, when added K ranging from 0.4 to 4.0g/L, the rate of K fixation increased with K addition. The values of K fixation capacity were different among five treatments. The K fixation value in the treatments with no application K treatments (CK and NP) was increased compared with initial soil value, while K fixation value in treatments with K application and rice straw application (NPK, NP+RS and NPK+RS) were decreased compared with initial soil value. The results indicated that the mail reason why affecting the increase of K fixation capacity was due to the long-term application of K fertilizer and rice straw, but the capacity of K fixation was relatively lower when application rate K fertilizer was higher. The long-term application of K fertilizer would be lead to K depletion of soil potassium, because a large amount of K was removed by rice crop from the soil under highly intensity rice-rice planting system. A long-term no application of K fertilizer would result in severe K depletion in soil, after that, K was applied into soil, a large amount of K was fixed by the soil, and reduced efficiency of K for crops in season. When the K was relatively higher in soil, it was not easily adsorbed and fixed by the soil. If the K can not be absorbed and utilized by crop in time, it was very likely to leach. After long-term application of K fertilizer and rice straw, the content of available K and slowly available K in soil was increased, and K fixation capacity was decreased. In addition, increasing K+saturation could also result in reduction of soil K fixation capacity under long-term application of K fertilizer and rice straw.Values of-ΔK°and ARek were decreased in the long-term treatments with no K fertilizer, while the value of PBCk was increased. That is, the easy release K become smaller, K adsorption capacity of soil was increased, and K supply capacity of soil reduced. A large number of K application was needed to raise K concentration of soil solution for long-term treatments without K fertilization. Effect of long-term application of rice straw on K behavior was an obvious in the 0-15cm soil layer. K behavior in soil was significantly affected by increasing organic matter and reducing pH value of soil. Through simulaton the equation (PBCk=CEC×KG), the sum of cations extracted BaCL2 or CEC of BaCL2 determination was the best predictive equation of Q/I curve. Organic matter could increase the high-affinity K+sites. The data of this study also proved that the release of a large number of K+into the soil solution was not controlled by K+-Ca2++Mg2+, but was controlled by the K+-NH4+.The average yields of the early rice under the NPK and NPK+RS treatments increased by 15.2% and 10.9%, respectively, and those of the late rice are increased by 17.2% and 9.1% compared with the NP and NP+RS treatments over 27 years. The trends of 27-year yield change in both the early and late rice are different in various treatments. There are negative trends of yield change with time in the CK and NP treatments, while there are the positive trends in the NPK, NP+RS and NPK+RS treatments. Rice straw return into soil increased significantly the rice production. Average rice yield with 27 years rice straw-returned treatment (NP+RS and NPK+RS) was increased by 12.2% and 6.7%, respectively, compared with treatments of NP and NPK application alone. The yield of grain was increased year by year after returning rice straw. Rice straw could partly replace chemical K fertilizer because K brought by rice straw had same nutritional function with that of chemical K fertilizer.