Degradation And Transformation Mechanism Of Controlled-Release Fertilizer Sulfur Coating In Soils And Plant Growth Responses

With continuous application of SCU (sulfur coated urea) in wide area, there are growing concerns about SC (sulfur coating) effect on the soil environment. Only by exploration of the migration and transformation process of SC in the soil, the impact of SC conversion on the soil can be theoretically explained. The research of degradation dynamics and transformation process of SC not only can explore SCU degradation effect on certain properties oil micro-sulfur domain, but also can ascertain the impact mechanism, which will provide scientific proof of CRFs (controlled release fertilizers) application and dissemination and guide SCU application and promotion.In this study, the migration and transformation process of SC of CRFs and the various times morphology and ultrastructure changes in soil SC were explored thoroughly by the indoor simulation training, sand culture and pool culture experiment combined with scanning electron microscopy, micro-domain technology and isotope tracer so on, the researches ascertain the intermediates during the degradation process and the final degradation products effect on soil environment, and reveal SC of CRFs specific destination in the soil sulfur specific destination, and also analysis SC effect on activation of soil nutrient elements, migration and transformation of N and the impact on crop growth and development. The main results are as follows:(1) Using the pot experiment of two consecutive years to observe feature and ultrastructure changes during the degradation of SC. In this research, the cross-section and surface of coating (SC, SC spraying paraffin sealant outer coating and packing thermosetting resins SC) were analyzed by scanning electron microscope in different cultivating stages, the electron microscope images show that only SC surface is rough, and has slits and holes all over. Along with the extending of time, the cracks and holes become gradually very big, whose reason is only the poor effect of SC control;â‘¡The surface of SC spraying paraffin sealant outer coating, on which some tiny cracks will be produced when coating because of temperature changes, has some relative smooth and thin coatings formed by paraffin, but it can better coat the slits and holes formed by sulfur. During the cultivating, the erosion holes, whose size enlarged increasingly and become darker in color from light, occurred on smooth and continuous paraffin coating. During the mid-late cultivation, sulfur of lack of sealant cracked into small pieces from coating condition and turned to powder.â‘¢Polymer-sulfur coating has relatively thin polymer layer and the gap of sulfur surface under coating can be seen, and with the degradation of the coating, the coating of sulfur under polymer coating can be found. Compared with SC spraying paraffin sealant outer coating, the degradation of packing thermosetting resins SC need a long time, which can control the release of nutrients better.(2) By the quartz sand sulfur-free culture experiment of 560 days, the intermediate product of the degradation of SC is thiosulfate, and at the end of culture the final product of is sulfate that is the main product of SC degradation. In matrix, SC main transformation approach is S⁰→S₂O₃^2-→SO₄^2-, and the rate of degradation of sulfur coating residual is higher than that of packing fertilizer coating. In the period when the concentration of sulfate ions of is higher, the total sulfur content in plants is also higher, which indicates that the degradation of SC can promote the sulfur absorption of plant.(3) The degradation of sulfur and SC in the soil gives rise to significant change of pH in each treatment micro-domain soil of brown soil, the less effect in non-micro-domain soil, but at the middle-late stage of culture, their PH value increase. Compared with the brown, PH value of fluvo-aquic soils descended in a narrow range. The each sulfur-containing treatment oxidation rate in brown and fluvo-aquic soil is Sulphur>SC>Polymer-sulfur coating. The application of SC and polymer-sulfur coating plays role of activation of trace elements in micro-domain soil. At the middle-late stage of culture the content of available ferrum, manganese and copper get exaltation, and the content of available sulphur in brown micro-domain soil has increase trend, also rising trend in non-micro-domain soil. In fluvo-aquic micro-domain soil, the content of available sulfur and phosphorus has a growing tendency, and with the extension of degradation time that in non-micro-domain soil also has rising trend.The experiment results show that the soil of sulfur micro-domain (≤5 mm) is significantly affected by the products of SC degradation, while the soil of non-micro-domain (>5 mm) is less affected. The above conclusions show that the controlled-release fertilizers application of SC and polymer-sulfur coating will dramatically affect the chemical properties of soil in a small range, no obvious effects were observed in the whole soil.(4) With brown pool culture experiment, the content change of soil sulfur components in the rotation system of summer corn-winter wheat shows the water-soluble sulfur content has a big margin in 0-20 cm layer, and the application of controlled-release fertilizers of SC and polymer-sulfur coating can significantly increase the water-soluble sulfur content in mid-late period of crop cultivation. In the process of crop growth, there exist the balance of mutual transformation between water-soluble sulfur and adsorbed sulfur. In brown soil, the water-soluble sulfur and soluble sulfur of hydrochloric acid is the main components of inorganic sulfur, and the content of soluble sulfur of hydrochloric acid has a gradually downward trend, which indicates when the adsorbed sulfur and water-soluble sulfur in soil is not sufficient, the soluble sulfur of hydrochloric acid can be timely supplied. The application of controlled-release fertilizers of polymer-sulfur coating increased the total amount of inorganic sulfur in soil, which guarantees the steady and continuous supply of sulfur required for crop growth in the whole growth period of corn. By the role of rainfall and irrigation-leaching, SO₄^2- has the migration trend to the deep soil layers.(5) Controlled release fertilizer of SC promotes absorption of sulfur in 12-leaf stage and milk ripe stage of summer wheat, the absorption of sulfur of each treatment is PSCU> SCU + PCU> PCU> Urea. The application of Controlled release fertilizer of SC increased total sulfur content of plant at flowering and mature stage of winter wheat. At flowering stage, total sulfur content of plant of PSCU and SCU+PCU treatment have the increase of 25.64% and 17.16% compared with Urea treatment respectively. The dynamic change of sulfur components in soil promotes the absorption of sulfur for crop.(6) In the rotation system of the summer corn-winter wheat, the isotope tracer technique is used to compare with N fate of Controlled release fertilizer of SC, the residual nitrogen in soil, the soil nitrate N migration in the profile and absorption and use of N in crop. The results show that in the summer corn, the application of controlled-release fertilizer has the higher availability of N fertilizer compared with urea, and N recovery rate, residue rate and loss rate of PCU treatment has the increase of 3.9% and 5.02% and the decrease of 7.67% compared with Urea treatment, respectively. In winter wheat, N recovery rate, residue rate and loss rate of PCU treatment has the increase of 1.88% and5.45% and the decrease of 7.32% compared with Urea treatment, respectively.(7) Application of controlled-release fertilizers of SC and polymer-sulfur coating can increase the plant dry matter accumulation and grain yield of crop maturity. Furthermore, the use efficiency of crop nitrogen and sulfur gain improvement synchronously, which will increase nitrogen and sulfur concentrations of grain, and accumulation amount of N and sulfur in plant. By increasing the harvest index of N and sulfur to promote the increase of grain yield, the yield of grain of summer corn of PSCU and PCU treatment has an increase of 16.0% and 13.43% compared with Urea treatment (11.85% and 9.79% for winter wheat, respectively). Application of controlled-release fertilizers of polymer-sulfur coating may realize the efficient use of nitrogen in rotation system of summer corn-winter wheat, and the lower price of PSCU will benefit large-scale use of field crops.(8) Path-coefficients of yield and yield components shows that when applied the mixture of PSCU and SCU(Sulfur coated urea), TGW(thousand-grain weight), R/E( rows per ear), ED (ear diameter) and G/E(the grain per ear) play a decisive role in yield of maize; in only application of PSCU, TGW, G/R(row grains), ED and G/E have the main role, while in that of PSU, TGW, G/R, ED and R/E play decisive role. Each key decision factors is TGW, and in limiting factor PSCU and PCU is R/E, SCU and PCU is G/R. Therefore, in high-yield maize cultivation, selection of combinations of different CRFs should coordinate the main factors of yield to guarantee the best combination of each factor.