Studies on the Preparation and Nutrient Release Characteristics of Low Molecular Weight Polylactic Acid Coated Urea
|Keywords||low molecular weight polylactic acid coated urea nitrification inhibitors urease inhibitors nitrogen release ammonia volatilization nutrient use efficiency|
Fertilizer is the vital input material for the sustainable development of crop production and plays an important role in food security. Nitrogen use efficiency is generally low in the single growing season. The nitrogen recovery ranged in 30% to 35% in major crops in China. Ammonia volatilization, nitrification-denitrification, leaching and runoff are regarded as main pathways of nitrogen losses, and the loss through ammonia volatilization is up to 40%-50%. The massive nitrogen losses not only cause energy waste and reduce economic efficiency, but also bring unprecedented environmental risks. Selecting the appropriate materials and developing the new coated urea will have important effects in the development of agriculture in China. Since coating materials used presently such as polymers are relatively expensive and may cause adverse environmental impacts, their application are limited. Developing and exploring coated fertilizers with environment-friendly, cheap coating materials is necessary.Based on the good environmental compatibility, low molecular weight (RMW) PLA was used as the main coating material in the present study. A series of coated urea were prepared by spouted-spraying methods. The structure of coating film was observed by scanning electronic microscopy (SEM), nitrogen release was investigated by a soil column leaching method, and ammonia volatilization was determined with an incubation experiment. Nitrogen use efficiency of the coated urea fertilizers was investigated by a pot experiment. The results were as follows:1. Coated with PLA only or modified with paraffin, nitrification inhibitors, urease inhibitors respectively, seven kinds of different coated urea as PLAUⅠ, PLAUⅡ, PLAUⅢ, PLAUⅣ, PLUAⅤ, PLUAⅥand PLUAⅦwere prepared, which made the foundation for studying the membrane structure, the sustained release characteristics, ammonia volatilization and nitrogen use efficiency.2. SEM images showed that both of RMW5000 and RMW10000 PLA as a film coated on the surface of granular urea. The film of RMW10000 PLA had less small gaps than that of RMW5000 PLA. Paraffin wax used as a sealant could fill up the micro holes or cracks in the coating film. The film of low molecular weight PLA coated on the surface of urea role as a physical obstacle for nutrient release.3. Adding the same doses of nitrogen, nitrogen release of prepared coated urea fertilizers were investigated by soil column leaching experiment. In the first leaching, the percentages of released nitrogen of PLAUⅠ, PLAUⅡ, PLAUⅢ, PLAUⅣtreatments to added nitrogen were 19.2%,15.1%,17.1%,14.0%, which significantly lower than that of urea treatment (27.3%). Compared with the normal urea treatment, the cumulative nitrogen release of PLAUⅠ, PLAUⅡ, PLAUⅢ, PLAUⅣtreatments were reduced by 12.7%,19.5%,17.5%, and 19.7%. The cumulative ammonia volatilization of PLAUⅠ, PLAUⅡ, PLAUⅢ, PLAUIV treatments were significantly less than normal urea treatment, which reduced by 45.8%、53.7%、58.5%、63.2%. The results indicated that PLA coated urea could effectively reduce the ammonia volatilization losses.4. The results of the soil column leaching experiment also showed that the cumulative leaching of nitrogen of UREA+N, UREA+D+N, PLUAⅤ, PLUAⅥ, PLUAⅦreduced 11.8 %,19.9%,18.0%,19.2%,19.5%, respectively, which were significantly less than that of UREA treatment. The difference between UREA+D and UREA on cumulative nitrogen leaching was not significant. The cumulative nitrogen losses of UREA+N, UREA+D+N, PLUAⅤ, PLUAⅥ, PLUAⅦthrough ammonia volatilization were significantly lower than that of UREA.5. Nitrogen fertilization could enhance ryegrass growth as the total shoot dry weights of two harvests of the treatments with N addition were significantly higher than that of the control (no fertilizer), as well as increase nitrogen concentrations in ryegrass shoots. With the comparison of the UREA treatment, addition of the nitrification inhibitor or the urease inhibitor, and application of coated urea fertilizers prepared could also significantly increase the shoot biomass of ryegrass. It was also found that N, P, K accumulation in ryegrass shoot and apparent recovery of N, P, K supplied of the treatments of UREA+D, UREA+N. UREA+N+D, PLAUⅠ, PLAUⅡ, PLAUⅢ, PLAUⅣ, PLAUⅤ, PLAUⅥ, PLAUⅦwere significantly larger that of the UREA treatment, implying that nitrification inhibitor or the urease inhibitor addition, coated urea application can increase not only nitrogen use efficiency, but also phosphorus and potassium use efficiency.