Preparation And Nitrogen Release Mechanism Of Hydrophobically Modified Bio-based Coated Nitrogen Fertilizers

Coated nitrogen fertilizer is considered as an effectively method to improve nitrogen use efficiency and crop yield while reduce agricultural environmental pollutions.It plays an important role in achieving agricultural sustainable development.However,the high price,non-renewable and non-degradable of coating materials used in conventional coated nitrogen fertilizer had limited their large-scale commercial applications.In order to solve these problems,the low-cost raw materials of polyvinyl alcohol（PVA）and agricultural waste bacterial residue were used to synthesize bio-based coatings.In view of the poor water resistance of bio-based coatings,a series of bio-based coatings and coated nitrogen fertilizers with different release periods were prepared by superhydrophobic modification technology.The effects were evaluated by measuring the water resistance and permeability of the coatings before and after modification.The nitrogen controlled release effects of bio-based coated nitrogen fertilizer were determined by soil incubation experiments and water incubation experiments.In addition,infrared spectroscopy,scanning electron microscopy,energy spectrum analysis and thermogravimetric analysis were used to explore the mechanisms of modification and controlled release.Finally,the effects of bio-based coated nitrogen fertilizers on maize yield were determined by field micro-area experiments.The main results were as follows:（1）When the concentration of water-based polymer was 6.0%,the amount of nano silica was 2.0%,the reaction temperature was 55℃,the reaction time is 1.0 h and the dosage of 1H,1H,2h,2h perfluorodedecyl triethoxysilane（FAS）is 1%,the water absorption of the hydrophobically modified water-based polymer coating material（BW8）is the lowest（21.4%）.Compared with other treatments,treatment BW8 reduced water absorption by 7.2%-70.1%,water permeability by 8.8%-13.0%,ammonium permeability by 0.6%-5.4%,respectively.It increased the hydrophobic angle from 33.3°to 120.9°.In addition,the water absorbent functional groups were disappeared,while Si-O and C-F bonds were appeared in hydrophobically modified water-based polymer coatings.Furthermore,a layer of nano rough structure was constructed on the surface of BW8 coatings,which was formed a barrier that could resist the pass of water molecules.The slow-release effects were significantly enhanced.The nitrogen release period of hydrophobically modified water-based polymer coated nitrogen fertilizer（SWCU）was increased from 4 days to 25 days,and the variation range of nitrogen release rate constant was 0.066 d^-1-0.191 d^-1.（2）Bio-based polyhydric alcohol was synthesized from agricultural waste bacterial residue by high temperature liquefaction technology,then reacted with polyphenylmethyl polyisocyanate and bisphenol A diglycidyl ether under the action of catalyst to synthesize bio-based polyurethane coatings and bio-based epoxy coatings,respectively.Furthermore,superhydrophobic modification technologies were used to modify bio-based polyurethane coatings and epoxy coatings.After modification,the micropores and loose structures on the surface of the bio-based coatings disappeared while a dense micro/nano rough structure appeared.The low surface energy substances were successfully grafted onto the surface of the bio-based coatings and then formed a hydrophobic structure similar to the surface of lotus leaves,which could greatly reduce the contact area between water and coatings while improve their water-resistant ability.The hydrophobic angle of SBCU and SBECU coatings increased by 53.0%and 55.6%respectively,the water absorption decreased by 82.1%and87.0%,respectively,and the ammonium adsorption equilibrium value decreased from 5.300mg g^-1 and 4.505 mg g^-1 to 4.141 mg g^-1 and 3.189 mg g^-1,respectively.When the coatings were buried in the field for 360 days,the mass loss rates of SBCU and SBECU coatings were41.71%and 32.42%,respectively.The results of water incubation experiments showed that the controlled release period of SBCU and SBECU increased from 5 and 28 days to 28 and56 days,respectively.Soil incubation experiments showed that when the soil temperature increased from 15℃to 35℃,the water content of field capacity increased from 60%to100%,the nitrogen release rate constants of SBCU and SBECU were 0.029 d^-1-0.070 d^-1 and0.019 d^-1-1.140 d^-1,respectively.（3）The nitrogen release characteristics and cost analysis of three bio-based coated nitrogen fertilizers showed that the controlled release periods of SBCU and SBECU were significantly higher while the costs were reduced by 15.3%and 15.5%than SWCU,respectively.The field experiments showed that the application of SBCU and SBECU could increase the content of soil nitrogen.SBCU could effectively meet the demand for nitrogen in early and middle stage of maize growth,while SBECU could effectively meet the demand for nitrogen in the middle and later stage of maize growth.The maize yield in SBECU treatment was the highest(13826.31 kg hm^-2),which was 23.83%and 6.62%higher than conventional fertilization and SBCU treatments,respectively.SBECU treatment increased the agronomic efficiency from 10.74 kg^-1 to 21.83 kg^-1in contrast to conventional fertilization treatment.In conclusion,the hydrophobically modified bio-based coated nitrogen fertilizers prepared by bio-degradable and widely sourced bio-based coatings could meet the needs of nitrogen in the process of crop growth,effectively improve crop yield and fertilizer utilization efficiency.In particular,the hydrophobically modified bio-based epoxy coatings are better and worthy of recommendation to agricultural practice.