Mechanism Of Soil Fertilization By Biochar And Ethylcellulose Coated Controlled Release Fertilizer

As a country with a large population,China has a prominent conflict of more people and less land.Together with the excessive use of chemical fertilizers and pesticides,it has caused serious soil damage problems.Improving nutrient utilization and reducing nutrient loss are of great significance for enhance soil productivity and ensure food security.In this study,two materials,biochar and ethylcellulose（EC）,were prepared from crop straws.The effects and mechanisms of fertilizer core（biochar）and slow-release fertilizer coating material（EC）in fertilizing soil were studied from the perspectives of“fertilizer supply”and“fertilizer conservation”,respectively.By using different alkaline solutions to soak straw,the effects of pretreatment methods and preparation atmosphere on nitrogen retention in biochar were investigated,and the optimal production conditions for nitrogen-rich biochar were determined.The migration and transformation process of nitrogen in soil-plant-microbial-atmosphere after biochar returning was investigated by using the isotope¹⁵N labeling method combined with high-throughput sequencing technology,and the mechanism of biochar-N absorption and utilization by crops was clarified.Different slow-release fertilizers were produced by adjusting the concentration of EC coating solution,and the factors affecting the slow-release of the nutrients were investigated,and the optimum concentration of the coating solution was analysed.In this study,a pH-sensitive material,sodium polyacrylate（PAANa）,was added to the slow-release fertilizer to produce the coated controlled-release fertilizer that could sense the nitrogen uptake demand of plants,and the sensing mechanism between fertilizer and plants was revealed by combining the data of pH value and nitrogen uptake of plants in hydroponic environment.The main conclusions are as follows.（1）The effects of alkaline solution concentration,alkaline solution type and preparation atmosphere on the nitrogrn retention in biochar during pyrolysis process were clarified.Compared with N₂ atmosphere,more biochar could be obtained under CO₂ atmosphere during the rice straw pyrolysis.When the concentrations of the three alkaline solutions（NaOH、KOH and Ca（OH）₂）were 0.15%,Na OH soaking could increase the nitrogen content in biochar to the greatest extent,and the higher the concentration of alkaline solutions,the more obvious effect of inhibiting the nitrogen release.This is because that alkaline solution immersion has removed some lignin and retained cellulose with strong thermal stability.Cellulose had an ordered crystalline structure and was not easily to become volatile gases during pyrolysis.When there was more release of nitrogen-containing gases,there was also more release of energy gases（CH₄,CO and H₂）,and the two showed a relatively consistent positive correlation.（2）The fate of biochar-N as well as the migration and transformation mechanism of biochar-N in the soil were investigated after biochar returned to the field.Most of the nitrogen（77.70%-99.06%）would be retained in the soil after biochar returning,and the nitrogen absorbed by plants was only 0.38%-6.91%of the biochar-N,and the remaining nitrogen was either taken up by microorganisms（0.23%-6.93%）or volatilized into the atmosphere（0.27%-8.46%）.Using plant nitrogen uptake as the evaluation basis,400°C-biochar returning to the field had the best effect,which were mainly related to the content and form of nitrogen at the pyrolysis temperature.In addition to providing available nitrogen,biochar returned to the field can also fix soil-N through its own special structure,reduce the volatilization of nitrogenous gases,stimulate the activity of soil microorganisms and improve the plant utilization of nitrogen.（3）The cellulose was extracted from the straw and then the EC coating solution was prepared.The slow release principle of coated urea and its role in reducing nitrogen loss were analyzed,and the optimal coating concentration was clarified.EC had good film-forming properties,and when the EC concentration was 0.1 g/m L,the prepared coated urea had a complete and smooth film layer with the slowest nutrient release curve,and the initial nutrient leaching rate was only 37.9%of the uncoated fertilizer.Compared with ordinary urea,EU10（coated urea with EC concentration of 0.1 g/m L）applied to soil increased plant uptake N by 17.69%and reduced the loss of soil leached N by 61.29%,indicating that EU10 can greatly improve the nitrogen utilization and the wheat cultivated by EU10 had the best growth potential.（4）Based on the coated slow-release fertilizer,the coated controlled-release fertilizer was prepared by adding PAANa,which successfully established an inductive relationship between fertilizer nutrient release and plant nitrogen uptake.Brassica napus was a plant that likes nitrate nitrogen（NO₃^--N）.When absorbing NO₃^--N,it would release OH^-and HCO₃^-,causing the pH value of the solution to increase from 5.5 to 8.3.The elevated pH promoted the absorption of water by PAANa,giving PAANa a maximum saturation swelling（38.8）at pH=8.0.The swelling behavior of PAANa led to an increase in the volume of the fertilizer core,causing nutrients to break through the envelope and release into solution.Since then,the induction effect of“high nitrogen uptake by plants”and“rapid nutrient release by fertilizer”has been established using pH as a signal.The prepared EPU5（coated controlled-release fertilizer with an EC concentration of 0.05 g/m L）had both slow release and induction properties,and also had a good fertilization effect.