Production And Nutrient Release Mechanism Of Yak Dung Hydrochar Slow-Release Fertilizer

In China,the nitrogen fertilizer utilization efficiency is low for a long time,serious environmental pollution is caused owing to a large amount of unused nitrogen is discharged into the environment.Slow and controlled release of nitrogen fertilizer can improve nitrogen utilization rate due to slow release of nutrients.However,high molecular weight polymer are commonly used as a coating material for slow and controlled release of nitrogen fertilizers,while their costly,difficult to degrade,and easy to form"secondary pollution".Hydrochar is one of the hotspots in the field of slow-release fertilizer research,it can be used directly to improve soil fertility and improve soil environment since its rich in nutrients such as nitrogen and phosphorus and oxygen-containing groups.At present,the research on the regulation of nutrient release from hydrochar itself is very limited,and it is difficult to control the nutrient demand of crops by regulating the release rate of hydrochar fertilizer itself.Based on these defects mentioned above,the effects of production conditions and hydrochloric acid modification on the performance,quality and nitrogen release rate of hydrochar produced from yak dung was researched in this paper.The physicochemical properties of hydrochar（BC）and acid-modified hydrochar slow-release fertilizer（HBC）were characterized by qualitative and quantitative techniques.The nitrogen release rate was evaluated by water dissolution rate method,and constructed the nitrogen controlled release model of yak dung hydrochar fertilizer nitrogen,revealed the nutrient release mechanism of hydrochar slow-release fertilizer.The main research conclusions are summarized as follows:（1）The effect of production conditions on the properties of BC was researched.It was found that BC produced under carbon dioxide had a high specific surface area ranging from 58.25 m²·g^-1 to 60.13 m²·g^-1,which had potential as a soil improving material.The nitrogen content of BC produced under nitrogen reached 22.05 mg·g^-1to 25.32 mg·g^-1,which had potential for fertilizer application.This study provided a theoretical basis and feasibility for the further development of hydrochar fertilizer.（2）The cumulative nitrogen dissolution rate of high temperature BC at 24 h was24.90%～26.20%,which was lower than that of the low temperature BC（34.87%）,indicating that the high temperature BC showed better slow-release performance.The cumulative nitrogen dissolution rate of HBC after 6 h acid treatment decreased the most at the 31^st day,and the higher acid treatment concentration decreased further by2.48%～4.27%.Therefore,shortening the acid treatment time and increasing the acid treatment concentration can enhance the regulation of nitrogen release ability.A nutrient controlled release model was constructed by polynomial model fitting based on Matlab-2018b.The equation was Y（x,y）=78.35-264.3x-0.232y+1625x²-2.506xy+0.008y².The degree of fitting was high with a correlation coefficient was0.859,which made it possible to develop a matched slow-release fertilizer according to the characteristics of specific crop growth nutrient requirements.（3）There were similarities and differences between the production conditions and hydrochloric acid modification on the regulation mechanism of nitrogen release rate of hydrochar fertilizer.The production temperature mainly reduced the nitrogen release rate by changing the physical structure of BC.The specific surface area and pore volume of BC at high carbonization temperature reached 25.71 m²·g^-1 and 0.08cm³·g^-1,respectively,while the physical structure was changed by the acid modification,the surface acid group was increased to hold nitrogen.The phenolic hydroxyl group content was increased to 1.48 mmol·g^-1 by the high acid treatment concentration,more surface active sites were provided,and the relative content of different nitrogen compounds were changed to achieve the purpose of delaying the release of nitrogen.