Study On Artificial Humification Of Rice Straw Assisted By Urea Under Room Temperature

China is rich in rice straw resources,with an annual output of 188 million tons.The current resource utilization of rice straw is mainly divided into fodder and energy,with limited capacity for consumption and low added value.The use of rice straw resources for the production of artificial humic acid can not only realize the high-value utilization of rice straw,but also realize the replenishment of soil carbon pool and reduce carbon emission.However,the current artificial humification technology often is adopted the "one-pot" transformation method,which can produce humic acid while losing the high value-added fraction(cellulose),and fails to realize the fine utilization of rice straw resources.Therefore,it is important to study how to retain the cellulose fraction to the maximum extent while artificially humifying the rice straw for high value and fine utilization.In this study,the effects of environmental factors on humic acid yield,fulvic acid yield and cellulose retention in the artificial humification process at room temperature were investigated by response surface methodology,and then the optimal process conditions were determined.The promotion mechanism of urea on artificial humification and the product modification mechanism were also revealed by various characterization means such as morphological structure analysis,spectral analysis,thermogravimetric analysis and particle size analysis.The effects of water-soluble humic acid fertilizer on the growth,soil physicochemical properties and soil microbial community of Brassica juncea were further investigated by potting experiments.The study will provide theoretical basis and technical support for the preparation,modification and application of artificial humic acid under mild conditions.The main findings are shown below:(1)The effects of different reaction times(0-4 h),liquid-solid ratios(6-8)and KOH concentrations(1-2 mol/L)on the liquid-phase and solid-phase products of artificial humification under room temperature conditions were investigated.The optimum conditions for artificial humification were obtained as follows: reaction time of 4 h,liquid-solid ratio of 6 and KOH concentration of 1.5 mol/L.At this time,the humic acid content was 30.33 g/L,the fulvic acid content was 1.20 g/L,the yield of artificial humic acid was 18.20%,the yield of crude cellulose was 66.62%,the retention of cellulose was 93.81%,the removal of hemicellulose was 76.16%,and the lignin removal rate was 81.52%.(2)The effect of adding different concentrations of urea(0-1.5 mol/L)on artificial humification was investigated under the conditions of 4 h reaction time,liquid-solid ratio of 6 and KOH concentration of 1.5 mol/L,and the optimum urea addition concentration was obtained as 1mol/L.The humic acid yield and fulvic acid yield were increased by 20.71% and 8.92%,respectively,under these conditions.The addition of urea did not affect the removal of cellulose from rice straw,and the removal of lignin and hemicellulose increased by 10.01% and 3.63%,respectively.(3)Natural humic acid,artificial humic acid and urea-modified humic acid were characterized.Artificial humic acid was more similar to natural humic acid in terms of functional group composition and atomic content;in terms of morphological structure,the urea-modified artificial humic acid showed a decrease in particle size and an increase in porosity in terms of morphological structure;in terms of chemical properties,urea increased the content of N elements and C-N bonds of artificial humic acid;the addition of urea increased the thermal decomposition temperature of artificial humic acid and improved the thermal stability.Urea weakened the degree of polymerization of humic acid and reduced the particle size distribution of artificial humic acid.It also improved the solubility of artificial humic acid under neutral or weakly acidic conditions and reduced the solids and viscosity of water-soluble humic acid fertilizer.The rice straw and crude cellulose were characterized.In terms of morphological structure,the addition of urea made the surface of coarse cellulose rougher and increased the pore structure;in terms of functional group composition,urea reduced the content of lignin and hemicellulose characteristic functional groups.(4)The results of the pot experiment showed that the application of water-soluble humic acid fertilizer promoted the growth and development of chard.Compared with the application of chemical fertilizers,the application of humic acid fertilizer increased the dry weight,fresh weight and plant height of chard by 10.89%,19.36% and 13.50%,respectively.The leaching treatment reduced the fertilization effect of chemical water soluble fertilizer and promoted the release of water-soluble humic acid fertilizer.The dry weight of cabbage in the water soluble chemical fertilizer group was reduced by 12.28% and that of water-soluble humic acid fertilizer group was increased by 12.59% after leaching.(5)There was an effect of applying water-soluble humic acid fertilizer on the properties of soil.The soil p H was closer to neutral after fertilization,and increased from 6.29 to 7.12 and 7.06 in the no leaching and leaching,respectively;the organic matter content of the soil was increased by5.61% in the water-soluble humic acid fertilizer application compared to the chemical water soluble fertilizer group;and the humic acid content of the soil was also increased by the application of water-soluble humic acid fertilizer,compared to the initial soil,the humic acid content of the no leaching and leaching groups increased by 12.59%.Compared with the initial soil,the humic acid content of the no leaching and leaching groups increased by 35.09% and 29.22%,respectively,and the retention rate of humic acid reached 60.47% and 50.33%,respectively.(6)The soil microbial communities under different fertilization treatments were analyzed.The application of water-soluble humic acid fertilizer affected the microbial community of bacteria and fungi in soil.In the community composition of soil bacteria,the application of water-soluble humic acid fertilizer increased the relative abundance of growth-promoting bacteria such as Glutamicibacter and Pseudomonas.In the community composition of soil fungi,the application of water-soluble humic acid fertilizer increased the relative abundance of growth-promoting fungi such as Mortierella and Gibberella and reduced the relative abundance of pathogenic fungi such as Fusarium.