Effects Of Weathered Coal Humic Acid Char-Based Nitrogen On Wheat Soil Nutrients And Bacterial Communities

The irrational application of fertilizers in agricultural production can easily lead to nitrogen loss and trigger problems such as eutrophication of water bodies.Ammonium nitrogen（NH₄⁺-N）is a common form of nitrogen in polluted water,as well as a nutrient that can be directly absorbed and utilized by crops.Currently,the use of char materials to adsorb NH₄⁺-N for application to soil to reduce nitrogen loss is a feasible way to utilize the resource.It is crucial to prepare a char material with good adsorption-retardation properties of NH₄⁺-N at a low cost.Weathered coal reserves are rich,because of the low calorific value can not be used as fuel was abandoned resulting in a waste of resources.How to resource utilization of waste weathered coal has attracted widespread attention.Weathered coal is high in humic acid（HA）and rich in oxygen-containing groups,the humic acid char（HAC）materials have excellent adsorption and storage properties for cations,which makes it a good choice for NH₄⁺-N adsorption.Few studies have been reported on the use of modified HAC for adsorbing NH₄⁺-N,and its effects on crops and microorganisms when applied to soil have rarely been reported.In this study,we used waste weathered coal as raw material,and screened the optimal conditions for HA extraction by catalytic treatment of weathered coal using ball milling.HAC was obtained by pyrolysis of HA（extraction under optimal conditions）and then graft-modified to prepare sulfonated acid humic acid char（SHAC）.The adsorption-desorption mechanism of NH₄⁺-N was studied using SHAC.Sulfonated humic acid char-based nitrogen（SHAC-N）was obtained after adsorption of NH₄⁺-N,and ammonium chloride（NH₄Cl）was selected for comparison of the application effect according to the form of nitrogen existence in SHAC-N.The effects of sulfonate-based humic acid charcoal on crop growth,soil nutrients,microbial metabolism,and bacterial communities were investigated in a pot experiment.The results of the study are presented below:（1）The best extraction conditions for humic acid extraction from weathered coal by ball-milling and catalytic treatment were obtained by orthogonal test,and the highest extraction rate was 52.11%.Compared to untreated weathered coal,the total and free humic acid content of ball-milling catalytic treated weathered coal significantly increases.Characterization showed that the oxygen content（29.90%and 28.68%）,O/C ratio（0.52 and 0.52）,and total acidic groups（7.88 and 7.64 meq/g）of catalytic treatment of weathered coal humic acid（C/WHA）and BC/WHA were higher than those of untreated weathered coal humic acid（WHA）（25.43%,0.44,and 6.46 meq/g,respectively）.PY-GC/M analysis displayed that the small molecule fatty acid content of C/WHA（79.07%）was significantly higher than that of WHA（66.03%）.BC/WHA appears to have high levels of the active substance sterols.It was demonstrated that the catalytic treatment improved the biological activity of C/WHA and BC/WHA,indicating thatα-Mn O₂/Kln played an effective catalytic oxidizing role.Overall,BC/WHA has high extraction rate and strong bioactivity,indicating that ball-milling combined with catalytic treatment for humic acid extraction is superior to ball-milling or catalytic treatment methods alone.（2）Structural characterization showed that the sulfonate group（-SO₃H）had been grafted onto SHAC successfully.SHAC has a 46.99%higher adsorption capacity for NH₄⁺-N（40mg/L）than HAC.The results showed that the adsorption of NH₄⁺-N by both HAC and SHAC was more in line with the quasi-secondary kinetic model and was a physicochemical synergistic adsorption process.The adsorption process was divided into fast adsorption,slow adsorption and equilibrium stages.The adsorption of NH₄⁺-N by SHAC reached equilibrium within 100 min,and the maximum adsorption amount was 77.24 mg/g in isothermal study.Adsorption mechanism:NH₄⁺-N was adsorbed mainly through electrostatic attraction,hydrogen bonding and pore volume sites.SHAC and NH₄⁺-N were weakly bonded.Desorption experiments showed that SHAC-N desorbed less NH₄⁺-N（14.49%）in 1 hour significantly less than NH₄Cl（>95%）,and the desorption rate was slow.Its desorption process conformed to the first-order kinetics and was characterized by slow release.Preliminary indications suggest that SHAC-N is suitable for use as a nitrogen fertilizer.（3）In the potting experiment,the fresh weight,dry weight,grain protein,starch and total nitrogen content of wheat treated with SHAC-N showed no significant difference compared to NH₄Cl treatment,while the apparent utilization rate of nitrogen had an improvement.The specific surface area,medium and large pores of P/SHAC-N were significantly increased,the pore volume was reduced,and the percentage of oxygen atoms was significantly increased.The C 1s bonding mode of P/SHAC-N shifted to a higher binding energy position,suggesting its structure is influenced by soil nutrients.Soil organic carbon（SOC）,soil NH₄⁺-N and total nitrogen（TN）were significantly higher in the SHAC-N treatment than in the NH₄Cl treatment at the maturity stage of wheat,and nitrogen loss rate was significantly reduced.This result indicates that SHAC-N can be used for sustainable utilization and management of nitrogen nutrients in soil.（4）Soil samples from a wheat pot experiment were collected and analyzed for carbon,nitrogen,and phosphorus cycle-related extracellular enzymes and nitrification functional genes（ammonia-oxidizing archaea（amo A-AOA）,ammonia oxidizing bacteria（amo A-AOB）,and nitrite oxidoreductase（nxr B））were determined.The results showed that the SOC:TN ratios of SHAC,SHAC+N and SHAC-N treatments were significantly higher than those of NH₄Cl treatment during wheat growth.The（BG+CBH）activity of SHAC-N treatment at tillering stage was significantly higher than that of NH₄Cl treatment,indicating enhanced decomposition of soil organic matter.Application of SHAC-N fertilizer treatment increased the microbial demand for nitrogen.The nutrient stoichiometric ratios SOC:TN and enzymes stoichiometric ratios EEA-C:N directly affected microbial nitrogen limitation.The amo A-AOA and amo A-AOB of SHAC-N treatment were significantly lower than NH₄Cl treatment.Soil TN showed a negative effect on nitrification function genes,and the SHAC-N treatment at maturity had higher soil TN,indicating that SHAC-N had fewer nitrification function genes,which could slow down nitrification.（5）The bacterial abundance and homogeneity were significantly higher in SHAC-N treatment than in NH₄Cl treatment,but the diversity of the bacterial community did not differ significantly among the fertilization treatments.The dominant phyla of the bacterial community in different fertilization treatments were Proteobacteria,Acidobacteriota,Firmicutes,and Actinobacteriota,with cumulative abundance>70%.The abundance of Actinobacteriota phylum was significantly lower in SHAC,SHAC+N,and SHAC-N treatments（33.72%,33.89%,and 33.35%）compared to CK and NH₄Cl treatments（38.93%and 38.33%）.In addition,the abundance of Myxococcota was significantly lower in the NH₄Cl treatment than in the other treatments.Bacillus was the dominant genus in each treatment.MND1 belongs to a group of ammonia-oxidizing bacteria（amo A-AOB）,and the abundance of this genus was significantly lower in SHAC-N treatment than in NH₄Cl treatment,that ammonia-oxidizing effect of SHAC-N is weaker.SOC and NH₄⁺-N were positively correlated with bacterial species abundance of SHAC-N treatment,and negatively correlated with CK and NH₄Cl treatments.It indicated that SHAC-N was favorable to increase soil species richness.The microbial bacterial were categorized into four modules,and the relative abundance of module 1 of CK was significantly higher than that of other treatments,whereas the relative abundance of module 2 of SHAC-N treatment was significantly higher than that of NH₄Cl treatment.There was a significant negative correlation between the key species（Proteobacteria）in module 1 and SOC,while there was a significant negative correlation between p H and the key species（Actinobacteriota）in module 3.In summary,this study obtained humic acid with high extraction rate and activity from weathered coal through ball-milling catalytic treatment,and prepared SHAC through humic acid pyrolysis and grafting modification.The mechanism of SHAC adsorption of NH₄⁺-N was elucidated as electrostatic attraction of oxygen-containing groups,hydrogen bonding with nitrogen-containing groups,and pore interactions.SHAC-N as a char-based slow-release nitrogen fertilizer had a certain slow-release and fixation effect on soil nitrogen nutrients.The study not only provides a new strategy for the resource utilization of weathered coal,but also has important practical significance for the application of sulfonated humic acid char for nitrogen nutrient management in agricultural production process.