Study On The Mechanism Of Humic Acid-magnetic Modified Biochar On The Restoration Of Mild Saline Land And Plant Growth Characteristics

The coastal soils have been irrigated by seawater for a long time and have a high water table,which has led to high salinity in the coastal soils,which in turn has led to serious degradation of the soil structure.These conditions have led to a decrease in crop yields and have severely restricted agricultural development.Therefore,it is urgent to improve coastal saline soils and make coastal areas an important agricultural reserve.The use of biochar for improving coastal saline soils has been paid attention by a wide range of scholars.Biochar is mainly used to solve soil degradation problems by improving soil nutrient retention capacity and reducing soil salinity,but there are still some drawbacks in imposing biochar to restore saline lands.In order to solve the drawbacks of biochar application in saline lands,measures such as proper modification of biochar and optimization of biochar dosage can be taken to ensure the restoration of saline lands while reducing the harm to the soil.In this thesis,we proposed to prepare rice straw biochar(RBC)by selecting rice straw as the raw material for pyrolysis at500?,magnetic biochar(MBC)by mixing RBC into solutions containing Fe²⁺and Fe³⁺and adjusting the p H,and humic acid-magnetic biochar(HA-MBC)by immersing MBC in humic acid(HA)solution.FTIR,SEM-EDS,elemental analysis,and BET were used to characterize the properties of the three biochars that were synthesized.The three biochar were applied to saline soils at different addition ratios,and the saline soils were drenched in a laboratory simulated soil column experiment.The basic properties of the leaching solution and the physicochemical properties of saline soils were determined.The mechanism of the effect of biochar on salt transport and crop growth in soil was also investigated in conjunction with Pak choi potting experiments.The main conclusions are as follows.(1)Characterization of biochar propertiesThe p H of all three biochar was greater than 7 and all were alkaline.Compared with RBC,the specific surface area and total pore volume of the modified MBC and HA-MBC were significantly increased,where the specific surface area were increased by 1.68 and 1.78 times,and the total pore volume were increased by 2.36 and 2.81 times,respectively.The results of infrared spectroscopy showed that the oxygen-containing functional groups of HA-MBC were significantly increased,which was due to the fact that humic acid itself contained a large amount of oxygen-containing functional groups.In addition,the O/C and(O+N)/C of MBC and HA-MBC were higher compared to RBC,indicating that the modified biochar had stronger hydrophilicity and more polar functional groups.SEM showed more loose and porous agglomerates deposited on the surface of MBC.And combined with the characterization results of FTIR,HA-MBC was presumed to loaded with irregular humic acid particles.SEM-EDS showed the appearance of iron peaks on MBC and HA-MBC,indicating the successful loading of iron oxides.(2)Mechanistic study of biochar for saline land restorationDue to the large pore volume of biochar,the application of different types of biochar led to a significant increase in the leaching rate in the soil column.The conductivity showed an increasing trend followed by a decreasing trend,and the conductivity in the leaching solution of the soil column with the addition of biochar was increased.The changes in the conductivity of the soil leaching solution were probably due to the soluble ions contained in the biochar itself,and the rate and conductivity of the drench solution were proportional to the amount of biochar added.After the addition of biochar,the p H of the soil leaching solution showed a slow increase,while the p H of RBC had no significant effect on the p H of the leaching solution,and the p H of MBC and HA-MBC leaching solution both showed a small decrease.The effect of biochar addition on the p H of the soil increased as the amount of biochar added increased.The cations in the leaching solution were mainly Na⁺,K⁺,Ca²⁺and Mg²⁺,of which Na⁺accounted for 80%,indicating that the salinity of saline soils mainly came from Na⁺,of which the RBC treatment also contained a large amount of K⁺.The trends of the contents of the four cations were similar to the conductivity.After the soil column leaching experiment,the physical and chemical parameters of the soil altered dramatically.Soil conductivity decreased to about 200?s/cm in all treated columns,and soil p H decreased to about 7.5 in all treated columns,with a decrease of about 1.1.The water holding capacity of soil in the columns with biochar was increased,with HA-MBC having the most significant increase of 3.8%.The Cation Exchange Capacity(CEC)of the soil also increased significantly,with 5%HA-MBC improving 1.99 times compared to the control Blank.The CEC of the soil increased significantly as the amount of biochar was added,and the loss of various nutrients by soil leaching was reduced.Biochar could effectively fix NH4⁺-N,but the application of RBC and MBC had no significant effect on NO₃^-retention,and HA-MBC could keep part of nitrate-N from being lost by leaching.Thus,it can be proved that humic acid-magnetic biochar can keep nitrogen from being lost by leaching,and humic acid and biochar have synergistic effects on improving saline land.(3)Study on the mechanism of biochar effect on the growth of Pak choiThe results showed that both drained and biochar-applied soils promoted the growth of Pak choi,and the best growth was achieved by draining and applying 5%HA MBC treatment.The chlorophyll content of cabbage grown in drenched saline soils was lower than that of non-drenched saline soils.The results of RBC and MBC treatments were similar,indicating that RBC and MBC had a weaker effect on the retention of chlorophyll synthesis elements,but the chlorophyll content was enhanced by the application of biochar compared to the drenched control,with AF-5%HA-MBC enhancing the chlorophyll content by about 120%.The soluble protein content in Pak choi was higher after drenching than before drenching,with the highest soluble protein content in AF-5%HA-MBC treatment,and the protein content increased with the increase of biochar dosing.The content of soluble sugars was lower in the pre-drench filtration treatment.After biochar application,the content of soluble sugars was lower than that of Blank,where the potted plants with AF-5%HA-MBC treatment were on par with Blank,demonstrating that the soluble sugar content of chard was inversely proportional to the growth of chard.In order to further investigate the migration pattern of salts in the soil,the 5%biochar treatment with better improvement effect and the control group Blank were selected to determine the distribution of salts in the soil after planting,and the results showed that the salt content was higher as the soil depth increased,and the salt content in each layer of the soil decreased after the application of biochar.The trend of sodium ions was similar to that of soil salts,while potassium and magnesium ions gradually increased in the 0-7.5 cm soil layer,and potassium and magnesium ions gradually decreased in the 7.5-12.5 cm soil layer,while calcium ions gradually decreased in the0-7.5 cm soil layer and calcium ions gradually increased in the 7.5-12.5 cm soil layer.Soil layers gradually increased.The sodium ions in the soil profile decreased and the potassium-calcium-magnesium ions increased after the application of biochar,indicating that biochar could adsorb sodium ions from the soil solution and release potassium-calcium-magnesium ions into the soil,with the most obvious effect being the HA-MBC treatment after drenching;the sodium adsorption ratio of the drained soil was lower than that of the soil before drenching,and the sodium adsorption ratios of the soil profiles of each treatment were as follows:HA-MBC