Effects Of The Addition Of Iron Oxides On The Decomposition Of Rice Straw And Changes In Characteristics Of Dissolved Organic Matter

Iron oxides are inorganic minerals with redox activity that widely exists in soils and sediments,which can catalytic decomposition or oxidation of organic matter,and conducive to the generation and condensation of precursor substances of humus.In view of these characteristics,they have the potential to be used as additives to strengthen the decay of organic materials.However,the physical and chemical properties of iron oxides vary greatly due to their different types and crystal structures,in order to explore the effects and differences of different iron oxides used as additives for organic material decay,rice straw was used as raw material for decay,and amorphous ferrihydrite（Fe OOH·n H₂O）,crystal goethite（α-Fe OOH）and hematite（α-Fe₂O₃）were selected as additives to carry out indoor simulated decay tests.There were treatments:the control（CK）,0.5%ferrihydrite（SF1）,1.0%ferrihydrite（SF2）,0.5%goethite（SG1）,1.0%goethite（SG2）,0.5%hematite（SH1）,1.0%hematite（SH2）.The effects of different iron oxides on the decomposition characteristics of rice straw and the transformation of dissolved organic matter（DOM）in the process of decomposition were compared and analyzed under the conditions of different iron oxides,so as to clarify the effect of different iron oxides on rice straw decay,and select the iron oxides with the best decay promoting effect and the appropriate addition amount.The main research results are as follows:（1）The addition of ferrihydrite,goethite and hematite（except SH2）all promoted the decomposition of rice straw to a certain extent,among them,SG1,SG2 and SH1 had better promoting effect.On the 180th d,the cumulative mass loss rate of each treatment was SH1（47.12%）>SG2（46.42%）>SG1（46.23%）>SF1（45.58%）>SF2（45.28%）>CK（45.10%）>SH2（44.86%）,and the mass loss of SH1 treatment was the highest.Through the function fitting of straw mass loss rate,it was found that SG1 treatment increased the cumulative mass loss rate of rice straw,and accelerated the decomposition rate of rice straw.With the extension of decomposition time,the carbon content of all treatments showed a decreasing trend,and on the 180th d,the carbon content of of SG1 treatment was the largest（13.82%）,the carbon loss of SF1 was the least（10.14%）.On the 180th d,the nitrogen content of SH1 and SG2 treatment was higher than CK treatment,and the nitrogen content of SH1 treatment was the highest,which indicated that SH1 treatment had a certain nitrogen retention effect.The C/N ratio of each treatment showed a decreasing trend with the decay process,on the 180th d,the C/N ratio of each treatment was less than 25,which basically reached the decomposed state.Based on the above results,SH1 treatment has the highest decomposition rate and the best nitrogen retention effect,but its decomposition rate was slow;SG1 treatment promoted the decomposition amount of rice straw and accelerated the decomposition rate of rice straw;SG2 treatment promoted the decomposition amount of rice straw,and the effect of nitrogen retention was better.（2）UV-Vis spectrum analysis of dissolved organic matter（DOM）showed that the addition of hematite increased the UV absorption intensity greatly at the later stage of decomposition（90-180 d）.On the 180th d,the UV absorption intensity of SH1 treatment was the strongest at 280 nm,and the value of SUVA₂₅₄ of SG1 and SH1 treatment was higher than CK treatment,the results showed that SG1 and SH1 had strong aromatic properties in DOM processing.The E₂₅₄/E₃₆₅ value of SG1 was the highest（3.54）,indicating that the addition of 0.5%goethite treatment enhanced the humification degree of DOM.The values of S_275-295/S_350-400 treated by SF1 were the lowest on the 180th d,indicating that DOM treated by SF1 at this time had strong aroma and high molecular weight.The fluorescence spectrum analysis of dissolved organic matter（DOM）showed that the relative content of protein-like substances decreased gradually and the relative content of humus-like substances increased gradually with the extension of decomposition time,indicating that protein-like substances gradually transformed into humus-like substances in the process of decomposition.On the 180th d,the fluorescence index（FI）of CK,SG1 and SH1 was lower,indicating the higher aromaticity of fulvic acid.The humification index（HIX）of SH1 and SG1 was higher than CK,indicating that the humification degree of DOM of SH1 and SG1 was deeper.The biogenic index（BIX）value of SH1 and SG1 treatment was lower on the 180th d,indicating that DOM autogeneic characteristics of SH1 and SG1 treatment were weak at this time.The P_V,n/P_III,n values of SF1 treatment were the highest,indicating that the humus component of DOM of SF1 was more stable.In conclusion,SG1 and SH1 promoted the humification degree of DOM.