Effects Of Iron Oxides In Degraded Black Soil On Organic Carbon Under Different Moisture Conditions

Iron oxide,whose form and content are easily affected by environmental conditions,is very active in the soil.Functioning as a protector and stabilizer,it can fix soil organic carbon through adsorption or co-precipitation.Under different moisture condition,the redox process of iron may affect the bind of soil organic carbon.In this experiment,two kinds of black soils with different degrees of degradation,middle layer and thin layer were chosen to be the study objects.We established three moisture conditions:field water holding capacity of 60%（FC）,flooding（F）,alternating dry and wet（AWD）,combined with three allogenic materials:glucose,goethite,and no material,indoor cultivating for 60 days to study the effect of glucose（Glu）addition on the change of soil iron oxide form and the effect of iron oxide（goethite,Goe）addition on soil mechanical composition,organic carbon content,enzyme activity and other properties,which could provide a theoretical basis for exploring the mechanism of the carbon retention of iron oxide on the black soil.The test results are as follows:（1）Different moisture conditions（FC,F,AWD）had effects on the form of soil iron oxides.The content of free iron oxide（Fed）,amorphous iron oxide（Feo）,and complex iron oxide（Fep）under different moisture conditions of the two soils fluctuated with the extension of the cultivation time;the content of Fed did not change during 0-30 days.Obviously,30-60d showed a fluctuating upward trend;Feo content reached the peak at the 10th day,reached the lowest at the 20th day,and changed gradually at 20-60d;Fep content increased significantly in the first 20d,and gradually decreased at 20-60d.The contents of Fed,Feo and Fep in the middle black soil range from 11.03-14.08,3.07-3.52,0.48-0.54 g·kg^-1;the contents of Fed,Feo,and Fep in the thin black soil range from 15.22-16.83,3.49-3.69,0.34-0.37g·kg^-1;the content of Fed and Feo in the two soils decreased under FC and F conditions,and significantly increased under AWD conditions（p<0.05）.The middle-layer black soil has high organic matter and low iron oxide content.The effect of water conditions on iron oxide is weaker than that of thin-layer black soil.（2）Compared with the control（CK）,the Fed content and iron freeness of the two kinds of soils were reduced when the glucose was added.The different water conditions were:AWD>F>FC,which had a greater impact on the middle black soil;the two soils Feo,Feo,Both Fep content and iron activation degree increased.Among them,the Fep content increased more in thin-layer black soil compared with middle-layer black soil,but the difference in iron complex degree was not obvious.Compared with CK,the addition of glucose treatment reduced the iron crystal gel rate of the two soils,and inhibited the aging of iron oxides,which had a greater impact on the thin black soil.（3）Adding goethite treatment significantly affects the soil particle size composition.Compared with FC conditions,F and AWD,soil median particle size（D50）decreased,>200μm particle size content decreased,soil fine particles increased,and the response of the middle black soil was more obvious;compared with CK,the middle black soil D50 was treated with goethite Increase,the content of clay particles of 50-200μm also increases,the soil particles are reorganized,and the fine particles agglomerate;the D50 of thin-layer black soil decreases,which is not significantly different from CK.（4）Compared with CK,adding goethite treatment increased the content of iron oxides in the soil,which promoted Fe（Ⅲ）to reduce Fe（Ⅱ）;soil p H,total organic carbon,iron oxide combined organic carbon content and its ratio Increase and decrease the soluble organic carbon content.Compared with FC conditions,F and AWD have reduced total organic carbon and increased soluble organic carbon.Compared with middle-layer black soil,adding goethite to treat the above indicators has more obvious effects;middle-layer black soil has a Fe-bound OC/Fe ratio of 0.67-6.68,the thin black soil Fe-bound OC/Fe is 0.55-0.63,the Fe-bound OC/Fe of the two soils are both less than 1,and the organic carbon is mainly combined with iron oxide by adsorption.Adding goethite treatment compared with CK,the thin-layer black soil Fourier infrared spectroscopy（FTIR）produced obvious differentiation,under the AWD moisture condition,the stretching vibration frequency of water and organic matter in the hydroxyl contraction peak at 3424 cm^-1Decrease,increase under FC and F moisture conditions;while the FTIR spectral characteristics of middle-layer black soil are relatively similar.（5）Water conditions and adding goethite treatment affect soil enzyme activity.Compared with CK,the invertase and urease activities of thin-layer black soil have greater changes in the treatment of adding goethite.The increase of invertase activity is 7.95%-39.44%under different water conditions,and the difference is more significant under AWD conditions（p<0.05）;Urease activity increased by 34.62-44.14%under FC and F conditions,and decreased by 57.97%under AWD conditions.The activities of invertase and urease in the middle black soil have small changes.The increase of invertase activity is 2.57%-9.90%under different water conditions;the increase of urease activity is 14.05-16.79%under FC and F conditions,and it decreases by 4.61%under AWD conditions.Compared with the thin-layer black soil,the catalase activity of the middle-layer black soil has greater changes,but the difference is not significant.