Status Analysis Of Soil Iron Oxide And Organic Carbon Forms In China’s Different Fertility Paddy Soils

Rice is an important food crop in China.Paddy soil has great carbon sequestration potential because of its special soil condition.Soil organic carbon and its forms reflect the level of soil fertility.Soil organic carbon content is affected by soil environmental factors,and the effect of soil iron content and its components on the change of organic carbon composition is the decisive factor of paddy soil organic carbon stability..In this study,paddy soil samples with high,medium and low fertility were collected from three typical planting rice regions(northeast plain,southern river network plain with double-cropping rice,and southwest mountainous Hilly area)in six provinces(Liaoning,Jilin,Sichuan,Guizhou,Jiangxi and Hunan).The status changes of iron oxide forms and organic carbon fractions in paddy soils with different fertility levels were determined and analyzed.The results are as follows:1.The content of free iron oxide in paddy soils with high,middle and low fertility showed a decreasing trend from south to north.The content of free iron oxide in paddy soil in four provinces of southern China showed the trend: medium fertility field > high fertility field >low fertility field.In northeast China,the free iron oxide content of paddy soil in Liaoning province(Dawa county and Liaozhong district)was higher than that of Jilin province.The content of amorphous iron and total iron in all paddy soils showed a trend of high fertility >middle fertility > low fertility.The content of complex iron in paddy soil with high fertility level in Dawa County,Liaoning province was significantly lower than that of other regions.The content of soil complex iron in Guizhou province with medium fertility level was significantly lower than that of other regions.The content of soil complex iron with low fertility level in Jilin Province was significantly lower than that of other regions.The content of various iron oxide forms in paddy soil of Jilin province was significantly lower than that of other provinces because of its high latitude and cold climate.2.The content of soluble organic carbon in high fertility paddy soil was significantly higher than that of low yield paddy soil except Sichuan province.The microbial biomass carbon content of paddy soil in all provinces showed a decreasing trend as the changing from high fertility,middle fertility to low fertility.The microbial biomass carbon content of high fertility was significantly higher than that of low fertility.The content of active organic carbon with high,middle and low fertility paddy soils in all provinces was significantly different and decreased with the decrease of fertility level.The active organic carbon content of high fertility paddy soil in Dawa County of Liaoning Province was significantly higher than that of other provinces and regions.The organic carbon content of paddy soil in different provinces was significantly different and decreased with the decrease of soil fertility level.Soil organic carbon content in high fertility level was significantly higher than that of medium and lowfertility level.3.In the three different regional soils,the content of free iron oxide and amorphous iron in paddy soil with different fertility levels in the Northeast Plain was significantly positively correlated with the content of soil active organic carbon,while the complex iron content had a very significant positive correlation with soil soluble organic carbon.There was a significant or significant positive correlation between soil complex iron content and soil active organic carbon,soluble organic carbon and microbial biomass carbon in Southern river network plain area.There was a significant positive correlation between soil free iron and soluble organic carbon in Southwest Hilly Region.4.Soil soluble organic carbon was positively correlated with free iron and total iron in high fertility paddy soil.There was no correlation between the form of iron oxide and organic carbon in medium fertility soil.The content of amorphous iron in soil with low fertility was positively correlated with the content of soluble organic carbon in soil.In summary,the differences of iron and carbon in paddy soils with different fertility levels were that the contents of free iron,amorphous iron,soluble organic carbon,microbial biomass carbon,active organic carbon and total organic carbon in high and medium fertility paddy soil were higher than those of low fertility soil.There was a significant positive correlation between free iron content and soluble organic carbon in the high and medium fertility paddy soil.However,there was a significant positive correlation between the content of amorphous iron and soluble organic carbon in low fertility soil.The results provide theoretical reference and data support for understanding the status of iron oxide and organic carbon and their transformation in paddy soils at different fertility levels in different regions of China.