Composition Of Clay Minerals And Their Relationship With Aggregate Stability Under Combined Application Of Chinese Milk Vetch And Chemical Fertilizer

In recent years,with the improvement of agricultural intensification,the problem of soil degradation caused by long-term unreasonable farming and fertilization has become more and more serious.As an excellent organic fertilizer commonly used in paddy fields in southern China,Chinese milk vetch（Astragalussinicus L.）can be used in combination with chemical fertilizers to improve soil structure and soil fertility,and can be widely used in my country.To this end,this study took the paddy soil that had been returned to the field for many years as the main research object,and the study focused on the effective improvement of soil structure by returning milk vetch to field and applying chemical fertilizer,selected the application of single application of Chinese milk vetch（MV）,the application of chemical fertilizer（F100）,and the application of 100%chemical fertilizer（MV+F100）after the milk vetch returned to the field.and 80%chemical fertilizer（MV+F80）after milk vetch returning to the field,4 treatments,The clay particles in the aggregate（>250,53-250,2-53 and<2μm）were separated by wet sieving and centrifugation,and the clay components were deep into the nanometer scale by overfiltration.X-ray diffraction（XRD）,synchrotron radiation XRD and other methods were used to study the types,contents and distribution characteristics of clay minerals in the aggregates of Chinese milk vetch and different amounts of chemical fertilizer.The changes of Fe/Al oxides and organic carbon in soil aggregates and clay particles were analyzed.And their relationship to the stability of aggregates.The main results are as follows:（1）The clay minerals in the aggregate particles（100-2000,25-100 nm）of different sizes were mainly illite,kaolinite,vermiculite and HIV.With the reduction of aggregate particles from 100-2000 nm to 25-100 nm,the relative content of illite increased significantly,and the content of kaolinite decreased significantly,but the types of clay minerals decreased,and the crystallinity became worse.Both MV+F100 and MV+F80treatments significantly increased the relative content of illite in aggregate particles<2μm（100-2000 and 25-100 nm）,as well as the relative content of vermiculite in aggregate particles of each size.The MWD of the agglomerates had a very significant positive correlation with the relative content of vermiculite in aggregates>250μm and illite in aggregates<2μm.However,it was significantly negatively correlated with the relative content of kaolinite in 2-53 and<2μm aggregates and HIV in>250,53-250 and 2-53μm aggregates.（2）The contents of Si O₂ in 100-2000 nm aggregates were the highest in single application of Chinese milk vetch.In contrast,the content of Si O₂ decreased while the content of Al₂O₃ increased when Chinese milk vetch was applied with different amounts of chemical fertilizer,and the ratio of silicon to aluminum decreased significantly,resulting in the phenomenon of desilication and aluminum enrichment.（3）The contents of free iron and aluminum in soil aggregates were the highest in53-250μm aggregates,while the contents of amorphous iron and aluminum increased with the decrease of aggregate size,and the contents of amorphous iron and aluminum were the highest in 2-53μm aggregates.Compared with MV,both MV+F100 and MV+F80treatments increased the contents of free iron and aluminum in>250μm aggregates,as well as the contents and activation degree of non-crystalline iron and aluminum in various aggregates.The contents of iron and aluminum oxides in the aggregate clay are mainly concentrated in the aggregate clay of>2μm,and the contents in the free clay of<2μm are low.MV+F100 and MV+F80 significantly increased the contents of free iron and free aluminum in the clay particles of>250μm aggregates,but decreased the contents of non-crystalline iron and aluminum in the clay particles of>250μm aggregates.（4）The content of organic carbon in soil aggregates and clay particles of different treatments decreased with the decrease of particle size.Compared with MV,MV+F100 and MV+F80 treatments significantly increased the content of organic carbon in 53-250μm aggregates and aggregates of various sizes are clay particles.In addition,MV+F80treatment significantly increased the organic carbon content in 2-53μm aggregates and free clay particles（<2μm）compared with MV+F100.In addition,the correlation analysis found that the content of organic carbon in the aggregates clay particles was more closely related to the stability of the aggregates than the organic carbon in the aggregates.（5）organic carbon,iron and aluminum oxides in aggregates and their clays,as well as layered silicate minerals in aggregates,together explained 64.3%of the differences in aggregate stability.Kaolinite,HIV,organic carbon in aggregated clay particles and free aluminum content in aggregates are the strongest factors affecting the stability of aggregates.In addition,the content of organic carbon,free aluminum and amorphous iron in the aggregates and their clay particles had a direct positive effect on the stability of the aggregate,and the effect of amorphous iron was the most significant.