Composition And Weathering Characteristics Of Clay Minerals Under Return Of Different Green Manures

The composition characteristics of clay minerals are closely related to the environment.Returning green manure,as an effective agricultural measure to improve the soil environment,can alter soil physical,chemical,and biological processes,thereby affecting the composition and weathering of clay minerals.In this paper,paddy soil developed from Quaternary red clay with 36 years returning of green manures in Qiyang,Hunan Province,and paddy soil developed from river and lake sediments with 7 years returning of green manure in Jingzhou,Hubei Province were studied.Four treatments were selected in Qiyang,including winter fallow with fertilization（CK）,milk vetch with fertilization（MV）,rape with fertilization（RP）,and ryegrass with fertilization（RG）.Five treatments were selected in Jingzhou,:winter fallow without fertilization（CK+N0）,milk vetch without fertilization（MV+N0）,winter fallow with fertilization（CK+N11）,milk vetch with fertilization（MV+N11）and rape then vicia with fertilization（VR+N11）.The composition of clay minerals and mineral structure characteristics were measured.It could provide a basis for proper utilization of green manure and improving soil productivity.The main research results are as follows:（1）Kaolinite and Illite were the main phyllosilicates of paddy soils in the two experimental sites,accounting for 80%.Compared with winter fallow,the content of Illite and vermiculite decreased after returning green manure,while the content of Kaolinite and HIV increased.Between the two test sites,the paddy soil developed from Quaternary red clay with a longer returning period had more obvious changes,in which RP has a greater impact on the composition of phyllosilicates,and the composition of phyllosilicates in MV and RG were relatively consistent.Paddy soil developed in river and lake sediments shows that compared with CK+N11,MV+N11 had greater influence on the composition of phyllosilicates than VR+N11.（2）The main interlayer hydroxyl mineral in the two paddy soil was interlayer hydroxyl vermiculite.The interlayer hydroxyl substances extracted from paddy soil developed from Quaternary red clay were mainly Al and Si,and the extraction amounts of MV and RG were low;The extraction amount of hydroxyl substances in the rice soil layer developed from river and lake sediments was relatively low,mainly Al and Fe,with the extraction amount of MV+N0 and VR+N11 significantly higher than other treatments.（3）The main iron bearing minerals of paddy soil developed from Quaternary red clay were goethite and hematite.The free and amorphous iron and aluminum contents of CK and MV were significantly higher than RP and RG,and the complex iron and aluminum contents were significantly lower than RP and RG.The manganese oxide content showed a downward trend after the three green manure returning.Compared with CK,MV has increased the iron and aluminum activity,and RG has increased the manganese activity;The main iron bearing mineral of paddy soil developed in river lake sediments is hematite,and the effect of fertilizer application on oxides was greater than green manures.The contents of iron,aluminum and manganese oxides in the treatments with fertilizer application（CK+N11,MV+N11 and VR+N11）were significantly lower than the treatment without fertilizer application（CK+N0,MV+N0）,of which the manganese content and activity of VR+N11 were the lowest.（4）The main mineral elements in soil were Si O₂（about 50%）,Al₂O₃（20%-30%）and Fe₂O₃（about 10%）.In paddy soil developed from Quaternary red clay,the silicon aluminum ratio（sa）and silicon aluminum iron ratio（saf）of CK and MV were lower than RP and RG.The soil weathering leaching index（ba）of RG was the highest,while the chemical alteration index（CIA）value was the lowest,indicating that the return of milk vetch promoted the soil desilication and aluminum enrichment,and the amount of base leaching was low after ryegrass returning;The paddy soil treatment of river and lake sediment development showed that the sa and saf of the treatments with fertilizer application were higher than the treatments without fertilizer application,with MV+N11 being the highest,MV+N0 having the lowest ba value,CIA value being the highest,and salt leaching amount being higher.（5）The Illite chemical index（ICI）of paddy soil developed from Quaternary red clay was 0.303-1.743,indicating strong chemical weathering.Compared with CK,illite crystallinity of MV and RG were weak and weathering degree was strong（Weaver index decreased,Kübler index and integral width increased）.RG’s Kaolinite crystallinity was strong,then MV,with the smallest in RP;The ICI value of paddy soil developed in river and lake sediments was 0.201-0.354,indicating that soil weathering was between physical weathering and chemical weathering,which has little impact on the structure of clay minerals.Among them,Illite crystallinity of VR+N11 and MV+N0 were higher,and Kaolinite crystallinity of MV+N0 was lower.