Composition Of Clay Minerals And Release Of Potassium In Soil Particles Under Long-term Straw Returning

Clay minerals are widely distributed in various types of soil and plays an important role in maintaining soil potassium fixation and release.Therefore,studying the composition of soil clay minerals helps to understand the behavior of soil potassium and the potential for potassium supply.In this study,straw returning to soil with different cultivation methods?CT,CTS,NT,NTS?and different fertilization measures?NP,NPS,NPK,NPKS?from long term experiment fields in Wuxue and Jingzhou city were selected for the research.The composition of clay minerals and release of potassium in farmland cultivated soil particles?<2000,450-2000,100-450 and 25-100 nm?were investigated by X-ray diffraction,synchrotron radiation X-ray diffraction and plasma atomic emission spectrum method,and combined with 0.5 mol/L oxalic acid simulated extraction test.The study combined with the chemical compositions and the contents of various forms of potassium of soil particles to discuss the evolution characteristic of clay minerals and their relationship with potassium nutrition in soil particles of farmland.Results indicated that:?1?The main clay mineral was kaolinite,which accounted for more than 60%,followed by illite,vermiculite and 1.4 nm transition mineral?HIV?,and also contained a small amount of goethite and hematite in the arable soil of the Wuxue test site.The clay composition was dominated by illite and it`s content was as high as 70%,followed by kaolinite and vermiculite and contained a small amount of HIV,chlorite and hematite in the plow layer soil of Jingzhou experimental site.The soil interlayer hydroxyl minerals?HIM?were all interlayer hydroxyaluminums in the Wuxue and Jingzhou test sites,and the interlayers were mainly hydroxyaluminum and contain only a small amount of hydroxyl iron.Among them,the Wuxue test site has more interlayer hydroxyapatite in the soil,while there are relatively few in Jingzhou test site.?2?Long-term straw returning and different fertilization can change the relative contents of soil clay minerals.The contents of illite and vermiculite reduced but HIV increased in no straw treatment.The contents of illite and vermiculite increased while those of HIV decreased in straw returning treatments.The contents of illite and vermiculite increased,but kaolinite,HIV and chlorite decreased in nitrogen and phosphorus fertilizer treatment.The contents of kaolinite increased,while vermiculite,HIV and chlorite reduced in straw returned to field with nitrogen and phosphorus fertilizers and nitrogen,phosphorus and potassium fertilizer treatment.The contents of kaolinite,vermiculite and HIV increased,while illite and chlorite decreased in straw returned to field with nitrogen,phosphorus and potassium fertilizer treatment.In addition,The oxide minerals?such as gibbsite and hematite?and primary minerals?such as feldspar?decreased in the soils after long-term straw returning.?3?With the difference of soil particles size,the types and relative contents of minerals decreases in soil particles.The types of clay minerals of 450-2000 nm and100-450 nm particles were the same as those of<2000 nm particles,while the types of clay minerals were few and the crystals were poor of 25-100 nm particles in different treatments.With the reducing of soil particles size,the contents of kaolinite increased,but illite,vermiculite and HIV decreased,and gibbsite and hematite also decreased in soil particles.There was a transition trend of clay minerals from 2:1 to 1:1 types.?4?The element composition of 450-2000,100-450 and 25-100 nm particles were similar to<2000 nm particles.With the decrease of soil particle sizes,the contents of SiO₂ and Fe₂O₃ decreased,and the content of Al₂O₃ increased,and the silica-alumina ratios and silicon-sesquioxide ratios decreased gradually in soil particles.Compared with no straw treatment,the silica alumina ratio and silicon sesquioxide ratio increased significantly in the soil particles of farmland.Furthermore,with the decrease of soil particle size,the contents of iron and aluminum oxides gradually increased.These were in agreement with the degrees of freedom and activation of iron.?5?The contents of total potassium,exchangeable potassium,and non-exchangeable potassium in soils and their various particles?450-2000,100-450,and 25-100 nm?were significantly increased after long-term straw return to field.With the decrease of soil particle sizes,the content of total potassium decreased,while those of non exchangeable potassium and exchangeable potassium increased in soil particles.Among them of100-450 nm particles are best for potassium supply in soil particles.There were a significant positive correlation between various forms of potassium and illite content in soils,and a significant negative correlation with kaolinite and HIV content.?6?The release quantities and the release rates of potassium of soil particles were maximum in straw returning treatments,and with the decrease of particle size,the release quantities and release rate of potassium increased.The total release of potassium was the highest at 100-450 nm for 7 weeks,followed by 25-100 nm,while the total release of potassium at 450-2000 nm was the lowest.The release rate of potassium was the highest in the first week of 0.5 mol/L oxalic acid treated soil.Afterwards,with the increase of the number of extractions,the release rate of potassium gradually decreased and remained stable after the 5th week.There was a significant positive correlation between soil potassium release and non-exchanging potassium and total potassium.Illite and HIV were the main clay minerals that affected the release of potassium in soils.