Study On The Characteristics And Phosphorus Fixation Potential Of Soil Colloids In Red Soils At Different Slope Positions Of Citrus Orchard

In this study,the soil samples were collected from the red soils at different slope positions of a citrus orchard located at Sunjia Watershed,Yingtan,Jiangxi Province,China.Water-soluble colloids sizes of 5-1μm,1-0.45μm,0.45-0.1μm,and<0.1μm were sequentially extracted using a combination of siphon settling and centrifugal separation.The surface morphology characteristics,mineral composition,composition of cementitious material,and surface functional groups of soil colloidal particles were observed.The stability of colloidal particles and their variation in KH₂PO₄ and NaCl dispersion systems were investigated.The phosphorus fixation mechanism and release capacity of soil colloidal particles were also clarified.The results are as follows:（1）The proportion of soil colloidal particles decreases as the particle size becomes smaller in red soils at different slope positions of the citrus orchard,and decreases with the decreasing of slope position.The colloidal particles in citrus orchard soils at different slope positions are mainly aggregated in block,layer,or flake shapes,and the arrangement becomes denser with smaller particle sizes.The main minerals in the colloidal particles are halloysite（19-33%）,kaolinite（30-42%）,vermiculite（8-11%）,and chlorite（8-15%）.Among them,the proportions of kaolinite,hematite,and goethite in the 0.45-0.1μm colloidal particles are significantly higher than in the others.The 0.45-0.1μm colloidal particles in the citrus orchard soils have the highest specific surface area,pore volume,and average pore size,and the specific surface area,pore volume,and average pore size of soil colloids decrease Significantly with increasing particle size.The cementing substances in the colloids of citrus orchard soils are mainly amorphous iron-aluminum oxides,with the highest content in the 0.45-0.1μm colloidal particles.The dominant surface functional groups of colloidal particles are HPO₄^2-,mainly present in orthophosphate and polyphosphate.（2）The all-size colloidal particles in citrus orchard soils show significant dispersibility.The dispersibility of 0.45-0.1μm and 5-1μm colloidal particles is the strongest and weakest,respectively,in the soils of the flat top slope,top slope,and mid-slope;while the smaller the colloidal particles,the lower their dispersibility in the soils of the bottom slope.In the KH₂PO₄and NaCl dispersion systems,all soil colloidal particles in citrus orchard soils are in a dispersed state and show fluctuating changes with increasing concentrations of KH₂PO₄ and NaCl.The mobility of all colloidal particles in citrus orchard soils exhibits strong mobility,which fluctuates with increasing phosphate concentration.The mobility is the highest when KH₂PO₄concentrations are 40-60 mg L^-1 and the lowest at 80 mg L^-1.The phosphorus（P）contents of all colloidal particles in citrus orchard soils are significantly higher than the soil total P.The contribution of colloidal P to soil TP increases with decreasing particle size,indicating a stronger P fixation ability of smaller colloidal particles.Only 1.5%-4.0%of the P fixed by soil colloidal particles can be desorbed,and most of it is in the form of electrostatically adsorbed P.The risk of colloidal P loss in citrus orchard soils at different slope positions is significantly higher than the risk of soil P loss,with the highest risk in the mid-slope soils,especially for colloidal P in the 0.45-1μm size fraction.In conclusion,the study found that all soil colloidal particles in citrus orchard soils are easily dispersed and mobile,serving as the main carriers of P loss in citrus orchard soils.Smaller colloidal particles exhibit stronger P fixation ability.Iron-aluminum oxides and organic matter are important cementing substances that regulate the fixation and release of phosphorus of colloidal particles in citrus orchard soils.