Genetic Characteristics Of Purple Mudstone-Developed Soils Dominated By Microtopography

Purple soil is mostly located in a hilly mountainous region in Sichuan Basin.The heat and water are redistributed in different location of topography,combined with severe soil erosion,resulting in differences of soil particle composition,nutrient content,water holding capacity,drought resistance and fertilizer retention in different location of topography,thereby the degree of soil development is different.Evaluating objectively and quantitatively changes of soil properties caused by soil weathering and the stage of soil weathering are the first prerequisite for interpreting changes of soil environment and improving soil environment.We cannot ignore the process of soil development behind changes of soil resources.How to solve effectively the soil erosion and its associated soil problems in purple soil region is an important issue we are facing at present.Therefore,this study took soils developed from purple mudstone as the research object.Through analyzing characteristics of soil morphology,physicochemistry and geochemistry,to study the degree of soil development in different slope positions?top,shoulder,waist,foot and valley?and to explore change rules of soil weathering and development under micro-topographic conditions,which provided quantitative basis for regulating the soil-forming process of purple soil under artificial conditions.The primary conclusions as follows:?1?Morphological characteristics of purple soilBy observing morphology of soil profiles in the field and describing their morphological characteristics,the results showed that:there was a significant correlation between soil samples and parent materials,and soils developed in different slope positions showed different morphological characteristics under micro-topographic conditions.From the top to the valley,the configuration of purple soil profiles changed from A-C type to A-B-C type,and the soil thickness increased significantly with the average values of 16.5 cm,35.5 cm,62.1 cm and 93.6 cm.The soils thickness of the valley were more than 100cm.Purple soils inherited from its parent rock.Soil hue was mainly 2.5YR,a few were 10R and 5YR,chroma ranged from 3 to 8,and value ranged from 4 to 6.Soil textures were mostly sandy loam,loam and clay loam and soil structures were mostly block structure,thereby there was little difference in soil texture and structure.Only a few illuvial horizons of profiles in the foot and the valley occurred a small amount of rust spots,manganese film and mucilage film.In addition,a few intrusions such as bricks and debris were found in plough horizons due to artificial cultivation.The degree of soil profiles development were analyzed from the perspective of soil redness rating?RR?:from the top to the valley,the average value of RR was 12.43,11.32,10.60,7.10 and 7.97.RR index among different horizons was C horizon>A horizon>B horizon,which indicated that soil redness rating at the foot of the slope was the lowest.RR index was negatively correlated with slope position,but soil profile development index?PDI?was positively correlated with slope position.The average value of PDI was 6.22,15.03,26.57,41.87 and 45.54 from the top to the valley which showed the degree of soil profiles development increased gradually,while the horizon index?HI?of soil profiles development in different slope positions was B horizon>C horizon>A horizon.RR had some limitations in indicating the soil age and soil development,while PDI had a better result in indicating the development degree of purple soil.Therefore,the morphological development of purple soil profiles were greatly influenced by topography and the degree of soil morphological development in the foot and the valley were relatively high.?2?Physicochemical characteristics of purple soilBy determining and analyzing physical and chemical characteristics of soil profiles and comparing their differences,the results showed that:the physical and chemical characteristics of soil profiles varied with the change of slope positions,especially between soils above the waist?top,shoulder and waist?and below the waist?foot and valley?.Soil bulk density ranged from 1.21 g/cm³ to 1.89 g/cm³,and porosity ranged from 25.59%to 52.48%.The bulk density increased with the decrease of slope position and the increase of soil depth,while the porosity was the opposite.The sand content was 43.70%,42.66%,40.98%,33.12%,24.59%,the silt content was 43.68%,41.75%,40.04%,39.70%,38.88%,and the clay content was 12.63%,15.59%,18.98%,27.17%,36.52%,respectively,thereby the soil particles were gradually refined from the top to the valley.The ratio of silt to clay was top>shoulder>waist>foot>valley,which the minimum value of 1.07 occurred in the valley with the higher degree of soil development.Therefore,soil particle composition had an impact on the process of soil formation and clay content was an important variable affecting the determinants of soil characteristics.The content of>0.25 mm water-stable aggregates in the top was the least with average value of 48.52%and that in the waist was the most with average value of 56.65%.Percentage of aggregate destruction showed top>shoulder>valley>foot>waist,that is,the soil agglomeration in the waist was relatively good.From the view of soil chemical properties,the pH of soils ranged 7.9 to 8.7,all of which were calcareous soils.Due to topographic effect and disturbance of artificial tillage,the pH of soils decreased by 0.2 to 0.4 relative to C horizon.From the top to the valley,the average content of soil organic matter was 5.62 g/kg,5.63 g/kg,8.31 g/kg,13.71 g/kg and 18.39 g/kg,and there were significant differences in soil organic matter among the foot,valley and above the waist.The content of soil organic matter in different horizons was B horizon>A horizon>C horizon which indicated that organic matter accumulated in illuvial horizon of the relatively low slope position.The contents of soil nutrient elements such as total nitrogen,total phosphorus,total potassium and cation exchange capacity were valley>foot>waist>shoulder>top,which nutrient accumulated in the foot and the valley.It reflected the results of high intensity of anthropogenic activities at relatively low topographic locations and strong soil erosion in hilly regions.?3?Geochemistry characteristics of purple soilThe geochemical characteristics of soils differed under different slope positions and horizons.The results showed that:the content of SiO₂ ranged from 50.00%to 54.62%,Al₂O₃ ranged from 13.47%to16.55%,TFe₂O₃ ranged from 5.48%to 6.99%,MgO ranged from 2.20%to 2.87%,CaO ranged from 4.80%to 8.11%,Na₂O ranged from 0.59%to 0.87%,K₂O ranged from 2.54%to 3.27%,Ti₂O ranged from 0.60%to 0.69%,MnO ranged from 0.08%to 0.12%and P₂O₅ ranged from 0.14%to 0.20%.The enrichment of Al and Fe and the leaching of Ca and Na existed in A horizon and B horizon of soil profiles in the foot and the valley,but the leaching of elements in purple soil was not strong.From the view of soil chemical weathering index,the CIA value was 71.78,73.17,72.51,74.55 and 74.50,the CIW value was 84.47,86.33,85.57,88.00 and 87.87,and the Na/K ratios was 0.44,0.38,0.40,0.33 and 0.34.The degree of chemical weathering was foot>valley>waist>shoulder>top and the CIA values and the CIW values in B horizons were significantly higher than those in A horizons and C horizons.In the A-CN-K triangular chart,the trend of soil chemical weathering was roughly parallel to the A-CN connection and illite and montmorillonite were still the main mineral components.The degree of chemical weathering was relatively weak during the whole process of soil formation,which indicated purple soil was at a moderate level of chemical weathering.From the perspective of soil element migration,the migration of geochemical elements in different topographic locations was different.For example,the average migration coefficients of Ca and Na at the top,the waist and the foo changed from positive to negative and the migration direction changed from enrichment to leaching,while Al,Fe and Mg elements changed from leaching to enrichment.However,the migration intensity of soil elements relative to Ti was weak,except the migration intensity of Ca in the foot was significantly higher than that of other elements.In summary,because of the good drainage and water shortage in the upper and middle positions of the hills,the weathering materials were transported and lot under the action of gravity and surface runoff,which often brought about the explosion of rocks,thus the degree of soils development were low.On the contrary,because of the abundant water conditions in flat terrain such as the foot or the valley of the hills,the chemical weathering occurred in long-term contact between water and soils and sediments,which results in a higher degree of soil development.The development of purple soil under micro-topographic conditions was not only affected by parent material,but also significantly related to topographic effect.Soil erosion not only refined soil particles,strengthened physical weathering,but also promoted chemical weathering of purple soil in hilly regions.