| Soil is the foundation of agricultural production, is the base for plant breeding and crop production, to provide nutrients for the plants, water conservation, and stability and buffer environmental change, while also regulating climate through the soil carbon pool. In pervious studies of soil, climate, topography, parent material, biology, and time are considered to be the five factors of soil formation. Pedogenic process is a series of materials transformation and migration process of parent material, which is products of geological process, under the effect of climate and biological factors, and also is a slow process. With the development of the industry, the increase of population and decrease in arable land, artificial cultivation intensity is increasing and intensive agriculture has become an important mode of modern agriculture. The influence of human effects on the soil is more and more profound. Sichuan basin is located in the southwest of our country, is the famous red bed basin. As for the subtropical monsoon climate and fertile purple soil, this region has become an important agricultural region of western China. At present, amounts of natural purple soil were developed for agricultural soil, and the development of large-scale farming became the trend of agricultural development. However, it is paid not enough attention to the effect of human activities on the purple soil formation. Taken of soil developed from purple parent material or sedimentary parent material of purple soil in Sichuan basin under natural conditions, conventional tillage conditions, and intensive tillage conditions as subject, the features of soil profile development, chemical weathering, and physical and chemical properties were studied, and then discussed the effect of human activities on the its pedogenic process, to provide a scientific basis for the sustainable utilization and transformation of purple soil in hilly region.1. Pedogenetic features under natural conditionsProfile development characteristics, chemical weathering, soil physical and chemical characteristics of different purple parent material and its developed soil under natural conditions was observed. The results showed as follow:The chemical weathering of purple soil under natural conditions was relatively weak and the status of leaching elements is at the initial stage of Ca, Na and Mg. The CIA, CIW and ratio of Na/K had no obviously difference of among soil layers. The distribution trend of data in A-CN-K triangular figure was parallel with A-CN.The purple soil was in the stage of plagioclase weathering and Ca and Na leaching, and the hierarchical data point distribution has no significant difference. As the progress of soil weathering, the gravel content became less, clay content and grain fractal dimension was increased, and soil particle became finer. The organic carbon content increased significantly, with the accumulation about9.27g-kg-1in the A layer and3.65g-kg-1in the B layer. The pH decreased a little, about0.2to0.3units. The N content was increased, with the accumulation about0.46g-kg-1in the A layer and0.19g-kg-1in the B layer. The leaching of P was also observed, about0.071g-kg-1in B layer.As the different chemical weathering degree of the sedimentary rock, the original weathering degree of purple parent material was difference, soil element composition, pH value and particle distribution were also different. These had important effect on the purple soil and weaken the effect of other factors on pedogesis. The differences of topography, climate and biology also had influence on other chemical properties of soil. Compare to the soil in shallow hill or hill top, the depth of soil in platform was larger, the accumulation of organic carbon and N was more, the loss of P was less, and the index of soil profile development was higher. As for the difference of climate and biology, soil depth, soil organic carbon, N, and P of soil in platform also were different.2. Pedogenetic features under conventional tillageProfile development characteristics, chemical weathering, and soil physical and chemical characteristics of soil that developed from parent material of J2s or sedimentary parent material of purple soil under conventional tillage conditions in Sichuan basin was observed, and then compared with that under natural conditions. The results showed as follow:Under conventional tillage conditions, the soil developed from J2s was in the stage of plagioclase weathering and Ca, Na and Mg leaching. Compared with soil under natural conditions, the tillage of dryland farming promoted soil chemical weathering. Under dryland tillage conditions, the CIA, CIW and ratio of Na/K had significant changes when compared with that of parent material. The intensity of chemical weathering under paddy tillage conditions also enhanced, the loss of soil elements was more, and the CIA and CIW was higher than that under dryland fanning conditions. Ratio of Na/K was less. Because of the effect of water in paddy, the oxidation-reduction of Fe and Mn was generally, for the W and G layer in almost soil layer, obviously Mn loss and Fe accumulation. Compared to the soil under natural conditions, the clay content and particle fractal dimension of soil under conventional tillage conditions was increased, with the increased order of dryland and paddy. The soil acidification was serious under conventional tillage conditions, and was more serious in surface soil under paddy tillage conditions. The accumulation of organic carbon and N was weakening under conventional tillage conditions. But the P content was enriched in surface soil, as for the fertilization of exogenous phosphorus. In paddy soil, the soil density decreased, organic carbon and N content increased, available nitrogen content enriched and soil aggregates status improved. The loss of P was promoted and the available P decreased. Under conventional tillage conditions, the parent material also had influence on soil pedogensis, topography and sampling point also affect the soil pedogensis by impacted on human activities and soil erosion. There were different of soil chemical weathering degree, soil texture, aggregates, soil organic carbon, total N, total P, available N, available P, available K, and CEC between soils developed from J2s purple rock and sedimentary parent material of purple soil. In half-slope of shallow hill, the human activities were intensive. The chemical weathering degree, clay content, particle fractal dimension value, and the content of soil organic carbon, total N, available N, and available K were highest in that region, and the P and available P content were least. Because the denudation and erosion was intensive in dryland, the chemical weathering degree and soil chemical and physical properties of paddy soil in the border on dryland and paddy was between that of dryland and paddy. And also for the soil denudation, transport, accumulate and bury, the soil chemical weathering degree and texture of B layer that in valleys or slope toe were similar with that of A layer, and soil organic carbon content, total N, total P and available were higher than that of B layer in half slope.3. Pedogenetic features under intensive tillageThrough comparing profile characteristics, chemical weathering, physical and chemical properties of dry land and paddy field before and after land consolidation which developed from purple parent material and deposition purple parent material in3intensive tillage agricultural regions (just finished land consolidation,7months’intensive farming after land consolidation and19months’ intensive farming after land consolidation), agricultural intensive farming disturbed soil and thickened soil layer artificially. Men artificially mixed soil to A layer in paddy land leading to increase the degree of development of A layer of soil. After land consolidation of dry land, chemical weathering, clay content and texture fractal dimension of A layer increased, soil gravel content increased, soil density decreased, the reunion situation was slightly worse, other physical and chemical properties of soil formation were within topsoil of dry land and paddy field before finishing their consolidations. There was a significant increase in soil gravel content of B layer, other soil formation properties changed little. Since paddy topsoil was mixed with the topsoil of dry land, the degree of development is artificially lowered. Due to anthropogenic mixed upland soil and parent rock blasting disintegrating from B layer of dry land, W layer became P layer, whose degree of retention of soil fertility reduced, degree of soil development have also been artificially lowered. After the land consolidation of paddy field, chemical weathering, clay content and texture fractal dimension of A layer and P layer decreased, soil gravel content increased, density slightly increased. Changes of soil chemical weathering, clay content, texture fractal dimension content of the gravel of A layer of A layer was less than P layer. Soil aggregates situation of A layer deteriorated significantly, other physical and chemical properties of soil formation in surface soil were similar to dry land and paddy field. Soil aggregates situation of P layer significantly became worse. Because there was less different physical and chemical soil formation characteristics between B layer of dry land and W layer of paddy field, there was little change before and after land consolidation. In addition, with rock blast into gravel and soil ridge covered in soil and other higher parts of the soil handled and covered, lm profile layer of G layer profile of paddy field in most profiles was not appeared. Overall, due to land consolidation progresses, soil has undergone significant agricultural intensification disturbance, increased the developing index of dry land profile, and reduced the developing index of paddy field within lm.Through comparing profile characteristics, chemical weathering, physical and chemical properties of dry land and paddy field developed from purple parent material and deposition purple parent material in3intensive tillage agricultural regions (just finished land consolidation,7months’ intensive farming after land consolidation and19months’intensive farming after land consolidation), agricultural intensive farming promoted the content soil organic carbon, total nitrogen, total phosphorus, nitrogen, phosphorus, potassium, and also contributed to the soil aggregate fonnation and accumulation of soil inorganic carbon. But these3regions are in weathered plagioclase and Ca, Na, Mg leaching stage. Within the limit time of agricultural intensive farming, soil chemical weathering and texture condition did not show significant changes in the study area.To sum up, the status and feature of soil-forming have different characteristics under natural conditions, traditional farming conditions and intensive fanning conditions in hilly region with purple soil. Under natural conditions, soil-forming factors as parent material, topography, climate, biology, etc. have different effects on soil characteristics in different aspects of purple soil Conventional tillage concludes different fertilization, tillage, irrigation practices which contribute to different soil profile characteristics, chemical weathering and physical chemical properties. Human activities have accelerated the chemical weathering of soil developed from purple parent material, promoted the change of soil texture and soil acidification, and also brought other changes in soil chemical properties of soil formation. And this effect influenced topography and other factors to some extent. In the progress of intensive farming, intensive disturbances significantly affect the characteristics of soil formation, change the structure of paddy soil, increase the developing degree of soil profile of dry land, but destruct soil aggregates. Intensive farming has significantly promoted the accumulation of soil carbon, nitrogen, phosphorus and aggregates conditions, improve the content of soil nitrogen, phosphorus and potassium. But within the time frame of intensive farming in study area, chemical weathering and soil texture and pH did not show significant changes. While tillage can accelerate soil chemical weathering, all in all, natural soil, soil under conventional tillage and soil under intensive farming conditions developed from purple parent material and deposition purple parent material were in plagioclase weathering and Ca, Na, Mg leaching phase, the overall degree of chemical weathering is still low. |
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