| Soil compaction caused by agricultural field vehicles can significantly affect soil physical and mechanical properties,soil structure,crop growth and yield,agricultural ecological environment,which has become one of the obstacles for sustainable agriculture.In view of the high soil water content,single cropping pattern,frequent soil tillage and high degree of mechanization in rice-wheat rotation system in southern China,the mechanized production of rice cropping system is more likely to lead to soil compaction.The establishment of evaluation and prevention technology system of paddy soil compaction is an effective measure to avoid the problem of soil compaction.There are many technologies,quantitative indices and compaction models for quantifying and evaluating soil compaction,but their application scope and limitations are obvious.Using multi-index soil compaction evaluation method can make up for the deficiency of different evaluation indices,and can comprehensively reflect the comprehensive information of soil compaction.Therefore,the establishment of a multi-index evaluation system of paddy soil compaction is the key to accurately evaluate the compaction of paddy soil,which has important guiding significance for formulating the field operation trafficability of agricultural machinery and the chassis design of related working machines.On the basis of systematically combing the existing research results of soil compaction,this paper focuses on three technical categories of soil compaction quantification and evaluation:the first is soil physical and mechanical index method,which uses physical and mechanical indicators to quantitatively evaluate the soil state before and after compaction;the second is soil compression method,which uses Gompertz function to analyze the data obtained from compression test;the third is soil fragmentation method,which is to describe,quantify and evaluate the compaction of paddy soil by establishing the functional relationship between the compactness of paddy soil and the crushing mechanical indices,the appearance shape indices and the mechanical indices of soil structures.The specific research work of this paper is summarized as follows:1.The soil physical and mechanical indices(water content,bulk density,porosity,soil penetration resistance,soil micro-relief)were established for the investigation of regional soil mechanical compaction,and this method was used to investigate the compaction status of harvesters in rice-wheat rotation area.A field survey toolbox for parameterization of agricultural system attributes was presented,which can be used to quantify the effects of crop seasons,soil physical properties,machinery specifications and farming practices on soil compaction caused by harvesters.The results of regional survey showed that the small harvester significantly compacted the paddy soil under the rice-wheat rotation system,and the mechanized harvesting increased the soil penetration resistance and reduced the soil porosity,but these changes only occurred in the topsoil layer,which indicated that the small harvesters had a good effect on the structure protection of the subsoil layer.The small harvesters seriously damaged the soil micro-relief of paddy field.The severity of harvester-induced soil damage in paddy harvesting was higher than in wheat harvesting.However,soil micro-relief damage caused by small harvester field operation was unavoidable.Therefore,it is necessary to improve management strategies to reduce the ratio of soil compaction area and reduce the disturbance of soil micro-relief.Local soil management strategies,such as pre-harvesting drainage,ultra-narrow wheels,floating chassis and puddling,were also investigated and these management strategies were identified effective to ameliorate or rehabilitate soil compaction caused by harvester.2.The compression characteristics of remolded paddy soil were studied by means of soil compression test.Twenty-five remolded soil samples were prepared to conduct uniaxial compression experiments.The compression data were fitted by a Gompertz function.The values of swelling index,compression index and pre-compression stress of remolded paddy soil under larger initial conditions were obtained.The effects of water content and bulk density on swelling index,compression index and pre-compression stress,and the relationship between compression characteristics and bulk density and water content were analyzed.The results showed that the swelling index values of remolded paddy soil ranged from 0.003 to 0.138,the compression index values of remolded paddy soil ranged from 0.115 to 0.839,and the pre-compression stress values of remolded paddy soil ranged from 33 to 127 kPa when the water content of remolded paddy soil was between 15%and 35%,and the bulk density was between 1.1 g/cm 3 and 1.5 g/cm3.The compression index of remolded paddy soil reaches the maximum value when the water content is 25%.The swelling index showed a significant positive dependence on water content,while bulk density was negative.A quadratic polynomial function was established between water content and compression index;the compression index showed a significant negative correlation with bulk density.The pre-compression stress showed a prominent negative correlation with water content and a significant positive correlation with bulk density.Based on the data obtained,the pedo-transfer functions between compression characteristics,water content and bulk density of remolded paddy soil were established.Combined with soil investigation data(water content and bulk density),pedo-transfer functions can be used to predict the compression characteristics of paddy soil in a large range.Meanwhile,the soil compression characteristics predicted by pedo-transfer functions can also be used as the input parameters of existing soil compaction models to evaluate the compaction risk of field operation of agricultural machinery.3.The compression characteristics of undisturbed paddy soil were also studied by means of soil compression test.Uniaxial compression experiments were performed on undisturbed soil samples,which were taken from four depths in the field and adjusted to five different water content in the laboratory.The compression data were fitted by a Gompertz function.The values of swelling index,compression index and pre-compression stress of undisturbed paddy soil under larger initial conditions were obtained.The effects of water content and bulk density on swelling index,compression index and pre-compression stress,and the relationship between compression characteristics and bulk density and water content were analyzed.The results showed that when the water content of undisturbed paddy soil was between 15%and 35%,and the bulk density was between 1.12 g/cm3 and 1.83 g/cm3,the swelling index values of undisturbed paddy soil ranged from 0.018 to 0.080,which was smaller than the swelling index of remolded soil;the compression index values of remolded paddy soil ranged from 0.07 to 0.23,which was smaller than the compression index of remolded soil;the pre-compression stress values of remolded paddy soil ranged from 58 to 140 kPa,which was higher than the pre-compression stress of remolded soil.Under the same water content and bulk density,the compaction sensitivity of undisturbed paddy soil is lower than that of remolded paddy soil,and the bearing capacity of undisturbed paddy soil is stronger than that of remolded paddy soil.The compression index of undisturbed paddy soil reaches the maximum value when the water content is 25%,the same as remolded paddy soil.The correlation between the compression characteristics of undisturbed paddy soil(swelling index,compression index,pre-compression stresses)and water content and bulk density is consistent with that of remolded soil.The pedo-transfer functions compression characteristics of undisturbed paddy soil is basically consistent with that of remolded paddy soil.4.The relationship between compaction degree and fragmentation characteristics of paddy soil was studied by means of soil fragmentation test.The field experiment set up five different applied stresses and one control group.Soil compaction test system was used to conduct four different compaction times in situ on paddy soil.Soil samples of 0-10 cm and 10-20 cm soil layers were collected and then taken to the laboratory.The water content of soil samples were adjusted to 0.9 WPL,and the soil samples were then fragmented.The effects of applied stresses and compaction times on mean weight diameter,specific surface area and fractal dimensions were analyzed.The experimental results prove that mean weight diameter of paddy soil in 0-10 cm and 10-20 cm soil layers increases with the increasing applied stresses and compaction times;while specific surface area and fractal dimensions of paddy soil in 0-10 cm and 10-20 cm soil layers decreases with the increasing applied stresses and compaction times.Mean weight diameter,specific surface area and fractal dimensions of paddy soil in 0-10 cm soil layer were significantly affected by the applied stresses and compaction times when applied stress is greater than 150 kPa.Mean weight diameter,specific surface area and fractal dimensions of paddy soil in 10-20 cm soil layer were not significantly affected by the applied stresses and compaction times compared with 0-10 cm soil layer.Mean weight diameter,specific surface area and fractal dimensions of paddy soil in 10-20 cm soil layer were significantly affected when the applied stress is greater than 150 kPa and compaction time is greater than six times.Mean weight diameter,specific surface area and fractal dimensions are three soil fragmentation indices that can reflect the different well among different compaction degree.Soil fragmentation method is a simple and easy-to-use method to quantify and evaluate soil compaction.It provides a new method and three quantitative indicators for the study of soil compaction.In this paper,the concept of soil fragmentation curve was proposed.The soil fragmentation curve can describe the soil fragmentation behavior well under different applied stresses.Based on the soil fragmentation curve,the concept of "pre-compression stress of soil structure"was proposed.The results show that the "pre-compression stress of soil structure" was significantly higher than pre-compression stress.From the perspective of soil fragmentation to formulate the protection criteria of soil structure,it is not accurate to use pre-compression stress to represent soil bearing capacity,but it is more accurate to use "pre-compression stress of soil structure" as soil bearing capacity.5.The effect of compaction degree on the appearance geometric parameters and mechanical properties of soil structure were studied by means of digital image method and indirect tensile test.The experimental results prove that rectangularity of paddy soil in 0-10 cm soil layer decreases with the increasing size classes of soil structure;while form index and angularity of paddy soil in 0-10 cm soil layer increases with the increasing size classes of soil structure.The rectangularity of paddy soil in 0-10 cm soil layer decreases with the increasing applied stresses and compaction times,while form index and angularity of paddy soil in 0-10 cm soil layer increases with the increasing applied stresses and compaction times.The rectangularity,form index and angularity of paddy soil in 0-10 cm soil layer were only significantly affected by the applied stresses and compaction times when applied stress is greater than 200 kPa and compaction timeis more than six times.The rectangularity,form index and angularity of paddy soil in 10-20 cm soil layer were not significantly affected by the applied stresses and compaction times.The experimental results prove that tensile strength and specific rupture energy of paddy soil in 0-10 cm and 10-20 cm soil layers decreases with the increasing size classes of soil structure.The tensile strength and specific rupture energy of paddy soil in 0-10 cm and 10-20 cm soil layers decreases with the increasing applied stresses and compaction times.The tensile strength and specific rupture energy of paddy soil in 0-10 cm soil layer were significantly affected by the applied stresses and compaction times,and the effect becomes more and more obvious with increasing size classes of soil structure.The tensile strength and specific rupture energy of 16-32 mm size class of paddy soil in 10-20 cm soil layer were only significantly affected by the applied stresses and compaction times when applied stress is greater than 200 kPa and compaction time is more than six times,which means significantly compaction occurs in 10-20 cm soil layer.Friability index ky and ke are negatively correlated with applied stress of paddy soil in 0-10 cm and 10-20 cm soil layers under four compaction times.Friability index ky is positively correlated with friability index ke of paddy soil in 0-10 cm and 10-20 cm soil layers under four compaction times,and friability index ke is 1.18 to 1.50 greater than friability index kY.The mechanical properties of soil structure,i.e.,tensile strength and specific rupture energy,are more sensitive to applied stresses and compaction times than appearance geometric parameters of soil structure,i.e.,rectangularity,form index and angularity.Therefore,the mechanical indices of soil structure obtained by indirect tensile test are more accurate than the appearance geometric indices of soil structure obtained by digital image method. |
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