Evolution Characteristics Of Fluvo-aquic Soil Structure Under Different Fertilization Measures

Soil is a porous mixed medium,and its pore structure profoundly influences and determines the basic properties and functions of soil.As a product of geological processes,soil pore structure has randomicity,complexity,and autocorrelation in spatial distribution.In addition,the actual soil structure is susceptible to changes in external environmental conditions(especially high-intensity agricultural production).Therefore,related research on the variation of soil pore structure under the background of different farmland utilization patterns has always been the focus and hotspot of soil physics research.However,limited by technical development,traditional methods were generally based on the “black-box principle” that is using the image nature parameters of soil porosity,aperture distribution,pipe moisture rate to build an equivalent structure system for research.In recent years,CT,a non-destructive porous medium research technique,has been used in soil structure-related research.Compared with the traditional soil structure research method,the main advantage of CT technology is that it can obtain the soil pore structure parameters without damage,and on this basis,carry out the three-dimensional reconstruction using digital image technology.The soil pore structure established according to this method can not only describe the geometric and topological characteristics of soil pore structure in spatial distribution intuitively and accurately but also help explain and quantitatively describe the specific change process of soil pore structure in different land-use modes.Therefore,this paper took the fluvo-aquic soil that widely distributed in southern Shanxi as the study object and carried out long-term different fertilization treatments(no fertilization: CK,NPK fertilizer: NPK,and organic fertilizer: OF),after that,obtained the pore structure parameters of the soil sample(soil column scale)through CT scanning,and then digital image processing technology and simulated annealing optimization theory were used to reconstruct the three-dimensional soil pore structure,to intuitively and quantitatively evaluate the evolution characteristics of the soil pore structure under long-term different fertilization modes.The main results are as follows:(1)We constructed the three-dimensional pore space structure of the three different treatments fluvo-aquic soil column level and obtained the geometric and topological characteristic structure parameters of the soil pores at each degree.(2)Changes in the basic physicochemical properties and pore structure parameters of fluvo-aquic soil:(i)After 22 years of fertilizer experiment,the soil physicochemical properties,organic carbon content,and aggregate stability were improved to varying degrees in the OF and NPK treatments,with the most significant changes in the OF treatment.From the variation of clay,silt,and sand content,fertilizer application has no significant effect on soil texture during the study period.(ii)Compared with CK and NPK treatments,after OF treatment,the number of large pores(>500 μm)in fluvo-aquic soil increased,and the number of small and medium pores(≤500 μm)decreased.Pore connectivity(the number of pore throats decreased,and the average pore throat surface area increased)and continuity(the proportion of long and narrow pores increased,and the average pore path length increased)were significantly improved.(3)The main characteristics of pore structure changes in fluvo-aquic soil: the CK treatment showed little significant variation in porosity between 5 and 10 years,while the large porosity gradually decreased and the small and medium porosity gradually increased between 10 and 22 years,with the path length mainly distributed in the range of 8-120 μm,and the pore path length,number of pore throats,surface area of pore throats and tortuosity did not show significant evolutionary differences throughout the period.Compared with CK treatment,NPK treatment favored the formation of secondary-large pores of 500-1100 μm and had little significant effect on large pores >1100 μm and small and medium pores <500μm.The path length,the number of pore throats,and the tortuosity had changed slowly between 5-22 years.The OF treatment was more likely to promote the formation of large pores that were larger than 1100 μm while reducing the number of small and medium pores and increasing maximum tortuosity and path length.This process occurred mainly between10 and 15 years of fertilizer application.