Effects Of Macropore On Soil Water Movement And Microplastics Pollution In On Typical Farmland And Woodland In Loess Region

Soil macropores and macropore flow commonly exist in cropland,which provides a priority path and driven force for the percolation of soil water and the transport of solute and pollutants.Vegetation coverage,field management,and soil animal activity are the main environmental factors for soil macropore development.Therefore,it is necessary to explore the development characteristics,soil hydraulic properties,macropore flow,and pollutant migration,which would benefit for the water use efficiency enhancement,macropore flow forecast,and pollution risk assessment.Considering the lack of the studies relating to macropore flow forecast and farmland soil microplastic pollution issues,this study chose four types of typical farmlands to investigate.In order to analyze the three-dimensional structures of macropores,the CT and photo analyze technology was applied;pedo-transfer function was built for forecasting soil water conductivities through applying machine learning algorithm;the characteristics of soil microplastics transporting and redistribution,and their effects on macropore flow and hydraulic parameters,were discussed.The main results obtained from the present study are as follows:(1)Four typical vegetation types were chosen and they are wheat cultivated field,WH;apple orchard,AT;kiwifruit orchard,KF;and Aesculus chinensis woodland,AC.The morphology and type of soil pores within the cultivated soil(0-37 cm)were recognized through CT scanning and three-dimensional reconstruction of soil pores.And the two-dimensional and three-dimensional characteristics of macropores were quantitatively studied.Results showed that for the four treatments,soil macroporosity decreased with the increase in soil depths.Besides,the distribution of macropores in the wheat field was punctate,with lower connectivity than that in the apple and kiwi fields.The macroporosity in the buckeye field was the lowest but the connectivity was better.The macropores in the apple and kiwi fields were regular and punctate with good connectivity,which was caused by earthworms according to the morphology and distribution characteristics of soil pores.Furthermore,the total surface area,network distribution density,length density,and node density of soil macropores were the highest in the apple field but the lowest in the buckeye field.Statistical analysis indicated that there were no significant differences in the average volume,average curvature,inclination,and fractal dimensional of macropores among the four treatments,but there were significant differences in total surface area,network distribution density,length density,and node density(p<0.05).(2)Some hydraulic parameters,such as infiltration rate,cumulative infiltration,saturated conductivity,soil water retention curve,horizontal diffusivity,unsaturated conductivity,were measured in the laboratory.Results showed that for the four treatments,the values of soil water hydraulic parameters for 20-40 cm depths were less than for 0-20 cm depth.Besides,the saturated conductivity,cumulative infiltration,infiltration rate,and diffusion rate were the highest for the AT treatment.And under the same suction condition,the trends of soil moisture and macroporosity were consistent among the four treatments.Furthermore,the order of magnitude of water capacity and unsaturated conductivity differed in different suction stages among the four treatments.When the suction was less than 500 cm,the changes of the water capacity and unsaturated conductivity with bulk density were opposite,which were consistent with the soil water retention curve and diffusion rate.(3)The basic parameters,such as bulk density,organic matter content,particle composition,were selected as input variables to construct the pedo-transfer function by using machine learning algorithms Gaussian process regression and Bootstrapping method(n=100).The obtained soil conversion functions were adopted to predict the saturated conductivity(K_s)and conductivity(K₁₀)under specific suction(-10 cm).Results showed that the pedo-transfer function had high prediction accuracy.And on this basis,the parameter K_j(Log(K_j)=Log(K_S/K₁₀))was introduced as the parameter to characterize the macropore flow probability.The trend of Kj among the four treatments was consistent with macroporosity and hydraulic parameters.This parameter had potential in predicting the probability of soil macropore flow.(4)The effects of earthworm quantity and earthworm incubation time on macropore quantity and distribution characteristics were quantified by using CT scanning technology in laboratory column experiments.Results showed that under the condition of the same earthworm number,the soil macroporosity and connectivity increased with the increase in the time of earthworm culture from 7d to 28d;under the same time of earthworm culture,the macropore amount and distribution density increased with the increase in earthworm number from 1 to 4.The bio disturbance generated by earthworms can rapidly and stably produce soil macropore networks and further change soil structure and soil water transport characteristics.(5)The distribution of soil microplastic driven by earthworm macropores was measured in laboratory column experiments,and their impacts on soil saturated conductivity and breakthrough curves were further explored.Results showed that earthworm activity led to microplastic accumulation in 40-50 cm depths.And the microplastics in the soil surface entered groundwater,increasing the risk of groundwater pollution.Moreover,earthworms'activities were thought of as the key factor impacting microplastics re-distribution along with soil columns.Besides,the distribution of microplastics was affected by earthworm activity.The microplastics with a particle size<250?m were easily leached into deeper soils.When there were no earthworms in the soils,the microplastics in the soil surface could not be transported by water flow,even though sandy soil has more micropores than other soil types.Furthermore,a low concentration of microplastics had no significant effects on water flow in macropores.The relative arrival time of 5%tracer material was positively correlated with the total volume of earthworm pores.The activity of earthworm was the dominant factor affecting the penetration curve,and the low concentration of soil microplastics had no significant effect on soil saturated water conductivity.