The Characteristics Of Preferential Flow Pathways And Its Effect On Nitrate Nitrogen Migration In Farmland Soil At Changping District In Beijing

Agricultural soils have got much attention in the procedure of development, utilization, improvement and protection as an important foundation for agricultural production. Irrigation and fertilization are two significant measures in the farming process. Because of the existence of the soil preferential flow paths, water and solute infiltrate to deeper soil quickly, which may not only result in the loss of water and nutrients but also cause groundwater pollution.Agricultural soils are the main object in the study. Dyeing tracer method was used to judge the preferential flow occurrence in the soils. Based on the judgment, water breakthrough curves and Photoshop software were used to make quantitative analysis on the number and distribution of soil macropores on the agricultural land respectively in this study. In order to specify the relationship between macropores and the influential factors that may have effects on the formation of macropores, some potential effecting factors were elected including soil texture, aggregates content, organic content and root density. Canonical correlation analysis was used to study the relationship between these factors and macropore proportions. With the use of40cm soil columns, the principles of soil water and solute migration under the influence of preferential flow in the agricultural soils were studied in the condition of a simulated heavy rain. The results of the research were as followed:Farming measures had a great impact on agricultural soils at Changping district. Preferential flow paths in soils with were concentrated around the large cavities with irregular distribution. The radii of macropores in the soils were mainly massing between0.5～2.8mm. The macropores showed a larger amount in upper layers and a smaller amount in lower layers on the vertical characteristics, and over98%of these macropores were less than1.5mm in radius. Silt particle content, aggregates content and root density are the main factors affecting the formation of preferential flow paths. In terms of soil texture, silt particle content had a significant negative correlation with macropores content. Aggregates content had a significant positive correlation with macropores content, and the connection between them mostly depended on the content of organic matter, which plays an important role in the cementation. Root density had a very significant positive correlation with macropores content, the impact of root density mainly concentrated on mechanical forces which root made to soil, and the ring-shaped channel around roots that contribute to macropore content immediately.The water flow rate with preferential flow in soil appeared larger degree of volatility, and was1.48～2.69times of that of balancing infiltration. The breakthrough time of undisturbed soil column which preferential flow exist in was12h with a57%reduction compared to the packed one. The preferential flow of NO3-in undisturbed soil column accounted for43.83%of total flux, but resulted to a97.60%accumulative lechate mass for NO3-in total mass. These results indicated that water could penetrate the soil fast under the promotion of soil preferential flow paths, resulting in the phenomenon of water and solute transport priority. In the process, nitrate nitrogen migrated in higher rates and greater numbers and water flowed with greater velocity and volatility.