Simulation Study on Migration and Transformation of Nitrogen in Layered Soil under Film-hole Irrigation

Layered soil is a common soil structure in nature, and film-hole irrigation is a new type of water-saving irrigation technology． Method of combining experiment and numerical simulation was used to explore migration and transformation law of nitrogen in free infiltration of fertilizing liquid under film-hole of layered soil． Three kinds of sand inter-layer positions were set up in experiment, and the HYDRUS-3D model was used to simulate and predict nitrogen transport and transformation process of layered soil under film-hole irrigation and fertilizer solution, and verified． According to simplified calculation method of film-hole infiltration, film-hole infiltration was divided into vertical one-dimensional infiltration and film-hole lateral infiltration． Influence of position of sand inter-layer was greater, and it increased with increase of burial depth of sand inter-layer, while position of sand inter-layer had little effect on vertical one-dimensional infiltration and infiltration reduction． Shape of wetting front of layered soil infiltration was significantly different from hemisphere or semi-ellipsoid of homogeneous soil, and there was obvious discontinuity at soil-sand interface． With increase of redistribution days, urea content in soil wet body showed a decreasing trend as a whole． In redistribution stage, ammonium nitrogen near center of membrane hole was larger than that at wetting front, and gradually decreased with distance from center of membrane hole． Ammonium nitrogen was mainly distributed in soil layer above sand layer, and in soil-sand interface content increased significantly． With time increased, nitrate nitrogen content at the same position increased． Nitrate nitrogen content at horizontal wetting front increased faster than that near center of film hole, and content was larger at soil-sand interface． In process of urea conversion and migration, measured value of urea nitrogen content was in good agreement with simulated value, indicating that selection of nitrogen chain reaction parameters was more reasonable, and model simulation effect was better． This study could provide a theoretical and methodological basis for simulating and predicting transport and transformation laws of nitrogen in free-flowing fertilization solution under condition of layered soil．