Simulation equations for soil nitrogen and phosphorus transfer andcrop nutrition in the EPIC model

EPIC(Erosion-Productivity Impact Calculator, now it is renamed as Environmental Policy-Integrated Climate Model)is a world famous comprehensive dynamic model developed by US. The major components in EPIC are weather simulation, hydrology, erosion-sedimentation, nutrient cycling, pesticide fate, plant growth, soil temperature, tillage, economics, and plant environment control. It can be used to assess the effect of management strategies on field soil and water resources and crop productivity. Because the systematical and functional simulation model related to processes of soil nitrogen and phosphorus transfer and transform and crop nutrition has not been developed in China yet, the principles and mathematical equations used for simulating soil nitrogen and phosphorus transfer and crop nutrition in the EPIC model were briefly introduced in this paper. The nutrient cycling process is affected by biological factors, such as crop and soil microbe, and weather factors, such as temperature and rainfall, and also soil management factors, such as fertilization, irrigation and soil tillage, hence, soil nitrogen and phosphorus spatially migrate in the soil profile and transform into (different) forms ceaselessly. More than 80 mathematical equations are used in the EPIC model to describe the detail (dynamic) processes of soil nitrogen and phosphorus mineralization and immobilization, nitrate and soluble P loss in surface runoff, organic N and P transported by sediment in soil erosion, nitration, detrification, NO3--N leaching and transported by soil water evaporation, ammonia volatilization, mineral P cycling among labile, active mineral, and stable mineral pool, and crop N and P demands and soil supplies, nitrogen fixation of legume etc. The flow rates and amounts of soil nitrogen and phosphorus migration and transformation and crop nutrition are simulated continuously with a daily time step for one to hundred years, and the simulated results can be outputted with daily, weekly, decade, monthly or annually (interval). It can be used to analyze the dynamic principle of soil nitrogen and phosphorus transfer and transform in soil profile and crop nutrition in crop production. This paper can be used as reference for field soil management and crop (nutrition) quantitative assessment research.