Analysis of nitrogen load to the environment based on nitrogen flow in food supply system of Japan

On the hypothesis that a crop uptakes a certain rate of available nitrogen in the soil, we developed a model for the analysis of the N (nitrogen) flow in the national food supply system combined with the soil N budget of the cropland in Japan. Using the model, the amount of N applied to soil in the recycled use of organic waste from the home and industry sector (recycled N) for crop nutrition was calculated to be 366, 418, 490, and 482 in Gg N for the years 1982, 1985, 1992, and 1997, respectively. The outflow of N from cropland was calculated as 491, 513, 499, and 458 in Gg N for those respective years, showing a little decrease in 1997 owing to the rapid reduction in the application of N by chemical fertilizer (chemical N). The total estimated load to the environment by the national food supply system was 1823, 1981, 1941, and 1862 in Gg N for the investigated years, which reflects the decrease of the outflow from cropland. In the case of all the food and foodstuff being imported, the total load to the environment reaches a minimum in the calculation without N outflow from the cropland. We found that by increasing the rate of recycled N in the total N supply for crop production (chemical N+recycled N) to the level of 68-69%, the total load to the environment reaches the minimum even if domestic crop production is sustained at the same level as in each investigated year. It gives a goal for environmentally friendly soil fertilization in modern Japanese agriculture. In the case of decreasing 10% of chemical N by increasing the rate of crop uptake of N from 0.5 to 0.52-0.53, the total load to the environment is lowered by 2.5-3.5% compared with the investigated value of each year. In the case of decreasing the same amount of chemical N and replacing it by increasing the use of recycled N, the total load to the environment is lowered by 3.5% each year in the same comparison. Increasing the domestic crop production, e.g. rice planting for silage in the vacant paddy field, can lower the load to the environment if the nutrition for increased crop production is supplied by the additive use of recycled N.