Numerical Simulation Of Soil Nitrogen And Phosphorus Circling In Peri-Urban Vegetable Farming Systems

Along with urbanized advancement speeding up and with the expansion of the city scale, the function of the soil in peri-urban area has a series of changes, fruit and vegetables are grown instead of grain primarily. For one thing, the non-point source pollution in the field increased; for another, the rubbish and the waste water produced in the city area also discharged into the soil in peri-urban area, massive nutrients that came from the leaching of fertilizer and pesticide and heavy metals were concluded in the contaminations. So the soil and the water environment pollution of the peri-urban area are inevitably occurred, then affects food quality and the water quality and causes the threat to the human health. Therefore, the studies for the soil environment in peri-urban area has the vital significance for the agriculture sustainable development, for the protection of the pere-urban environment, for the improvements of the humanity quality of life and guarantee the continuing development of the entire state economy.GLEAMS(Groundwater Loading Effects of Agricultural Management Systems) is a computer model used to simulate the nutrient loss and evaluate the non-point source pollution in the field, and it's developed by scientists of U.S. Department of Agriculture. This paper introduced the elements, the configuration, the application condition and the development history of the model. It also introduced its application in America, East Europe, Vietnam and the middle and China. Then, the prospect of the modal application on the non-point source pollution problems of China was analyzed. Now the model is used to simulate circling of soil nitrogen and phosphorus in the peri-urban areas of Nanjing. Results showed during the period from July 2003 to July 2005, nitrogen input reached 1289 kg.hm^-2, of which 360 kg.hm^-2 came from fertilization and 899 came from animal waste, the rest 31 kg.hm^-2 were from rainfall and irrigation, nitrogen output was composed of leaching 484 kg.hm^-2, runoff 33 kg.hm^-2, crop uptake 269 kg.hm^-2, volatilization 178kg.hm^-2 and denitrification 263 kg.hm^-2, thus forming a surplus of 76 kg.hm^-2 left in the soil as residue. On such a basis, nitrogen recovery ratio was figured out to be merely 20.8%～21.6%, the phosphorus uptake ratio was 15.5%～17.0%. The results indicate that the agricultural system is rather low in fertilizer use efficiency, but causes much pollution to the groundwater.Besides, based on several scenarios, the model was used to simulate the influence of reducing fertilization and irrigation to the N and P uptake ratio. Results showed when the organic fertilization reduced to 45.2%～53.4% and the chemical fertilization reduced to 13.3%～96.9%, the N and P uptake ratio increase 9.9%～14.4% and 9.60%～13.1% separately, the N leaching decreased 26.5%～37.6% and the cost of fertilization would be saved 1040～2200Yuan hm^-2. The results also showed the NO₃^—N leaching reduced along with the reduction of irrigation, although the concentration of NO₃^—N increased. The findings of the study may serve as scientific basis for exploration of optimal fertilization recommendation and ways to improve the eco-environment in the region.