Modelling analysis of nitrogen removal from paddy water with high infiltration rate

An understanding of nitrogen processes in a paddy field, characterised by large water flux for irrigation and outflows under continuous irrigation, is required to manage adequate nitrogen inputs and outputs. This study identifies the effect of large water flux, especially high infiltration rate, on nitrogen processes in a paddy field under continuous irrigation. The developed nitrogen process model in this study was applied to two paddy fields having different infiltration rates (216 and 106 mm day- 1 on average), and simulated physicochemical and biological nitrogen processes in ponded water, soil water and soil, including whole water flows as well as organic, and inorganic nitrogen forms. In each field, irrigation was found to be the major nitrogen input (153.2-461.5 kg N ha- 1 year- 1), and nitrogen outflow (65.2-284.3 kg N ha- 1 year- 1) found to be smaller than the input from irrigation. The irrigation water was primarily contaminated by dissolved organic nitrogen (DON) and nitrate. Nitrogen transportation from ponded water to soil water was four times greater under high infiltration condition than under low infiltration condition. High nitrogen transport to the soil layer increased air emission via denitrification and decreased outflows. In particular, DON and ammonium transported to soil water are sources of nitrite and nitrate, and denitrification was five times higher under high infiltration than low infiltration. The results of this study imply that paddy fields with high infiltration rates have a greater possibility of nitrogen removal from paddy water, rather than being a pollutant source for the water environment.