Water Balance and Nitrogen Dynamics in Paddy Field Planted with Forage Rice

Abstract

We aimed to evaluate the impacts of the water environment and nitrogen removal on paddy fields planted with forage rice in the research field of the Prefectural University of Kumamoto. Water balance (inflow: irrigation and precipitation; outflow: surface runoff, evapotranspiration, and underground seepage) and nitrogen dynamics (inflow: irrigation, precipitation, and fertilizer; outflow: surface runoff and underground seepage; others: plants and uncollected fractions of nitrogen) were investigated for two years (2019 and 2020) in paddy fields planted with forage rice. We found that most of the nitrogen load to the water environment in paddy fields planted with forage rice was attributable to nitrogen outflow caused by groundwater seepage, and that the sums of groundwater seepage and surface runoff were 9.1 kg N ha^-1 in 2019 and 34.1 kg N ha^-1 in 2020, which are very small compared with the nitrogen inflow in paddy fields. Higher than 90% of the nitrogen inflow was removed in paddy fields planted with forage rice. In 2020, 184.8 kg N ha^-1 (about 40% of the total nitrogen inflow) was removed by denitrification and 163.1 kg N ha^-1 (about 40% of the total nitrogen inflow) by forage rice harvest.