Is rice cultivation sink or source of nitrogen for a Japanese river

Abstract The balance of nitrate nitrogen (NO3–N) in the Ohno River basin on Kyushu, southwest of Japan was investigated based on observed flow discharges and NO3–N concentrations in river water over the period from 1969 to 2018 (half a century). The dependence of NO3–N load on flow discharge at Shirataki in the river was quantified for the cultivation season (May–September) and the fallow period (October–April) in order to evaluate the impact of rice cultivation on the water quality in the river. Measured results in the 50 year period show that NO3–N loads for the cultivation season were smaller than those for the fallow period for a given flow discharge which was less than 100 m3/s. This indicates that the rice cultivation contributes to the decrease in the NO3–N load in the effluent water discharged from paddies which is 18% of the whole basin to the river. Rice cultivations in a watershed can be a source of nutrients induced by excess fertilizer application or a sink because nutrients in inundating water can be assimilated by rice plants to aid their growth. A model presented in this study incorporates the management of water and fertilizer in the paddies, and describes the balance of NO3–N in the basin. The model assumes that rice plants assimilate nitrogen in the soil resulting from fertilization. This occurs preferentially when it is sufficient and, when the rice plants themselves utilize nitrogen in the inundated water after the nitrogen in the soil is depleted. The model was found to substantially reproduce measured NO3–N loads in the Ohno River for the cultivation season in the period (1969–2018). This suggests that the difference in the NO3–N loads for a given flow discharge between the cultivation season and the fallow period at Shirataki in the river was induced through the uptake of nitrogen by rice plants.