Stream Water Nitrogen Eutrophication During Non-Irrigated Periods in a Paddy-Dominated Agricultural Basin in a Snowfall Area in Japan

We measured the concentrations of dissolved inorganic nitrogen (DIN; nitrate, ammonium, and nitrite) in stream water in a paddy-dominated agricultural basin in a snowfall area from August 2009 to October 2010 to facilitate evaluation of stream water eutrophication from nitrogen during the non-irrigated period. We compared the nitrogen budget in a paddy field between irrigated and non-irrigated periods, from information about nitrogen fertilizer, denitrification, harvested rice, and atmospheric nitrogen deposition. We also estimated stream nitrogen exports from DIN concentrations and stream flow rates. DIN concentrations in stream water were higher during the non-irrigated period (October–March) than during the irrigated period (April–September). Stream flow was also higher during the non-irrigated period (5.9 mm day⁻¹) than during the irrigated period (2.5 mm day⁻¹), which possibly reflects snow melting. Although nitrogen fertilizer was applied during the irrigated period, the amount of nitrogen removed by the rice harvest and denitrification was sufficiently large to reduce nitrogen exports from paddy fields. Atmospheric nitrogen deposition was higher during the non-irrigated period (755 kg N km⁻²) than during the irrigated period (410 kg N km⁻²). DIN exports were also higher in the non-irrigated period (860 kg N km⁻²) than in the irrigated period (120 kg N km⁻²). The higher exports in the non-irrigated period may reflect the lack of nitrogen removal by a rice harvest and denitrification and increased runoff and higher atmospheric nitrogen deposition. Our study highlights the important contribution of the non-irrigated period to nitrogen eutrophication in stream water in this particular environment.