Monthly change and mass balance of nitrogen in river water and spring water in the Shira river basin

Abstract

The aim of this study is to investigate the monthly variation of nitrogen concentration and nitrogen mass balance in the Shira river basin, Kumamoto. The average concentration of total nitrogen was 1.11±0.34 mg L⁻¹ (n = 205), ammonium was 0.084±0.12 mg L⁻¹ (n = 238), nitrate nitrogen was 0.0071±0.010 mg L⁻¹ (n = 236) and nitrite nitrogen was 0.65±0.31 mg L⁻¹ (n = 241). The concentrations of total nitrogen and nitrate nitrogen in the river tended to be low from June to July during the rainy season. However, the amount of nitrogen increased from June and July, and then was relatively stable from August to the next May. The amount of nitrogen in the river was estimated to be about 62±32 t/month at St.8 which is located in downstream. The input of total nitrogen in the basin was estimated at about 3010 t N, and the runoff ratio to the river was about 0.66. As a result of mass balance analysis for the agricultural land in the basin, excessive nitrogen was estimated at about 66 kg N ha⁻¹ year⁻¹.