Effects of subsurface flow and paddy fields on water and nutrient dynamics in the Nasu alluvial fan II. Nutrient dynamics

Abstract

We measured nutrient (N, P, K) and Cl concentrations in surface water for 3 years and 4 months in 23 instrumented river basins in and surrounding the Nasu alluvial fan. The procedures that calculate nutrient budget were built into the water dynamics model documented in Part I of this paper, and the dynamics of nutrient transport in the Nasu alluvial fan were interpreted. The calculated annual nutrient discharges from the individual basins were linearly related to measured values, with correlation coefficients ranging from 0.84 to 0.98 and regression line slopes ranging from 0.6 to 1.4. The predicted annual changes in nutrient concentrations in surface flow downstream of the whole study area were similar to the measured values. The annual N load from livestock and fish farms in the Nasu alluvial fan was estimated to be 36 kg ha⁻¹ y⁻¹, accounting for 62％ of the total input of 58 kg ha⁻¹ y⁻¹. The annual level of denitrification in paddy fields was estimated at 19.8 kg ha⁻¹ y⁻¹. The largest source of primary Cl input was mineral fertilizer at 125 kg ha⁻¹ y⁻¹, equivalent to 65％ of the total input of 191 kg ha⁻¹ y⁻¹. Nutrient concentrations in surface water tended to be lower in summer as a result of higher precipitation. However, in basins in the middle–lower areas, the N concentration of surface water increased in the summer–fall season, presumably due to increased input from subsurface flows containing high N concentrations. The discharge rates of nutrients from the whole study area were N 51％, P 6％, K 93％, and Cl 91％ of the annual load (resulting from processes such as organic binding, adsorption, and denitrification), and of the total, approximately 60％ was carried by subsurface flow.