Impact of rice cultivation to concentrations of silica and major inorganic ions in river water

Abstract

While river water flows from mountainous region to flatlands, the concentration of dissolved inorganic components in river water changes due to the influence of the basin. In this study, we investigated the concentrations of dissolved silica and inorganic ions in small rivers in Hirakawa City, Aomori Prefecture, and their seasonal changes. Dissolved silica decreased significantly within the short period after planting rice, and then increased in concentration but fluctuated at a fairly lower concentration than that of irrigation water.There became no concentration difference in irrigation water and paddy water by the rice harvest season.From the time series change, it was found that the silica absorption by the rice was most active in the early stages of rice growth, when paddy water silica was depleted.The growing rice continued to use silica in irrigation water, but paddy water silica was not be depleted. In the rice agriculture area, dissolved silica concentration in small rivers used for paddy field irrigation showed significant decrease in downstream area than that in the upstream area, indicating the influence of rice cultivation on the river water quality.