Impact of agricultural land use on N and P concentration in forest-dominated tea-cultivating watersheds

The increase in nitrogen (N) and phosphorus (P) and the decrease in dissolved silica (DSi) in aquatic environments attributed to human activities has been the focus of many studies. The resultant increases in the N/DSi and P/DSi ratios have been associated with the disturbance and deterioration of aquatic ecosystems. We investigated the impact of land use on riverine N and P as well as their ratio to DSi in 4 mountainous forest-dominated tea-cultivating areas in Shizuoka, central Japan. More than 50% of the drainage basins investigated were under forested land, while the land under tea (Camellia sinensis) fields varied from 0 to 18% and orchard fields reached up to 30% in some cases. The total nitrogen (TN) concentration was 5 mg L ⁻¹ at maximum, the majority of which was dominated by nitrate-N. An increase in the tea field area (%) within the basin resulted in significant increase in the TN concentration (p