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

Abstract

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 < 0.001) and in a considerably higher impact factor of 0.1–0.2 for the tea field. Orchard fields also affected the TN concentration in corresponding areas. An increase in the tea field area significantly contributed to an increase in the TN/DSi mole-based ratio. These results indicated that tea fields are a major source of riverine N. To maintain the TN/DSi below the critical level (TN/DSi ratio <1.0), tea fields should be cultivated only in a maximum of 30% of the watershed area, and the TN concentration should be limited to 2.5 to 4.0 mg L−1. Based on the level of precaution (TN/DSi <0.3), the TN concentration should be strictly limited to less than 1 mg L−1, which is identical to the Japanese critical standard for environmental conservation of lakes and coastal seas. On the other hand, TP concentrations were almost always below 0.1 mg L−1 and the TP/DSi ratios were considerably lower than the critical level (TP/DSi <0.07). Excess N load in the tea-cultivating watersheds might accelerate the P shortage in aquatic ecosystems. N concentration should be reduced to prevent future problems. In most of our mountainous tea-cultivating watersheds, 1 mg L−1 of TN concentration could be achieved, and generally a minimum of 2.5 to 4.0 mg L−1 should be achieved. To reduce the riverine TN concentration, the fertilizer N rate should be reduced by using modern practices to enhance N fertilizer use efficiency, such as slow-released coated urea combined with cultivar selection, without producing inferior quality tea leaves.