Influence of landscape mosaic structure on nitrate and phosphate discharges: An island-wide assessment in subtropical mountainous Taiwan

Abstract

Landscape composition and configuration, which runoff flows through, have substantial effects on nutrient export of humid mountainous watersheds. The relationships between landscape metrics and annual and seasonal NO3-N and PO4-P fluxes are developed for 43 watersheds distributed island-wide in Taiwan during 2015–2016. The results show that the watersheds with higher concentrations of NO3-N and PO4-P mainly distributed on western Taiwan whereas watersheds with lower concentrations of NO3-N and PO4-P scattered across mountain regions and eastern Taiwan. Multiple stepwise regression models are used to realize the effects of landscape pattern on NO3-N and PO4-P exports at watershed level and riparian buffers of various width (100 m, 200 m, 500 m, 1000 m, 1500 m, and 2000 m). Our findings show that the average annual exports of NO3-N and PO4-P across various watersheds are 3.8–50.8 and 0.04–7.7 kg ha−1 yr−1, respectively, much higher than reported values worldwide. The class-level metrics IJI (interspersion and juxtaposition) of agricultural land and LPI (largest patch index) of buildup area obtained from 200 m buffer explain 85% of variation of NO3-N flux, and ED (edge of density) of buildup area and LPI of water body or percentage of water body at 200 m buffer combined contribute 90% of variation in PO4-P exports. The highly mixed development of urban and agriculture distributed along with the limited extent of riparian buffers within watersheds in Taiwan make the watershed susceptible to loss of a large quantity of nutrients and sediments into streams. Management practices such as diminishing fragmented agriculture and buildup area are recommended as they should help to reduce water pollution, and better designs of developed area and water body can further improve water quality.