Influence of Watershed Landscape Structure on Nitrate and Phosphate Exports: An Island-wide Scale Assessment in Subtropical Mountainous Taiwan

Abstract:

Landscape composition and configuration have a more substantial effect on nutrient export of humid mountainous watersheds. The relationships between landscape metrics and NO₃^- and PO₄^3- fluxes are developed for 43 watersheds island-wide Taiwan, annually and seasonally during 2015–2016. The results reveal that the watersheds with higher concentrations and fluxes of NO₃^- and PO₄^3- mainly distributed on western Taiwan whereas watersheds with lower concentrations and fluxes of NO₃^- and PO₄^3- scattered across mountain regions and eastern Taiwan. Multiple stepwise regression models are used to disentangle the effects of landscape pattern at watershed level and riparian buffers of various width (100-2000 m) on NO₃^- and PO₄^3- exports. Our findings show that the annual exports of NO₃^- and PO₄^3- cross various watersheds are 16.8-225.0 and 1.3-249.8 kg ha^-1 yr^-1 respectively, which are much greater than reports worldwide. The class-level metrics IJI (interspersion and juxtaposition) of agricultural land and LPI (largest patch index) of buildup area derived from 200 m buffer can explain 80% of variation in NO₃^- flux, and ED (edge of density) of buildup area and LPI or percentage of water body (PLAND_WAT) at 200 m buffer combined account for 90% of variation in PO₄^3- exports. Therefore, management practices at the suggested riparian buffer such as reducing fragmented agriculture and buildup area will help to moderate water pollution, and better designs of developed area and water body can further benefit to water quality.