Influences of topography on nitrate export from forested watersheds on Yakushima Island, a Natural World Heritage site

Abstract

In East Asia, high levels of atmospheric nitrogen are deposited onto land. This could elevate the nitrate levels in coastal waters via river runoff, even from areas where anthropogenic sources are minimal. It is important to identify NO₃⁻ sources in river water and the mechanisms involved in NO₃⁻ runoff. Yakushima Island, Japan, is a Natural World Heritage site featuring numerous watersheds with diverse topography and rivers. The area receives significant precipitation, with up to 10 000 mm in mountainous regions. Its proximity to coastal urban areas in China (∼800 km) leads to substantial atmospheric nitrogen wet and dry deposition in the island's forests. The study aimed to clarify regional water quality characteristics by conducting long-term monitoring of dissolved ion components (Na⁺, K⁺, Mg²⁺, Ca²⁺, F⁻, Cl⁻, NO₃⁻, and SO₄²⁻) in river waters, and to determine the effects of NO₃⁻ sources and watershed topography on NO₃⁻ behavior. Dissolved ion concentrations were obtained from a long-term monitoring (2011–2014) dataset. Cluster analysis classified runoff water from the central mountainous region into three groups: western region, other regions, and groundwater. The average NO₃⁻ concentration in the western region was 10.2 μmol L⁻¹, which was higher than the 6.24 μmol L⁻¹ observed in the other regions. Stable isotope analysis in December 2018 showed that river water NO₃⁻ (1.39 μmol L⁻¹) in the western region had a high proportion of atmospheric NO₃⁻. Topographic analysis indicated that NO₃⁻ and atmospheric NO₃⁻ increased in smaller watersheds and steeper terrain. This study conclude that NO₃⁻ output is controlled by topography.