Nitrate leaching and its susceptibility in response to elevated nitrogen deposition in Japanese forests

Understanding the variability in nitrogen (N) leaching from forested ecosystems and its susceptibility to elevated atmospheric N deposition is essential for assessing the impact of atmospheric N deposition on forested ecosystems. This study explored the site variability in nitrate (NO₃⁻) leaching from forested areas and its susceptibility to elevated atmospheric N deposition from five sites in broadleaved forests grown on Andosols and Cambisols from northern to southern Japan. Site variability of NO₃⁻ leaching and its possible contributing factors, including soil parent material and net nitrification rate, were evaluated using a generalized linear model with Akaike’s Information Criterion used for model selection. Three-year experimental N fertilization (50 kg N ha⁻¹ yr⁻¹ as ammonium nitrate) experiments were conducted at each site to evaluate the susceptibility of NO₃⁻ leaching to elevated atmospheric N deposition. Soil solution NO₃⁻ concentration below the rooting zone (50 cm below the ground surface) was variable among the five sites and was the highest in Shibecha, one of the Andosols sites in northern Japan where soil net nitrification tended to be higher than elsewhere. In model selection for NO₃⁻ leaching, the soil parent material was not selected but soil net nitrification was selected, indicating that soil net nitrification could explain the variability of NO₃⁻ leaching. A significant increase in NO₃^– leaching after the 3-year N fertilization experiment was observed at the site with the highest levels of NO₃^– leaching (Shibecha), indicating that the susceptibility of NO₃^– leaching is high in locations where N leaching was originally high.