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.

明确森林生态系统氮（N）淋溶的变异特征及其对大气氮沉降增加的响应敏感性，是评估大气氮沉降对森林生态系统影响的关键前提。本研究以日本南北梯度上5个分别发育于火山灰土（Andosols）与始成土（Cambisols）的阔叶林样地为研究对象，探究了林地硝酸盐（NO₃⁻）淋溶的样间变异特征，及其对大气氮沉降增加的响应敏感性。本研究采用以赤池信息准则（Akaike’s Information Criterion）进行模型筛选的广义线性模型（generalized linear model），对NO₃⁻淋溶的样间变异及其潜在影响因子（包括土壤母质与净硝化速率）进行了评估。各试验样地均开展了为期3年的氮添加试验（以硝酸铵为氮源，施用量为50 kg N·ha⁻¹·yr⁻¹），以此明确NO₃⁻淋溶对大气氮沉降增加的响应敏感性。根系层以下（地表下50 cm处）的土壤溶液NO₃⁻浓度在5个样地间存在显著差异，其中位于日本北部的火山灰土样地标茶（Shibecha）浓度最高，该样地的土壤净硝化速率整体高于其他样地。在NO₃⁻淋溶的模型筛选过程中，土壤母质未被纳入最优模型，而土壤净硝化速率则被选中，这表明土壤净硝化速率可以解释NO₃⁻淋溶的空间变异特征。为期3年的氮添加试验结束后，原本NO₃⁻淋溶水平最高的标茶样地出现了NO₃⁻淋溶量的显著提升，这表明原本氮淋溶水平较高的区域，其NO₃⁻淋溶对大气氮沉降增加的响应敏感性也更强。