Increased nitrogen deposition contributes to plant biodiversity loss in Japan: Insights from long-term historical monitoring data

Atmospheric deposition of reactive nitrogen compounds (Nrs) has been recognized as a threat to plant diversity in terrestrial ecosystems. As a first attempt to investigate the relationship between Nrs deposition and plant diversity loss in Japan, we collected and analyzed the available long-term nationwide monitoring data on annual Nrs deposition and plant (tree) species, and evaluated the relationship between Nrs deposition and plant species loss at corresponding sites. Analyses of the available data showed that the amount of Nrs deposited annually tended to decrease at two monitoring sites (Yusuhara, Hedomisaki) and increase at six monitoring sites across Japan (Rishiri, Sadoseki, Tokyo, Aichi, Oki, and Ogasawara) during the late 1980s to 2011, especially at Aichi (11.8-21.6 kgN·ha^-1·yr^-1), Tokyo (10.0-23.5 kgN·ha^-1·yr^-1), Oki (6.63-14.1 kgN·ha^-1·yr^-1), and Rishiri (4.52-7.82 kgN·ha^-1·yr^-1). Another long-term study, the Monitoring Sites 1000 Project, investigated the growth of tree species at 20 core sites across Japan during 2004-2012. The sites with higher potential plant diversity loss were close to those sites where Nrs deposition had markedly increased over the 20 years, such as Tokyo, Aichi, and Oki. Analyses of long-term monitoring data for tree species in the Tokyo University Forest in Aichi revealed that 22 of the 273 tree species (8.05%) disappeared during the period of 1990-2010, and twelve out of the 22 lost species were shrub species less than 5 m tall. Although our study obviously has some limitations in quantitatively presenting the relationship between the loss of plant diversity and increased atmospheric Nrs deposition in Japan, our findings provide evidence for this relationship based on analyses of historical nationwide monitoring data. These findings will be useful for establishing N critical loads for Japanese forests.