Recent advances of stable isotope analysis providing indicators for nitrogen saturation of forest watersheds

KOBA, Keisuke* (Tokyo Univ Agri Tech), FANG, Yunting (CAS)

In this paper, we introduce the applications of stable isotopes (15N and 18O) to explore N flows in a forest ecosystem. Using the denitrifier method to constrain δ18O values for nitrate produced from nitrification in 9 temperate forest soils in central Japan, we found that δ18O values of nitrate produced from nitrification may have been overestimated in many previous studies, indicating that the contribution of anthropogenic N input like fertilizer and atmospheric deposition to a given ecosystems and the progress of denitrification in N removal are larger than we thought. The δ18O values for nitrate in stream water from three N-saturated forested watersheds showed unexpected small seasonal fluctuations, but were caused by different mechanisms with large contribution of nitrification in the watershed in southern China whereas large nitrate removal of denitrification in the two watersheds in central Japan. This new analytical methods can also allow us to measure δ15N of total dissolved N, and we present some preliminary data from forests in Japan and China that can imply the importance of gaseous N loss from the forest ecosystem.