[H-TT21] Development and applications of environmental traceability methods

[HTT21-07] Natural abundance of ¹⁵N in Japanese forest soils

*Keisuke Koba¹, Bin Hiratsuka², Rieko Urakawa³, Kazuya Nishina⁶, Kazuo Isobe⁷, Hideaki Shibata⁴, Nobuhito Ohte⁵ (1.Center for Ecological Research, Kyoto University, 2.Faculty of Agriculture, Tokyo University of Agriculture and Technology, 3.Asia Center for Air Pollution Research, 4.Field Science Center for Northern Biosphere, Hokkaido University, 5.Graduate School of Informatics, Kyoto University, 6.National Institute for Environmental Studies, 7.Graduate School of Agricultural and Life Sciences, The University of Tokyo)

Keywords:Stable isotopes, Forest soils, Nitrogen dynamics

Natural abundance of ¹⁵N of bulk soil N has been investigated in many studies with emphasis on soil N decomposition, N loss, and chronosequence of N dynamics in forests. δ¹⁵N of bulk soil N generally increases with soil depths, which is considered as a consequence of the loss of ¹⁵N-delpleted N via leaching and gaseous loss both coupled with N mineralization and nitrification as well as the incorporation of ¹⁵N-enriched N into bulk soil N via microbial biomass. However, the details on how ¹⁵N of bulk soil N can be discriminated during these processes are not clear.
We analyzed the δ¹⁵N of bulk soil N collected in the GRENE (Green Network of Excellence) environmental information project and the ReSIN (Regional and comparative Soil Incubation study on Nitrogen dynamics in forest ecosystems) project (Urakawa et al. 2014, 2016). Mineral forest soils were collected in each watershed from five soil profiles with different soil depths down to 40 or 50cm depth. We used a modified EA-IRMS in TUAT with higher sensitivity to measure the δ¹⁵N of bulk soil N with low N concentrations. We analyzed the soil samples (488 samples from 32 watersheds) with higher N concentration than ca. 0.1% (with less than 8mg sample weight to run) to reduce the risk of incomplete combustion.
δ¹⁵N of bulk soil N ranged from -3.2 to +10.2 permill with N concentrations ranging from 0.1 to 1.3%. Isotopic fractionation factors for bulk soil N, estimated from the relationship between N concentrations and δ¹⁵N values, ranged from 0.6 to 7.2 permill. The differences in mycorrhizal association of plants would influence the isotopic fractionation factor (Hobbie and Ouimette 2009), although we found no significant relationships between mycorrhizal associations (ECM, AM) and the isotopic fractionation factors. Climate factors such as MAT and MAP suggested as the factors affecting δ¹⁵N of bulk soil N (Amundson et al. 2003) were not significantly correlated with δ¹⁵N of bulk soil N. In the presentation we will discuss the possible factors influencing δ¹⁵N of bulk soil N and isotopic fractionation factors in the forest watersheds.