A Sequential Diffusion Method for 15N Natural Abundance Measurement of Ammonium, Nitrate and Total Dissolved Nitrogen in Water Samples

Haruka Kiba1, Fujio Hyodo2, Yuichi Asano3, Morihiro Maeda1.

1 Okayama University, Graduate School of Environmental and Life Science, 3-1-1 Tsushima-Naka, Okayama 700-8530, Japan

E-mail: [email protected]

2 Okayama University, Research Core for Interdisciplinary Science, 3-1-1 Tsushima-Naka, Okayama 700-8530, Japan

3Oyo Corporation, 2-907 Seko-Higashi, Moriyama-Ku, Nagoya 463-8541, Japan

Abstract 

Natural abundance of nitrogen-15 (δ15N) is a useful tool to estimate sources of nitrogen (N). The objective of the study was to clarify practical conditions of a sequential diffusion-based method for 15N natural abundance of inorganic N and total dissolved N (TDN) with relatively high concentration. We tested the sequential diffusion method at 40ºC for 24 hours for recovery of NH4-N and NO3-N (0–40 mg L-1) and for that of TDN (0–4 mg L-1). Results showed that the complete recovery was achieved for inorganic N with 0.3–30 mg L-1 and for TDN 0–3 mg L-1. Furthermore, recovery rates for TDN declined when the amount of N exceeded 120 μg N. The time required for N recovery can be shortened to 24 hours by increasing temperature to 40ºC. No discrimination of 15N occurred during the whole process under the above conditions. In conclusion, the sequential diffusion method for 15N natural abundance measurement can be applied to water samples including 0.2–20 mg L-1 for NH4-N or NO3-N, and 0.25–3 mg L-1 for TDN. The volume for TDN recovery must be adjusted so that amount of TDN in solution is less than 90 μg N.