[A-HW28_30PM2] Water and material transport and cycle in watersheds: from headwater to coastal area

[AHW28-23] Rapid procedure for d15N and d18O determination and identifying nitrate sources in agricultural watershed

*Saeko YADA¹, Yasuhiro NAKAJIMA¹, Hikaru UNO¹, Sunao ITAHASHI¹, Kei ASADA¹, Seiko YOSHIKAWA¹, Sadao EGUCHI¹ (1.National Institute for Agro-Environmental Sciences)

Keywords:agriculture, irrigation, IRMS, nitrate, stable isotope, watershed

The natural abundance of nitrogen (δ¹⁵N) and oxygen isotopes (δ¹⁸O) of nitrate (NO₃‾) can be a powerful tool to discriminate the source of NO₃‾ in agricultural watersheds. This dual isotopic approach has been used successfully to evaluate the denitrification process in an upland vegetable-field dominant watershed. Recently, determination of δ¹⁵N and δ¹⁸O of NO₃‾ has been updated using a autosampler for automatic analysis. In this study, we developed a further time-saving procedure, which advanced the time-event efficiency by controlling sample traps and 6-port valves. Moreover, the procedure was used to identify sources of riverwater NO₃‾ in a rice paddy watershed in Tsukuba, Japan, where the irrigationwater was supplied from out of the watershed.The δ¹⁵N and δ¹⁸O values were determined by isotope-ratio mass spectrometry (IRMS) after converting NO₃‾ to N₂O gas using the denitrifier method. We conducted sample purge and determination at the same time. Our developed procedure doubled the sample throughput, saving the amount of He carrier gas and liquid N₂. It also engaged the versatile utility of IRMS, since changeover of equipment was not required. This rapid procedure can be applied to other trace gas analysis, which require cryofocus e.g. CO₂, and will contribute for GWG dynamics studies.Using the developed procedure, we identified principal sources of NO₃‾ in mainstream riverwater of the watershed. The δ¹⁵N—δ¹⁸O relationship during irrigation period indicated that NO₃‾ in mainstream riverwater were mainly provided from mountainstream and irrigationwater, and that significant effects of denitrification on the decrease in NO₃‾ concentration were locally limited at some irrigationwaters and drainagewaters in the watershed.