[M-IS21_28PO1] Biogeochemistry

[MIS21-P04] Sources of hydroxyl radical photochemically produced in headwater streams from nitrogen-saturated forest

*Masaaki CHIWA¹, Naoko HIGASHI¹, Kyoichi OTSUKI¹, Hiroki KODAMA², Tohru MIYAJIMA², Kazuhiko TAKEDA³, Hiroshi SAKUGAWA³ (1.Kyushu University Forest, 2.Graduate School of Science and Engineering, Saga University, 3.Graduate School of Biosphere Science, Hiroshima University)

Keywords:hydroxyl radical, dissolved organic matter, nitrate, photo-Fenton reaction, stream, photoinduced processes

Hydroxyl radical (●OH) is the most oxidative reactant among the active oxygen species and oxidation reactions with ●OH are involved in important biogeochemical processes. In this study ●OH photoformation rate (R_OH) was determined in headwater stream samples from nitrogen (N)-saturated forests, 1) to quantify the sources of ●OH in headwater streams and 2) to evaluate the nitrate (NO₃^-)-induced enhancement of ●OH formation in stream water caused by N saturation in forested watersheds. Stream water fulvic acid extracted from the forested watersheds was used to quantify the contribution of dissolved organic matter (DOM) to R_OH. The results showed that almost all (97%; 81-109%) R_OH sources in our headwater stream samples were quantitatively elucidated; the photolysis of NO3- (55%; 34-75%), nitrite [N(III)] (2%; 0.5-5.2%), and DOM-derived ●OH formation, from which photo-Fenton reactions (18%; 12-26%) and the direct photolysis of fluorescent dissolved organic matter (FDOM) (22%; 10-40%), was successfully separated. FDOM, which accounted for 53% (24-96%) of DOM in total organic carbon bases, was responsible for ●OH formation in our headwater streams. High NO₃^- leaching caused by N saturation in forested watersheds increased R_OH in the headwaters, indicating that N-saturated forest could significantly change photoinduced and biogeochemical processes via enhanced ●OH formation in downstream water.