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Origin and chemical properties of dissolved humic substances in Okitsu river water(Summaries of Researches Using AMS)

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  • 興津川河川水中の溶存腐植物質の起源および構造特性(タンデトロン加速器質量分析計業績報告2004(平成16)年度)

Abstract

Dissolved humic substances (DHS) in river water function as carrier of trace bioelements. Although land use change in the basin of a river, such as deforestation, has been considered to affect the ecosystem at estuary and coastal area through changes in the quantity and existing form or structural property of bioelements, DHS, and their complexes supplied from soil to river, it has not been proved yet. In the present study, possible origin of DHS in Okitsu River (Shizuoka) was investigated to estimate the contribution of major land uses there, secondary forest, tea garden, and rice field, as a server of DHS. The DHS were collected at three sites by adsorbing them to Amberlite XAD-8 under acidic conditions. Mineral materials attached to DHS were removed by shaking them with 0.3 M HF at room temperature, and finally powder samples of DHS in the H^+-form were obtained. The yield, ^<13>C nuclear magnetic resonance (NMR) spectra with cross polarization/magic angle spinning method (CPMAS), stable C isotopic ratio (δ^<13>C), and ^<14>C age were measured. Whereas the dissolved organic C (DOC) concentrations showed seasonal variations from 0.4 to 1.9 mg C L^<-1> without significant difference between the sampling sites, the concentration of DHS was constant between 0.09〜0.13 mg C L^<-1> at all the sites except for the forest in October (0.03 mg C L^<-1>). Although the proportions of alkyl (33〜43%) and aromatic C (29〜36%) in total C estimated from ^<13>C CPMAS NMR spectra were larger than those of O-alkyl (13〜17%) and carbonyl C (12〜18%) as common characteristics of Okitsu river DHS, the higher alkyl C content in DHS at the forest site than at the other sites suggested the replacement of DHS between forest and tea garden areas. The δ^<13>C of DHS was-27.4〜-28.0‰ at the forest site, which was lower than that at paddy field site (-26.8〜-26.9‰). The DHS at the tea garden site showed similar δ^<13>C to those at the forest site in January and April, whilst the values in July and October were close to δ^<13>C of DHS at paddy field site. The similarity in δ^<13>C between DHS and water-soluble HS in the topsoil especially at the rice field site suggested that the major origin of DHS is WSHS eluted from coastal soil. The ^<14>C age of DHS at the tea garden site was -940 yBP and those at the forest and paddy field sites were -190〜-205 yBP, which indicated the recent generation of DHS and denied their coming after long-time percolation through soil layers. Since the generation of DHS in Okitsu River may be minor because of shallow depth (30 cm) and the covering of the bottom with rocks and stones, the top layer of soils could be the major origin of DHS. The variation in ^<14>C age also indicated the replacement of DHS.

タンデトロン加速器質量分析計業績報告 Summaries of Researches Using AMS 2004 (平成16)年度

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