[A-HW22] 流域の物質輸送と栄養塩循環－源流域から沿岸海域まで－

[AHW22-09] ベトナム中部の野菜畑における浅層地下水のアンモニア態窒素汚染

★招待講演

*前田 守弘¹、藤村 拓也¹、Hoang Ngoc Tuong Van ²、井上 大介³、近森 秀高¹、兵藤 不二夫¹ (1.岡山大学、2.フエ大学、3.大阪大学)

キーワード：アンモニア態窒素、異化型硝酸還元アンモニウム、窒素無機化

Nitrogen contamination in shallow groundwater occurs in a coastal area in Central Vietnam. In agricultural fields close to the residential area, high concentrations of NH₄⁺ and NO₃⁻ in groundwater was attributed to unintended inflow of livestock wastes and/or domestic wastewater. On the other hand, NH₄⁺ concentration reaches to several milligrams N per liter even in vegetable fields away from the residential area. The objective of this study was to determine NH₄⁺ contamination mechanisms of the shallow groundwater without domestic wastewater inflow. In this study, we tested three hypotheses: (i) NH₄⁺ derived from fertilizer or manure applied to the fields was transported to groundwater thorough soil, (ii) nitrate (NO₃^-) produced as a result of nitrification in topsoil was transferred to deep soil and again transformed to NH₄⁺, or (iii) nitrogen mineralization occurred in deep soil. Soil samples were collected up to 240 cm from a vegetable production field in Quang Thang Commune, Central Vietnam in June (dry season) and November (rainy season) of 2016 and 2017. The groundwater table existed at 57 or 67 cm in June and 33 cm from the soil surface in November. Concentrations of NH₄⁺ in PVC pipe wells fluctuated between 1 and 3 mg N L^-1 in accordance with the detection of soil NH₄⁺ content (5-20 mg kg^-1) in the 150-240 cm layer. On the other hand, soil NH₄⁺ was not detected in the 30-80 cm layer whereas it was 2-6 mg kg^-1 in the 0-10 cm layers, indicating that NH₄⁺ was not transferred to layers deeper than 30 cm. The absence of a functional gene of dissimilatory nitrate reduction to NH₄⁺ (DNRA) in the 150-240 cm soil layer shows that DNRA did not occur in this deep layer. Natural abundance values of ¹⁵N (δ¹⁵N) for soil NH₄⁺ (+3.6 to 5.5‰) were similar to those in soil total N (+1.9 to 3.6‰) in the 150-240 cm layer. In addition, anaerobically incubated soil from the 140-160 cm layer was mineralized by 1-2 mg NH₄⁺-N kg^-1 for 8 weeks. These experimental results suggest that the high concentration of NH₄⁺ found in wells was originated from mineralization of deep soil.