[A-HW22] Materials transport and nutrient cycles in watersheds; from headwaters to coastal seas

[AHW22-09] Ammonium contamination of shallow groundwater beneath vegetable fields in Central Vietnam

★Invited Papers

*Maeda Morihiro¹, Takuya Fujimura¹, Van Ngoc Tuong Hoang², Daisuke Inoue³, Hidetaka Chikamori¹, Fujio Hyodo¹ (1.Okayama University, 2.Hue University, 3.Osaka University)

Keywords:ammonium , dissimilatory nitrate reduction to ammonium, nitrogen mineralization

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.