人工濕地的氮循環和氧化亞氮排放：中型生態箱研究

Abstract

人工濕地為具有潛力的污水處理技術，然而氮污染處理過程可能產生溫室氣體 N₂O，其污染處理以及溫室氣體排放之權衡關係仍待釐清。本研究探討不同氨氮入流濃度對人工濕地氮循環機制之影響，研究目標包含：(1) 建立氮循環反應動力學模型；(2) 評估 N₂O 排放特性；(3) 以 ¹⁵N 同位素追蹤量化氮轉化路徑。 本研究設置 18 組體積約 200 L 之中型生態箱，種植蘆葦 (Phragmites australis)，配置三組人工污水的氨氮入流濃度 (2.2、22.0、100.0 mg NH₄⁺-N L⁻¹)，以 5 日水力停留時間處理入流污水，並結合 N₂O 通量連續監測與 ¹⁵N 同位素添加實驗進行分析。 動力學分析顯示，氨氮移除效率隨入流濃度增加而顯著下降，由低濃度組 97.36% 降至高濃度組 24.90%，中、高氨氮入流濃度組之硝酸氮與亞硝酸氮呈現累積 (k < 0)，顯示出硝化-脫氮作用失衡。N₂O 通量連續監測顯示，低、中、高濃度組別平均 N₂O排放通量分別為 6.18、109.83、264.11 μg N₂O-N m⁻² hr⁻¹，N₂O 通量與氨氮濃度呈高度正相關 (r = 0.74)，高濃度組約為低濃度組之 43 倍。¹⁵N 同位素添加實驗顯示，添加 ¹⁵NO₃⁻ 之低濃度組有 86.89% 經氣體途徑移除，而高濃度組僅 16.24%，且 54.12% 滯留於土壤；添加 ¹⁵NH₄⁺ 組別氣體消散比例相對穩定 (42.28 ~ 63.43 %)，顯示高氨氮入流下脫氮作用受抑制。 本研究整合反應動力學、連續通量監測以及同位素追蹤技術等三種方法，驗證高濃度氨氮入流會導致硝化-脫氮作用耦合失衡，可能降低污染移除並增加 N₂O 排放。建議污水處理型人工濕地之氨氮入流濃度應控制於 22 mg NH₄⁺-N L-1 以下，以維持氮移除效率並降低溫室氣體排放之環境風險。

Constructed wetlands represent a promising wastewater treatment technology; however, the nitrogen removal process may produce the greenhouse gas nitrous oxide (N₂O), and the trade-off between pollution treatment efficiency and greenhouse gas emissions remains unclear. This study investigated the effects of different influent ammonia nitrogen concentrations on nitrogen cycling mechanisms in constructed wetlands, with the following objectives: (1) establishing a nitrogen cycling kinetic model; (2) evaluating N₂O emission characteristics; and (3) quantifying nitrogen transformation pathways using ¹⁵N isotope tracing. Eighteen mesocosms (approximately 200 L each) were established and planted with common reed (Phragmites australis). Three influent ammonia nitrogen concentrations (2.2, 22.0, and 100.0 mg NH₄⁺-N L⁻¹) were applied with a 5-day hydraulic retention time (HRT). Continuous N₂O flux monitoring and ¹⁵N isotope addition experiments were conducted for analysis. Kinetic analysis revealed that ammonia nitrogen removal efficiency decreased significantly with increasing influent concentration, declining from 97.36% in the low-concentration treatment to 24.90% in the high-concentration treatment. Nitrate and nitrite accumulated (k < 0) in the medium- and high-concentration treatments, indicating nitrification-denitrification imbalance. Continuous N₂O flux monitoring showed mean fluxes of 6.18, 109.83, and 264.11 μg N₂O-N m⁻² hr⁻¹ for the low-, medium-, and high-concentration treatments, respectively. N₂O flux was strongly positively correlated with ammonia concentration (r = 0.74), with the high-concentration treatment approximately 43-fold higher than the low-concentration treatment. The ¹⁵N isotope addition experiments demonstrated that 86.89% of added ¹⁵NO₃⁻ was removed via gaseous pathways in the low-concentration treatment, whereas only 16.24% was removed in the high-concentration treatment, with 54.12% retained in soil. The proportion of ¹⁵NH₄⁺ removed via gaseous pathways remained relatively stable across treatments (42.28–63.43%), indicating that denitrification was inhibited under high ammonia loading conditions. By integrating reaction kinetics, continuous flux monitoring, and isotope tracing techniques, this study demonstrated that high influent ammonia concentrations lead to nitrification-denitrification coupling imbalance, potentially reducing pollution removal efficiency while increasing N₂O emissions. It is recommended that influent ammonia nitrogen concentrations in treatment wetlands be maintained below 22 mg NH₄⁺-N L⁻¹ to sustain nitrogen removal efficiency and mitigate environmental risks associated with greenhouse gas emissions.