應用結構方程模型評估亞熱帶山地集水區硝氮和銨氮吸收的控制因子

Abstract

描述水生生態系統狀況的營養鹽吸收指標在很大程度上取決於流域的陸地和水生生態系統，但是在陡峭的上游地區進行的調查較少。本研究進行了保守示踪劑（Cl-）和非保守示踪劑（NO3-和NH4+）的瞬時示踪劑添加實驗，估計福山實驗林（FEF）和坪林地區兩種氮營養鹽的吸收指標（NO3-N和NH4-N）。並利用結構方程模型識別環境因子之間的直接和間接關係，從而確定控制因素。 研究結果顯示，硝氮和銨氮的吸收速度分別介於0.1和2.2及0.5和12.1 mm min-1之間，表明在這種環境中銨氮是被優先選擇的氮營養源。而硝氮和銨氮的單位面積吸收量分別為38.4至1558.3和3.9至103 μg m-2 min-1。對比銨氮，硝氮的吸收量表明硝氮是主要的氮營養來源。然而與其他研究相比，這幾個河段的吸收效率很低，而高單位面積吸收量但低吸收效率是養分吸收指標的特徵表明溪流是處於貧營養的狀態。我們應用了結構方程模型來模擬氮吸收指標與環境變量之間的關係。流量和氮營養鹽的濃度與吸收長度和單位面積吸收量都密切正相關。農業對氮營養鹽吸收有很強的影響。農業土地利用比例直接影響硝氮的吸收長度和吸收速度，而且分別透過增加流量和減小河流寬度間接影響硝氮吸收長度和硝氮的吸收量。出乎意料的是，在我們的案例中，生態系因子並未顯示為重要因素，唯有對硝氮的吸收速度有些微影響。了解不同形式的氮在河流轉化和運輸的過程對於維持生態系統健康至關重要，尤其是台灣的無機形式的氮（如NO3-）。本研究的結果增進我們對亞熱帶流域氮循環的理解提供基礎的見識。

Nutrient uptake metric, describing the status of aquatic ecosystems, strongly depends on terrestrial and aquatic ecosystems in catchments, but fewer investigations have been done in the steep headwaters. This study performed instantaneous tracer addition experiments of conservative tracer (Cl-) and non-conservative tracers (NO3- and NH4+) to determine the nutrient uptake metrics (NO3-N and NH4_N) in Fushan Experimental Forest (FEF) and Ping-Lin area, Taiwan. Besides, the structural equation model, which can identify the direct and indirect relationship between variables, was used to identify the controlling factors. Results showed that the ambient uptake velocities (Vf-amb) of NO3-N and NH4_N ranged from 0.1 and 2.2 and 0.5 and 12.1 mm min-1, respectively, showing the preference of ammonium in this environment. Ambient areal uptake (Uamb) for NO3-N and NH4_N ranged from 38.4 to 1558.3 and 3.9-103 μg m-2 min-1 , respectively. The high ambient areal uptakes of NO3–N presents that NO3–N is the dominant N source. Compared with other studies in temperate and tropical area, our low efficiency of N uptakes is likely due to the steep slope and high velocity. However, the higher areal uptake values in our streams indicate the streams are oligotrophic. The high ambient areal uptake but low uptake efficiency characterizes the nutrient uptake metrics in small mountainous rivers. We applied structural equation modeling the relationship between nitrogen uptake metrics and environment variables. Stream discharge and nitrogen concentrations strongly related with both uptake length and Uamb. The biological dimension factors didn’t appear as significant factors in our cases which only showed a small effect on NO3-N Vf-amb.