副熱帶半鹹水人工濕地碳氮循環之探討

Abstract

人工濕地是近年廣泛應用的生態工法，具備各項生態系統服務功能，然而人工濕地運作機制複雜，入流水營養鹽濃度若過高，可能伴隨二氧化碳、甲烷、氧化亞氮等溫室氣體產生，尤其後兩者為高全球暖化潛勢之氣體，若大量排放可能導致碳吸存速率小於溫室氣體排放之二氧化碳當量，使得人工濕地成為碳源。 為了探討人工濕地的碳收支情形，本研究於社子島濕地進行為期一年之現地研究，採集水質、土壤與植體樣本進行分析，並以光聲譜溫室氣體監測儀量測土壤與生態系統溫室氣體交換量，搭配長石粉標記法與分解袋實驗，分別量測沉積速率與分解速率。藉由量化濕地內碳、氮循環的各個途徑，以深入了解濕地生物地質化學過程。 研究結果發現社子島濕地為碳匯與氮源，可分別吸存11.19 ton C ha-1 yr-1，並釋放0.56 ton N ha-1 yr-1。其中土壤碳變化貢獻了66 %之碳吸存量，速率為7.42 ton C ha-1 yr-1，植物初級生產量則貢獻34 %之碳吸存量。土壤的氮損失為主要氮源，其損失速率為0.68 ton N ha-1 yr-1。溫室氣體測量結果以正值表示濕地排放，負值表示濕地吸收，生態系統交換之CO2平均通量為-149.2 mg CO2 m-2 hr-1，CH4平均通量為-1255.8 μg CH4 m-2 hr-1，N2O通量為-138.2 μg N2O m-2 hr-1。土壤之CO2平均排放通量為91.9 mg CO2 m-2 hr-1，CH4平均通量為13193.9 μg CH4 m-2 hr-1，N2O通量為-187.1 μg N2O m-2 hr-1。計算溫室效應之平衡，社子島濕地仍為溫室效應匯，削減CO2eq之速率達41.56 ton CO2eq ha-1 yr-1。 此外，藉由長石粉標記法發現社子島濕地之沉積速率高，其平均值為6.0 cm yr-1，沉積物為濕地帶來的碳量達9.67 ton C ha-1 yr-1，氮量為0.87 ton N ha-1 yr-1。分解袋實驗則發現植物枯落物經過一年的分解會留下生物量中60%的碳與63%的氮。 本研究之結果量化了副熱帶半鹹水人工濕地之碳、氮動態，對於濕地管理提出建議，並可作為環境條件相近濕地之參考。

Constructed wetlands have been widely applied in recent years as an ecological engineering method, providing various ecosystem services. However, the operational mechanisms of constructed wetlands are complex. If the nutrient concentration in the inflow water is too high, it may lead to the production of greenhouse gases such as carbon dioxide, methane, and nitrous oxide, with the latter two being gases with high global warming potential. Excessive emissions of these gases may result in the carbon sequestration rate being less than the carbon dioxide equivalent emissions of greenhouse gases, thereby turning the constructed wetland into a carbon source. To investigate the carbon balance in constructed wetlands, this study conducted a year-long field study at Shezidao wetland. Water, soil, and plant samples were collected for analysis, and greenhouse gas exchange from soil and ecosystems was measured using a photoacoustic gas monitor. The feldspar marker method and litter bag experiments were used to measure sedimentation and decomposition rates, respectively. By quantifying various pathways of carbon and nitrogen cycling within the wetland, we aimed to gain a deeper understanding of the biogeochemical processes in wetlands. The results showed that Shezidao wetland is a carbon sink and a nitrogen source, sequestering 11.19 tons of carbon per hectare per year and releasing 0.56 tons of nitrogen per hectare per year. Soil carbon changes contributed 66% of the carbon sequestration at a rate of 7.42 ton C ha-1 yr-1, while primary plant production contributed 34% of the carbon sequestration. Soil nitrogen loss was the main source of nitrogen, with a loss rate of 0.68 ton N ha-1 yr-1. Greenhouse gas measurements indicated that positive values represented wetland emissions and negative values represented wetland absorption. The average ecosystem exchange rates of CO₂, CH₄, and N₂O were -149.2 mg CO₂m ² hr⁻¹, -1255.8 μg CH₄ m² hr⁻¹, and -138.2 μg N₂O m² hr⁻¹, respectively. Soil exchange rates for CO₂, CH₄, and N₂O were 91.9 mg CO₂ m² hr⁻¹, 13193.9 μg CH₄ m² hr⁻¹, and -187.1 μg N₂O m² hr⁻¹, respectively. In terms of greenhouse effect balance, Shezidao wetland remains a greenhouse gas sink, reducing CO₂eq at a rate of 41.56 tons per hectare per year. Additionally, using the feldspar marker method, it was found that Shezidao wetland has a high sedimentation rate of 6.0 cm yr⁻¹, bringing 967 g m² yr⁻¹ of carbon and 87 g m² yr⁻¹ of nitrogen through sedimentation. The litter bag experiment revealed that plant litter, after a year of decomposition, retains 60% of its carbon and 63% of its nitrogen. This study quantified the carbon and nitrogen dynamics in a subtropical brackish constructed wetland, provided management recommendations, and can serve as a reference for wetlands in similar environmental conditions.