臺灣西部紅樹林沉積物中有機碳含量研究

Abstract

在全球暖化與氣候變遷日益加劇的情勢下，如何有效降低大氣中二氧化碳濃度並尋求長期穩定的碳匯機制，已成為環境科學領域的重要課題。紅樹林因具厭氧沉積環境與光合作用帶來的高生產力，能促進有機碳之長期累積與儲存，因而在全球範圍內被視為藍碳系統中最具潛力的碳匯之一。尤以紅樹林地下部沉積物在分解速率較低且不易被分解的條件下，能維持高含量的有機碳並延長封存時間。本研究旨在定量評估臺灣西部紅樹林區域沉積物之有機碳含量，並進一步探討其控制因子與對比國際上其他紅樹林沉積物有機碳含量。為達成此目標，本研究選取臺灣西部由北至南之四處代表性紅樹林區域-關渡自然公園、挖子尾自然保留區、新豐濕地，以及台南安平四草鹽水溪濕地，採集沉積物岩芯樣本，並透過元素分析儀分別量測總有機碳(TOC)、總氮(TN)、總無機碳(TIC)、易揮發有機碳(LOC)及難降解有機碳(ROC)，以量化各區域紅樹林沉積物的碳匯潛能。同時為釐清控制機制並佐證樣區間之差異，利用雷射繞射粒徑分析獲得沉積物之顆粒大小分布(黏土:<4 μm、粉砂:4–63 μm、極細砂與細沙:63–250 μm、中砂:>250 μm)，並配合穩定碳同位素(δ¹³C)資料以闡明有機碳來源，並將測量數據進行Spearman相關係數矩陣以求各比值之間的關係。最後再配合氣候條件、風速、河川流量等的自然因素，以評估其對紅樹林沉積物有機碳含量之影響。 結果顯示淡水河口關渡純林岩芯 TOC 平均 1.18 %，岩芯δ¹³C 與 C/N 比值推測藻類來源特徵；淡水河左岸挖子尾TOC除了表層樣品1.49%，其餘平均僅 0.25 %， δ¹³C 與 C/N 比值推測也幾乎藻類來源特徵；新豐內陸樣區缺乏潮汐動力，沉積物以粗砂為主；粗顆粒的稀釋效應使總有機碳（TOC）平均僅 0.36 %。臨港沿岸升至 0.91 %；安平黏土至粉砂混合表層 TOC 最高 1.7%，無植被沙洲則僅 0.51 %。黏土+粉砂比例高的關渡與安平有助富集碳(r >0.4, p <0.01)，而砂質環境（挖子尾、新豐內陸）則限制碳累積。LOC(易揮發有機碳)占 TOC 55–70 %，顯示臺灣紅樹林以易分解碳為主，長期封存潛能可能有限。 與國際資料相比，臺灣各樣區的 TOC 均顯著低於全球紅樹林沉積物 2.2 %的中位值(Kristensen et al., 2008)，亦落後於東南亞泥質河口紅樹林普遍的 3–5 % 水準。全球紅樹林 0-100 cm 剖面平均 SOC 密度約 337 t C ha⁻¹，本研究各站估值僅約其三分之一。全球碳埋藏速率均值為 171 g C m⁻² yr⁻¹，而挖子尾僅 46.8 g C m⁻² yr⁻¹，僅達全球均值三分之一。上述落差顯示，除有機碳來源差異外，台灣紅樹林所處的高能量水動力環境與頻繁極端氣候事件同樣是限制有機碳累積的關鍵因素。

In response to the escalating impacts of global warming and climate change, the urgent need to reduce atmospheric carbon dioxide concentrations and establish long term, stable carbon sequestration mechanisms has become a central focus in environmental science. Mangrove ecosystems, characterized by high primary productivity and anoxic sedimentary conditions resulting from tidal inundation, are recognized as highly effective blue carbon systems due to their capacity for substantial organic carbon accumulation and long-term storage. In particular, subsurface mangrove sediments exhibit high levels of organic carbon with relatively low decomposition rates, ensuring long-term sequestration. The present study quantitatively assesses the organic carbon content of mangrove sediments along Taiwan’s western coastline and investigates the primary environmental factors influencing carbon dynamics. Sediment cores were collected from four representative mangrove sites spanning north to south—Guandu Nature Park, Wazihwei Nature Reserve, Xinfeng Wetland, and Anping Wetland. An elemental analyzer was used to determine total organic carbon (TOC), total nitrogen (TN), total inorganic carbon (TIC), labile organic carbon (LOC), and refractory organic carbon (ROC), thereby evaluating the carbon sequestration potential of each region. To further elucidate carbon dynamics and inter-site variability, grain-sized distributions were measured using laser diffraction. Stable carbon isotope (δ¹³C) analysis was conducted to trace organic carbon sources. Measured data undergo Spearman correlation analysis to examine relationships among the calculated ratios. Climatic conditions, wind speed are incorporated to assess their influence on organic carbon content in mangrove sediments. The results indicate that the pristine mangrove core from Guandu, has a mean total organic carbon (TOC) content of 1.18 %. Combined δ¹³C signatures and C/N ratios imply a predominantly algal origin. At Wazihwei on the left bank of the Tamsui River, TOC averages only 0.25 % throughout the core—apart from the surface sample 1.49 %—and δ¹³C and C/N likewise point to an almost exclusively algal source. In the inland sector of Xinfeng, where tidal forcing is absent and sediments are dominated by coarse sand, the dilution effect of large grain sizes reduces mean TOC to 0.36 %; values rise to 0.91 % along the adjacent coast. In Anping, surface TOC reaches 1.7 % in clay-to-silt mixtures, whereas the unvegetated sandbar contains only 0.51 %. High clay + silt fractions at Guandu and Anping favor carbon enrichment (r > 0.4, p < 0.01), while sandy settings (Wazihwei, inland Xinfeng) constrain carbon accumulation. Labile organic carbon (LOC) constitutes 55–70 % of TOC, suggesting that Taiwanese mangrove sediments are dominated by easily decomposed carbon fractions and therefore have limited long-term storage potential. Compared to international data, TOC values from all Taiwan sampling sites were significantly lower than the global mangrove sediment median value of 2.2% reported by Kristensen et al. (2008), and also fell behind the typical 3-5% levels found in Southeast Asian muddy estuarine mangroves. The global average soil organic carbon (SOC) density for 0-100 cm profiles in mangroves is approximately 337 t C ha⁻¹, while estimated values from study sites were only about one-third of this figure.The global carbon accumulation rate (CAR) averages 171 g C m⁻² yr⁻¹, while Wazihwei achieved only 46.8 g C m⁻² yr⁻¹, reaching merely one-third of the global average. These disparities indicate that beyond differences in organic carbon sources, Taiwan's mangroves exist in high-energy hydrodynamic environments with frequent extreme climatic events, which are equally critical factors limiting SOC accumulation.