以穩定碳氮同位素探討雙連埤浮島養分來源

Abstract

浮島是由水生植物的根系以及有機物殘質所構成，在全球熱帶與亞熱帶溼地中常見的地景。浮島的增長可能會限制湖泊的可達性，佔據沉水植物和挺水植物適合的棲地，然而浮島在改善環境與水質上有重要的作用。近來，浮島被認為是一種環境議題的自然解方，不但能增加湖域的水質、生物多樣性更能提供美學價值。浮島上的水生植物氮的主要來源究竟是浮島的基質或者是湖水與其對環境變遷如大氣沉降的反應有密切的關係，但少有研究觸，故為本研究欲探討的議題。不同來源的氮素常具有不同之穩定同位素比值(δ)，因此透過分析植物、浮島基質以及湖水的氮穩定同位素，可估算浮島植物運用了多少湖域來源的氮和浮島本身產出的氮素。另外除了測量穩定同位素比值的天然含量外，主動添加高濃度氮穩定同位素比值的追蹤劑，透過定量的實驗的投放，可以增加穩定同位素的訊號，有助於進一步釐清氮素在浮島上的移動狀況，並可以與穩定同位素的天然含量做比較是否有相同的趨勢存在。另外本研究收集了環境因子與植物功能性狀，希望能了解造成δ15N變化的原因。結果顯示，δ15N在物種間有顯著的差異，鄰近水域的物種有較高的δ15N，顯示浮島與湖域之間的氮素因為不同的循環路徑而有不同的訊號；物種間δ15N的含量有顯著差異，藉由多元線性回歸分析發現δ15N與，物種、比葉面積、葉乾物質量和菌根類型有顯著的相關，並且也受到土壤水pH、電導度等環境因子的影響δ15N有顯著的影響，物種間δ15N的差異或與其功能性狀的差異有關，而所量測的功能性狀和植物經濟譜(plant economics spectrum)有密切的關係。雙來源混合模型的結果顯示，浮島上的植物約有56.9%的氮源來自浮島的土壤，43.1%來自湖水中。δ15N的追蹤劑實驗結果示縱劑很快速的進入湖水、土壤跟水中的有機物中。比較2022與2024的兩次實驗可以發現，雙連埤湖域的同位素結構有著非常劇烈的變化，值得進一步探討造成其變化的是因為外來的汙染，或是湖水養分循環的變化。Floating islands are composed of the root systems of aquatic plants and organic debris, and they are commonly found in tropical and subtropical wetlands around the world. The expansion of these islands may limit lake accessibility and occupy habitats suitable for submerged and emergent plants. However, floating islands play an important role in improving the environment and water quality. Recently, they have been regarded as a natural solution to environmental issues, as they not only enhance water quality and biodiversity but also provide aesthetic value. How macrophytes growing on floating islands acquire nutrients in this unique environment is an important question because it helps to predict the response of the macrophytes to environmental change such as atmospheric deposition of nutrients but is rarely studied. Different sources of nutrients such as nitrogen have different abundance of stable isotope ratios so that through stable isotope analysis, we can distinguish nitrogen sources, mainly the island and the lake sources. In addition to measuring the natural abundance of stable isotopes, adding nitrogen enriched in 15N ratio (δ) can help to better understand nitrogen cycling. I also collected environmental factors and plant functional traits and explored their relationships to δ15N variations.The results showed significant differences in δ15N among plant species. Species near the water had higher δ15N, indicating different nitrogen cycles between the floating islands and the lake. Multiple linear regression analysis revealed that δ15N was significantly correlated with species, specific leaf area, leaf dry matter content, and mycorrhizal type. δ15N was also significantly affected by environmental factors such as soil water pH and conductivity. The differences in δ15N among species are probably related to differences in their functional traits, which are closely related to the plant economics spectrum.The results of the dual-source mixing model showed that approximately 56.9% of the nitrogen source for the plants on the floating islands came from the soil/substrate of the islands, while 43.1% came from the lake water. The tracer experiment results showed that the tracer quickly entered the lake water, soil, and organic matter. Comparison of the experiments conducted in 2022 and 2024 reveals significant changes in the isotopic structure of the Shuang-lian-pi Lake area. It is worth further investigating whether these changes are due to external pollution or variations in the nutrient cycling of the lake water.