流況轉移對溶解性有機氮輸出之碳氮耦合關係的影響： 以福山試驗集水區為例

Abstract

溶解性有機氮(DON)作為一種在氮循環中生物可利用的活性氮，由於其量測上的困難度，導致在過往的研究中常被忽略，可能對氮營養狀態飽和的評估有所低估，因此了解溶解性有機氮的輸出量以及輸出行為有其必要。本研究在福山試驗林集水區，藉由及每週以及颱風事件時採集的雨水、河水、地下水、土壤水資料，透過平日與颱風事件中的營養鹽輸出狀況的比較，進一步討論當中的溶解性有機氮輸出的模式以及碳氮耦合關係的改變。研究結果顯示在福山試驗林集水區中，溶解性有機氮(DON)的平均濃度為~0.31 mg-N L⁻¹，年輸出產量為~18.96 kg ha⁻¹ yr⁻¹，暴雨約佔了全年20%的溶解性有機氮輸出佔比;溶解性無機氮(DIN)的平均濃度為~0.53 mg-N L⁻¹，年輸出產量為~24.04 kg ha⁻¹ yr⁻¹，暴雨約佔了全年17%的溶解性無機氮輸出佔比;溶解性有機碳(DOC)的平均濃度為~0.32 mg-N L⁻¹，年輸出產量為~23.97 kg ha⁻¹ yr⁻¹，颱風約佔了全年38%的溶解性有機碳輸出佔比。福山試驗林集水區的氮輸出在溶解性有機氮和無機氮上呈現負相關，另外溶解性有機碳和溶解性有機氮卻呈現解耦的狀態，並顯示出遠低於世界平均的河川碳氮比(約為0.88)。本研究認為流況的轉移特別是颱風，可能在福山試驗林的碳氮耦合模式貢獻上扮演一個重要的媒介，在不同流況上會改變平常福山試驗林的碳氮耦合，導致集水區呈現出極低的碳氮比，讓我們對於溶解性有機氮的輸出假設以及看法提供了更近一步的了解。

Dissolved organic nitrogen (DON) is a kind of reactive nitrogen in the nitrogen cycling processes, which has been neglected for decades because of the difficulty in measurement, leading to underestimating the nitrogen saturation in the ecosystem. This study compares the pattern of DON losses to previous studies, focusing on the carbon-nitrogen coupling between weekly normal flow and rainstorm event data. We analyze the stream water, groundwater, rainfall, and soil water collected at the Fushan Experimental Forest (FEF) in northeastern Taiwan. Results show that the mean DON concentration of ~0.31 mg-N L⁻¹, the mean DON yield of~18.96 kg ha⁻¹ yr⁻¹, and the rainstorms accounted for 20% of the annual DON yield; the mean DIN concentration of ~0.53 mg-N L⁻¹, the mean DIN yield of~24.04 kg ha⁻¹ yr⁻¹, and the rainstorms accounted for 17% of the annual DIN yield; the mean DOC concentration of ~0.32 mg-N L⁻¹, the mean DOC yield of~23.97 kg ha⁻¹ yr⁻¹, and the rainstorms accounted for 38% of the annual DOC yield. Furthermore, the pattern of DON losses shows the negative correlation between DIN and DON concentration, the relationship between carbon and nitrogen shows decoupling, and the riverine CN ratio of~0.88 is lower than the global average. Our study considers that the shift of flow regimes, especially typhoons/rainstorms, may play an essential role in the change of pattern of the DON losses, which causes the meager CN ratio and provides valuable insights for improving our understanding of carbon-nitrogen coupling in the Fushan Experimental Forest (FEF).