應用INCA-N模式模擬山地小集水區氮循環作用

Abstract

溶解無機氮(DIN)是評估水域生態系統營養狀態的重要指標。臺灣的河川DIN輸出率大約是3,800 kg-N km-2 yr-1，比世界平高出許多，有導致優養化的潛在風險。因此了解臺灣的氮循環過程有其必要。本研究藉由每周採集的雨水及河水資料，以INCA-N (Integrated Nitrogen Catchment Mode) 模式模擬臺灣北部福山試驗林的河川DIN輸出，並以此推測其背後陸域氮循環的過程。模式結果顯示INCA-N可以有效地模擬河溪流量與硝酸氮(NO3-)的輸出，其模擬值與觀測值相近 (效率係數EC=0.80)。根據模擬結果顯示在福山試驗林最重要的三個氮移除作用依序為植物吸收、河川運輸以及反硝化作用。而每年高達4,920 kg-N km-2 yr-1的植物吸收理應導致大量的生物量累積，然而觀測顯示福山試驗林相對於其他熱帶森林的生物量而言是相對偏低的。推測原因可能是颱風的干擾，進而有效的移除許多枯枝落葉所導致。福山試驗林DIN濃度低但輸出率高顯示了水文控制了DIN的輸出，特別是在颱風侵襲期間。福山試驗林估計反硝化作用約為750 kg-N km-2 yr-1，相對於其他熱帶森林同樣偏低，有可能是由於本地孔隙較大的土壤所導致。若以大氣沉降對河川輸出比例而言(約0.45)，對比於世界其他河川而言偏高，可能顯示福山試驗林可能面臨了初步氮飽和的現象。本研究的模擬對於副熱帶森林集水區的氮循環及建立監測計畫提供了更進一步的了解。

Riverine dissolved inorganic nitrogen (DIN) is an important indicator of trophic status of aquatic ecosystems. High riverine DIN export in Taiwan, ~3,800 kg-N km-2 yr-1, which is several folders higher than the global average, urges the need of thorough understanding of N cycling processes. This study applied INCA-N (Integrated Nitrogen Catchment Model) to simulate riverine DIN export and infer terrestrial N processes using weekly rainwater and streamwater samples collected at the Fushan Experimental Forest (FEF) of northern Taiwan. Results showed that the modeled discharge and nitrate export are in good agreement with observations (EC = 0.80), suggesting the validity of our application. This modeling work revealed that the three main N removal processes, in the order of descending importance, were plant uptake, riverine N transport and denitrification at FEF. The high plant uptake rate, 4,920 kg-N km-2 yr-1, should have led to accumulation of large biomass, but biomass at FEF was relatively small compared to other tropical forests, likely due to periodic typhoon disruptions. The low DIN concentration but yielding high export highlights the hydrological control on DIN export, particularly during typhoons. The denitrification rate, 750 kg-N km-2 yr-1, at FEF was also low compared to other tropical forest ecosystems, which likely resulted from quick water drainage through the coarse-loamy top soils. The high DIN export to atmospheric deposition ratio, 0.45, suggests that FEF may be in advanced stages of N excess. This simulation provides useful insights for establishing monitoring programs and improves our understanding N cycling in subtropical watersheds.