臺灣西南部鰲鼓海岸溼地造林及農地土壤鹽化與生態系碳吸存之權衡

Abstract

隨著氣候變遷與全球暖化日益嚴重，它們對自然與人類生存的影響也日漸顯著。其中，海平面上升與海水入侵已經造成沿海環境劣化的危機，包含土壤鹽化、海岸侵蝕，以及糧食安全等，對沿海的生態與人類生命財產生存帶來威脅。本研究以鰲鼓濕地森林園區及其鄰近地區為研究對象，旨在探討海水入侵與地層下陷後，海岸森林及農地土壤鹽化的程度、兩者的差異，還有評估生態系碳儲存的能力。鰲鼓濕地森林園區為60年前海埔新生地，後因海水入侵鹽化而逐漸廢耕。本研究將此區域分為五個樣區，由沿海至內陸大致上分別為鰲鼓濕地西側(AG West)、鰲鼓濕地東側(AG East)、笨港港口宮(BG)、港墘(GC)，以及高鐵嘉義站(HSR)，並在每個樣區中選擇造林地與持續耕作之農地採集土壤。每個樣區土壤採集由淺至深分為0-20 cm、20-40 cm、40-60 cm、60-80 cm、80-100 cm ，除了高鐵樣區由於土壤質地較黏，所以採樣深度僅至20-40 cm)。將土壤依照分層深度進行基本性質與鹽分含量化學分析，包括飽和水抽出電導度(saturated water extract electrical conductivity, ECe)、交換性鈉飽和度(exchangeable sodium percentage, ESP)，以及土壤有機碳(soil organic carbon)濃度，此外也使用樣區表層土壤種植玉米進行盆栽試驗。 研究結果顯示，鰲鼓濕地森林園區及鄰近地區之林地、農地土壤pH皆屬於鹼化範圍，與土壤母質和海水入侵有關。在海岸森林土壤方面，在水平方向上以最靠海岸的AG West 及AG East的鹽化程度最嚴重，屬於強烈鹽化(ECe大於16 dS m-1)其次是BG和GC，屬於中度鹽化(ECe：4.1-8.0 dS m-1)，鹽化程度最輕的樣區則是最內陸的HSR，屬於無鹽化土壤；垂直方向上，則是在大部分樣區中呈現下層土壤鹽化較嚴重的現象，海岸林土壤ECe明顯超過多數植物之耐鹽極限，其土壤交換性鈉含量和ESP等指標也都普遍呈現較靠海較高，顯示此區域的海水入侵方向有由沿海向內陸、由下層往上層的趨勢。農地土壤方面，所有樣區的土壤都呈現無鹽化的狀態，顯示人為控管、洗鹽措施，以及土地利用型差異等，會導致土壤鹽化程度的不同，並且人為干預自然環境，在某種程度上能有效減緩土壤因海水入侵而鹽化的現象。玉米盆栽試驗也反映出類似的結果與相關趨勢，造林與農地土壤所種植的玉米地上部高度、生物量皆是越內陸越高，且相同樣區中，農地土壤的玉米地上部高度和生物量普遍較造林土壤的玉米高。代表鰲鼓濕地土壤鹽化的程度，對於該區常見作物生長會產生明顯的影響。土壤有機碳儲存方面，林地土壤的有機碳濃度雖然較農地土壤高，但其總體密度卻較農地土壤低，因此一來一往下，兩者整體地下部有機碳儲存並無明顯差異。不過，林地生態系除了土壤碳儲存外，再加上林木生物量以及枯落物層的碳儲存後，整體碳儲存會明顯高於農地生態系，約可多出72.5-187.0 ton C ha-1，顯示土地利用對於生態系碳吸存能力的影響巨大。 綜合以上，由本研究結果可得知，在海水入侵、土壤鹽化的情境下，沿海土地利用呈現兩難的情況。若以造林的方式利用鹽化土壤，可得到碳吸存與生物多樣性等生態系統功能的服務，但卻無法阻止土壤鹽化，而犧牲糧食生產的可能；但是另一方面，若持續以洗鹽與灌溉的方式維持土壤於無鹽化的狀態，可以確保可耕地的利用與糧食安全，卻會放棄森林可能提供的各樣生態系統功能。如此的兩難困境，是未來沿海土地管理者需要面對與抉擇的重要議題。

As climate change and global warming intensify, their impacts on natural and human systems are becoming increasingly evident. Among these, sea-level rise and seawater intrusion pose serious threats to coastal environments, leading to ecosystem shifts, soil erosion, and soil salinization. This study investigated the forest and cropland soils of the Aogu Wetland Forest Park in southwestern Taiwan to assess their salinization status, differences between land-use types, and ecosystem carbon storage. Five study sites were selected along a coastal–inland gradient: the west side of Aogu Wetland (AG West, nearest to the coast), the east side of Aogu Wetland (AG East), Bengang-Kangkao Temple (BG), Gangcian (GC), and the Taiwan High-Speed Rail Chiayi Station (HSR, farthest inland). At each site, soil samples were collected from both afforested land and croplands at depths of 0-20, 20-40, 40-60, 60-80, and 80-100 cm, except for the HSR site where clayey conditions limited sampling to 20-40 cm. Soil physical and chemical properties, including electrical conductivity (EC), exchangeable sodium percentage (ESP), and soil organic carbon (SOC) content, were analyzed. Additionally, a maize pot experiment was conducted in a greenhouse to evaluate the impacts of soil salinity on plant growth. The results indicated that forest soils in AG West and AG East were strongly saline (ECe greater than 16 dS m-1), while BG and GC showed moderate salinity (ECe：4.1-8.0 dS m-1), with salinity generally increasing with depth, suggesting inland and upward seawater intrusion. Cropland soils, however, showed minimal to no salinization, indicating that irrigation and management practices can effectively mitigate soil salinity. The pot experiment supported these findings, as maize grown in saline coastal forest soils exhibited reduced height and biomass compared to maize grown in inland or cropland soils. Furthermore, although forest soils had higher organic carbon concentrations than cropland soils, their lower bulk density resulted in similar soil organic carbon storage overall. Yet, the total ecosystem carbon storage of forests in this study was significantly higher than that of cropland ecosystem due to contributions from tree biomass and litter layers. Compared to croplands, the forest ecosystem in this study stores 72.5 to 187.0 ton C ha-1, highlighting the significant influence of land use on ecosystem carbon sequestration potential. In conclusion, while coastal soils are vulnerable to salinization under seawater intrusion, active management can reduce this issue to an extent. These results suggest a trade-off in managing coastal saline soils: whether to prioritize afforestation for enhanced ecosystem services, such as carbon sequestration and biodiversity, at the cost of reduced agricultural productivity, or to favor agricultural land management to secure food production while compromising some ecosystem functions.