宜蘭太平山地區淋澱化土之特性化育與分類

Abstract

本研究選取宜蘭縣境內太平山地區10個淋澱化土壤樣體，研究其土壤形態特徵、理化特性、礦物組成及元素分佈狀態，並且探討其化育作用與分類。選取的土壤樣體可以分類為弱育土、具薄膠層土壤及極育土三類。研究地區海拔約為1900至2000公尺，母質為黏板岩，主要植生種類為紅檜(Chamaecyparis formosensis Matsum)以及台灣杉(Taiwania cryptomerioides Hay.)，年降雨量約3000公釐且雨季集中於4到10月，土壤水分境況為濕潤(udic)而土壤溫度境況則屬於溫和(mesic)。土壤物理特性顯示洗入作用為研究地區薄膠層生成之前最為優勢之土壤化育作用，化學性質方面則顯示極育土中主要由有機態-鐵錯合物為主要成分，而薄膠層則以游離態之氧化鐵所組成。此外分析薄膠層鐵物質之成分則發現主要由弱結晶性的針鐵礦(goethite)、纖鐵礦(lepidocrocite)、水合氧化鐵礦(ferrihydrite)及少許的赤鐵礦(hematite)所組成，研究地區土壤中之主要黏土礦物種類為蛭石、蛭石-伊萊石混層礦物以及伊萊石，其風化序列為綠泥石及伊萊石→蛭石及蛭石-伊萊石混層礦物→水合層間蛭石(HIV)及高嶺石→水鋁氧。 研究結果顯示，具薄膠層土壤與極育土之間存在著側向洗出洗入作用(lateral eluviation-illuviation)，可見地形為兩者化育差異的控制因子。而薄膠層之生成機制為：(1)強烈的洗入作用導致土壤樣體出現上粗下細之質地變化，並在該層界形成暫棲水位導致鐵物質呈現還原狀態；(2)處於略微傾斜的地理位置之土壤會因側向流動作用導致暫棲水位快速變動，提供快速且劇烈的氧化與還原交替作用；(3)待土體呈現氧化狀態時，鐵物質即在粗質地下部層界氧化、累積並且包覆粗顆粒物質形成薄膠層下部主體；(4)吸附來自表土之有機質形成上黑下紅之薄膠層。而值得注意的是較高之土壤pH值有利於薄膠層之生成，但是會抑制淋澱化作用之進行，研究地區pH值≧5.0似乎是薄膠層成因的一個重要因子，同時也可能是淋澱化作用之限制因子。研究地區土壤風化序列為新成土→弱育土→極育土或具薄膠層極育土。 土壤分類方面，研究地區具薄膠層土壤及極育土的土壤分類結果皆無法明確表現出其土壤特性，因此建議在美國分類系統(Soil Taxonomy)增加薄膠濕潤弱育土(Placudepts)大土類、薄膠浸水型簡育濕潤極育土(Placoaquic Hapludults)亞類及淋澱型簡育濕潤極育土(Spodic Hapludults)亞類，而在世界土壤分類系統(WRB system)增加薄膠型極育土(Placic Lixisols)與薄膠型變育土(Placic Cambisols)之亞類，以因應類似研究地區之土壤。 依據各類元素在土壤剖面不同深度含量之分佈狀態可將元素種類分類為：(1)累積於土表之元素：鈉、鉀、鎂、錳；(2)累積於B層之元素：鎵、鉻；及(3)與薄膠層生成有關之元素：銦。由於在薄膠層的生成過程中對於元素銦(In)具有強大的吸附能力，因此元素銦(In)的含量將可作為薄膠層的生成指標。

Ten podzolic soil pedons were selected to study the soil morphology, characteristics, material composition, pedogenesis, elements distribution, and classification in Tai-Ping mountain region of Taiwan. The podzolic soils in the study area can be divided into three classes including Inceptisols, soils with placic horizon, and Ultisols. The selected pedons had an elevation ranging from 1900 to 2100 m and were derived from slate. The vegetation types are dominated by red cypress (Chamaecyparis formosensis Matsum) and Taiwan Chinese fir (Taiwania cryptomerioides Hay). The annual rainfall is about 3000 mm and mostly falls from April to October. The soil moisture regime is udic and the soil temperature regime is mesic. The soil physical properties showed that the illuviation process of clay particle was the dominant pedogenic process which occurred in the study area before the formation of the placic horizon. The chemical properties showed that the Ultisols appeared to be dominated by the organic-Fe types but the placic horizons appeared to be dominated by free iron oxides. On the other hand, the placic horizons are composed of poor crystalline goethite, lepidocrocite, ferrihydrite, and some hematite. All soil pedons are dominated by vermiculite, interstratified vermiculite-illite minerals, and illite. The weathering sequence of clay minerals in the study area is proposed as: illite and chlorite → vermiculite and interstratified vermiculite-illite minerals → HIV and kaolinite → gibbsite. Lateral eluviation-illuviation occurred between the soils with placic horizon and the Ultisols. This result showed that the relief is the important controling factor of the genesis variation between soils with placic horizon and Ultisols. The genesis of placic horizon in the study area is proposed to have four stages. First stage, the perched water was formed on the surface after heavy rainfall owing to the abrupt textureal change due to strong illuviation. Second stage, the lateral flow which was induced from the slightly sloping area caused the perch water table to fluctuate rapidly thereby ehancing the fast re-oxidation alteration environment. Third stage, the soluble iron was re-oxidized, precipitated, and coated on coarse particles to form the placic horizon at the boundary between the eluvial and illuvial horizons after the perched water was drained. Fourth stage, the organic matter was adsorbed by iron oxide as shown in the characteristic morphology of the upper part of the placic horizon which is black and red in its lower part. The high pH value (>5) of the study area may be one important factor that favored the placic horizon genesis but retarded podzolization. The soil sequence in the study area is as follows: Entisols→Inceptisols→Ultisols or Ultisols with placic horizons. The results of soil classification can not show the properties of placic horizons in the study area. I propose that a Placudepts should be included in the Great Group of Udepts. Moreover, Spodic Hapludults and Placoaquic Hapludults should be included in the subgroup of Hapludults in Soil Taxonomy. On the other hand, Placic Lixisols and Placic Cambisols should also be included in the subdivisions of Lixisols and Cambisols in the World reference base for soil resources system (WRB system). Based on the distribution of elements in the soil profiles, the elements can be divided into three classes, such as (1)elements concentrated in the topsoil: sodium, .potassium, magnesium, and manganese; (2)elements concentrated in the B horizon: Gallium and Chromium; and (3)element concentrated in relation to the placic horizon genesis: Indium. Because Indium (In) can be strongly adsorbed in the placic horizon during the formation process, it would be a good indicator for placic horizon genesis.