兩個不同風化時間的地形序列中火山土壤特性比較

Abstract

臺灣早期火山活動以北部的大屯火山群為主，受成土因子影響大屯火山群生成的土壤多歸類於灰燼土綱(Andisols)，隨海拔降低逐漸轉變成弱育土綱(Inceptisols)與極育土綱(Ultisols)。從火山噴發活動的時間來看，紗帽山(Sms)在大屯火山群中最晚停止(30萬年前)，土壤化育過程受母質影響較大；丁火朽山(Dhx)約79萬年前停止噴發，土壤受時間與氣候的影響較大。本研究選擇大屯火山群土壤化育時間差異最大的兩地區，探討火山土壤之特性差異與化育作用。分別在紗帽山與丁火朽山採集共六個土壤樣體，進行土壤形態描述、基本理化性質與選擇性化學抽出性質測定。調查結果顯示，相似的土壤溫度與水分境況下，風化時間的長短會影響兩地區土壤化育程度。Sms樣體土層薄(<100公分)、色黑(2.5YR 2/1)、結持度易脆且構造以團粒為主。Dhx樣體土層厚(>250公分)、色呈紅棕(2.5-7.5YR 4/4)、結持度緊實且構造以塊狀為主，高海拔地區有黏粒膜(Clay film)生成。Sms樣體總體密度(0.3-0.7 Mg/m3)、磷酸結持度(> 85%)與草酸抽出鋁＋1/2鐵含量(3-6%)符合火山灰土壤性質(Andic soil properties)且厚度達到標準，屬於灰燼土綱。其中Sms-Summit與Sms-Backslope-2樣體有機碳含量高(>6%)且符合Melanic Index <1.7，分類為典型烏黑濕潤灰燼土(Typic Melanudand)；Sms-Backslope-1樣體不符合melanic epipedon要求分類為典型黑瘠濕潤灰燼土(Typic Hapludands)。Dhx樣體化育時間長，經過劇烈的風化及淋洗作用已經不具火山灰土壤性質，化育成弱育土或極育土受地形與氣候影響，300公尺以上地區風化程度高、淋洗作用強烈，土層厚、有黏粒聚積層及低鹽基飽和度等特性，屬於典型厚育濕潤極育土(Typic Paleudults)。300公尺以下地區土壤不具上述特性，也不符合火山灰土壤特性標準，屬於具火山灰特性的低鹽基濕潤弱育土(Andic Dystrudepts)。風化時間短的紗帽山與風化時間長的丁火朽山，兩地區火山土壤特性明顯不同，隨風化時間增加有從灰燼土化育成弱育土或極育土的趨勢。本研究區域中，長時間風化作用在巨觀上會減弱火山灰土壤性質，地形與海拔則是影響其他土壤分類項目的主要因子。

Tatun Volcano Group was major active volcano in Taiwan. Volcanic soils are always classified as Andisols based on Soil Taxonomy. Andisols were transformated to Inceptisols or Ultisols with decreasing the elevation of the Yangminshan National Park. Shamaoshan (Sms) and Dinghuoxiu (Dhx) mountain are both belong to Tatun Volcano Group. Sms soil is relative younger (0.3 Ma) and mainly affected by parent material and Dhx soil is relatively old (0.79 Ma) and mainly affected by time and climate. The objectives of this study of two region volcanic soils with different pedogenic processes time are (1) to integrate soil morphological characteristics and chemical analysis to identify the soil properties and soil classification, and (2) to investigate the pedogenic processes with weathering time. Total six pedons were sampled from Sms and Dhx regions. Soil characteristics include soil morphology, physical and chemical properties, showed that the major differences of pedogenic processes were major affected by different weathering time. In this study, all Sms pedons had thiner soil depth (<100 cm), black soil color with high organic matter, granular soil structure and friable consistence. All Dhx pedons had thicker soil depth (>250 cm), brown color, subangular or angular blocky soil structure and firm consistence. Sms pedons were classified as Andisols with andic soil properties. Sms-Summit, and Sms-Backslope-2 pedons with melanic epipedon and it can be classified as Typic Melanudand based on Soil Taxonomy, however Sms-Backslope-1 pedons without melanic epipedon was classified as Typic Hapludands. Dhx pedons had longer weathering time and stronger leaching processes to show the soils without the andic soil properties. Soil classification of Dhx soils was affected by the landscape positions. Dhx pedon with the elevation higher than 300 meters above sea level (asl) had the characters of thick argillic horizon and low base saturation, which can be classified as Paleudults. Dhx pedon with the elevation lower than 300 meters above sea level had cambic horizon without andic soil properties, which can be classified as Andic Dystrudepts. An weathering time gap was found between Sms and Dhx pedons that resulting in significantly different andic soil characteristics, according to pedogenic processes and soil weathering time, the pedogenic processes of soils of two volcanic region are time dependent and shown as the soil formation sequence as: Andisols, Andic Inceptisols, Inceptisols, and finally Ultisols. Prolonged weathering has a significant effect on pedogenic processes, but the morphology and elevation are the main factors of detailed soil classification.