重複森林火燒對台灣中部大肚山保安林土壤性質之影響

Abstract

森林土壤性質的維持，是森林永續利用的重要因子。不過，森林的農業利用以及森林的火燒干擾，常是改變森林土壤性質的兩個重要因子。過去的研究，往往將這兩個因子分開進行討論，很少將兩個干擾因子放在同一樣區內進行比較。本研究以台中大肚山地區保安林為例，比較長期頻繁火燒 (頻繁火燒時間約20 – 30年) 與長期農地耕作 (森林轉換時間約100年) 等干擾，其土壤性質與鄰近相思樹森林的差異。研究結果顯示，土壤 pH 值受到灰分及肥料施入的影響，使得火燒樣區及農地的 pH 值在表土 0 - 5 cm 分別為 5.6 及 5.3，皆較相思林高 (pH 4.5)。火燒及耕作行為亦影響土壤物理性質，頻繁火燒及農地利用均造成土壤容積密度 (bulk density) 的上升。頻繁的火燒也明顯降低土壤有機碳含量，火燒樣區有機碳含量較相思樹林表層土壤減少50%，而長期耕作則影響更甚，有機碳減少量更高於 80%。農業耕作對於土壤總氮含量所造成的影響亦大於火燒干擾，使得長期耕作地的土壤總氮含量低於火燒樣區。火燒與耕作除了減少土壤中有機物的「量」，更造成土壤有機物質「質」的改變，本研究以土壤孵育試驗與粒徑大小方法分離分析土壤有機碳，並配合 13C同位素分析，探討長期火燒與農業利用對土壤有機碳的來源，組成與分解速率的影響。整體而言，頻繁的火燒與耕作對土壤性質有很大影響，例如土壤物理化學的改變、土壤肥力與土壤有機碳的下降等，這些改變如何影響後續土壤的生態功能與生物地球化學性質，值得進一步研究。

Soil property is one of the factor in maintaining forest sustainability. However, forest soil properties are strongly affected under the disturbance of forest fire and haman cultivated. Previous research works seldom consider these two factors in the same time. This research is conduceted in Datu tableland. We compare the disturbances of long-term repeated fire and long-term human cultivated on basic soil properties between nearby Acacia forest. Our results showed that soil pH is affected by the adding of ash after forestfire and fertilizer practice. Soil pH in grassland (GR) and cultivated land (Farm) in surface soil are 5.6 and 5.3, respectively. The soil pH values in disturbed lands are higher than unburned Acacia forest (pH = 4.5). Soil physical properties are also affected by forestfire and human cultivation. Soil bulk density is increased disturbance. Frequent fire events decrease soil organic C stock. Long-term human cultivation has more severe impact on soil N stock. The decrease of soil N stock is higher in Farm than fire-affected GR. Forestfire and cultivation both change SOM both quantitively and qualitively in the same time. We also use soil incubation method and particle-size fractionatino method to further investigate SOM, source, its composition, and decomposing rate. Soil properties were changed after disturbance both physically and chemically, which results in the depletion of SOM and fertility. More research works should be applied to further understand the mysterious role soil plays in earth ecological biogeochemical function.