長期果園廢耕地與造林地之生態系統碳儲存量與土壤性質

Abstract

隨著全球暖化問題的日益惡化，如何減緩溫室氣體效應的負面危害，成了現今最重要的環境議題。其中，對農地或是劣化土地的造林，是減緩溫室氣體效應危害有效且經濟可行的方法之一。台灣自2002年起開始推動「平地景觀造林及綠美化方案」，迄至2009年為止，造林面積已超過一萬公頃。不過，目前仍缺乏直接證據，用來評估這些新植造林地可能對碳蓄積潛能、蓄積碳量於生態系統的分布情形與土壤性質變化的長期影響。本研究利用現有長期果園廢耕地與造林地（造林時間14~33年，造林面積>0.5 ha）為例子，以現地造林的實際資料，作為推估平地造林長期的碳蓄積潛能與碳庫分布。研究結果顯示長期造林地的生態系統碳含量約介於123.3~205.7 ton C ha-1間，碳量分布以林木生物碳量與土壤有機碳量為主，約占生態系統碳量的90%以上。不過，造林地碳含量的差異，受到造林時間、造林樹種、經營管理與土壤性質的影響。若與鄰近持續耕作農地比較，除了鄰近水田造林地的碳蓄積量無顯著增加外，其餘造林地均有顯著性碳蓄積量的增加，增加碳量介於123.3~205.7 ton C ha-1之間。造林生態系統所增加的碳量，以林木生物碳量為主，約占了80%，另外死有機物質（木質殘體與枯枝落葉）與土壤有機碳量則各占了10%的碳增加量。相反地，水稻田土壤則由於浸水影響，使得農地土壤有機碳量則明顯高於造林地。造林後，增加的土壤有機碳主要在表層0-20 cm，更深層礦質土壤則沒有明顯差異。土壤有機碳粒徑分析進一步指出土壤有機碳以活性大有機物（砂粒粒徑）為主，但部分造林地也顯示，將隨造林時間增加活性大有機物仍可逐漸轉變至低活性粘粒粒徑有機碳，而增加土壤有機碳的穩定性。此外，造林後表層土壤pH值下降，陽離子交換容量與交換鹽基則上升，不過林地與農地在這些土壤性質的差異，受到周圍農地利用影響甚大，農地的施肥，可將影響造林效益的預測。對現有造林地可能的碳吸存量潛力與土壤性質，本論文結果將是至目前為止，最接近對平地造林長期影響潛能的推估。

Climate change mitigation is an important global issue nowadays. Afforestation in the crop lands has been proposed to be an economical and effective method way to mitigate global warming. In Taiwan, the afforestation policy in the crop lands was began in the year of 2002 and more than 10,000 hectares have been successfully implemented by 2009. However, we still do not have enough information, especially no field data, to assess the carbon storage potential for the afforestation policy. In the present study, we used the in-situ long-term abandoned orchards and afforested lands (afforestation age 14~33 years and afforestation size > 0.5 ha) to estimate the carbon storage potential, the relative carbon pools distribution, and soil properteis. Our results showed that the ecosystem carbon storages in the long-term afforestation sites was between 123.3 to 205.7 ton C ha-1, of which carbon pools in living tree and soil organic matter comprised more than 90% of ecosystem carbon storage. The differences of the ecosystem carbon storages could be attributed to afforestation age, tree species, field management and soil properties. Compared to adjacent crop land, carbon storages at afforested sites were significantly higher by 123.3~205.7 ton C ha-1, except of the paddy field where soil organic carbon could be conserved by saturated water. Most carbon was sequestered in the pool of tree biomass (80%) while dead organic matter (woody debris and litter) and mineral soil organic accounted for remaining 10% apiece. The soil organic carbon (SOC) between afforested and cultivated soils was significantly different at the topsoil of 0-20 cm and no difference for the soil layers of 20-100 cm. Soil particle size fractionation further indicated that these differences were mainly in the labile (sand) fractionation, while some sites might prove that SOC may be further stabilized into recalcitrant (silt and clay) fractionations over time. Moreover, soil pH was decreased and cation exchange capacity and exchangeable bases were increased at afforested sites. These soil properties could be masked by the fertilization in the adjacent crop lands. Till now, our results are the only pratical estimation for predicting the long-term effects of afforestion in carbon sequestration potential, carbon pools distribution, and soil properties in the cropland of Western Taiwan.