臺灣農地廢耕造林對土壤有機碳儲存量及形態劃分的影響

Abstract

農地廢耕 (abandonment) 與造林 (afforestation) 被視為可以增進陸域碳儲存量、抵消人為碳排放，進而減緩氣候變遷的方法之一。當農地廢耕造林時，不僅地上部的樹木可以儲存碳，土壤亦為重要的碳儲存庫。本研究選擇全臺11個具廢耕或造林15年以上的廢耕造林地樣區的地點 (梅峰、武陵、梨山、清境、西寶、新社、名間、瑞穗、關西、二水、臺南，海拔介於14-2056 m)，並於相鄰或相近處選定農地樣區 (分為耕犁農地及無耕犁農地兩類)，分別採取深度0-10 與10-20 cm的土壤進行分析，以推估當農地轉換為林地後，土壤有機碳 (soil organic carbon，SOC) 儲存量的變化。除了測定土壤有機碳總儲存量，土樣還進行有機物密度劃分實驗 (soil organic matter density fractionation)，探究土壤有機物形態劃分 (分為輕質部、團粒內有機物及重質部) 的碳含量分配，以了解SOC儲存的穩定度，並藉碳13核磁共振光譜輔助了解有機物劃分的成分差異。 結果顯示土壤有機碳濃度與土壤有機碳儲存量均會受到地點、土地利用型及深度的影響。表層 (0-10 cm) 土壤有機碳變化通常比下層 (10-20 cm) 劇烈。不論是耕犁農地 (tilled field，如：菜園、鳳梨園) 或無耕犁農地 (no-till field，如：茶園、果園)，廢耕造林都可顯著提高土壤有機碳濃度及土壤有機碳儲存量。低海拔 (低於500 m) 的廢耕造林地，淺層0-20 cm的土壤有機碳儲存量比相鄰的耕犁農地多了15.90-25.86 ton C ha-1；比無耕犁農地多7.97-25.38 ton C ha-1。中海拔 (海拔介於1015-2056 m) 的廢耕造林地，表層有機碳濃度比相鄰耕犁農地高了22.8 ± 12.8 g C kg-1，但土壤有機碳儲存量受到含石率及總體密度的干擾，導致廢耕造林地與農地間的有機碳濃度高低趨勢，不一定同樣展現於有機碳儲存量。三種土地利用型的土壤有機濃度均會隨海拔提升而增加，且海拔可建立配適度高的線性迴歸模式，但土壤有機碳儲存量受含石率與總體密度的影響，未能隨海拔提升而顯著增加。 低海拔樣區的三種有機物劃分碳含量皆隨廢耕造林而顯著增加，中海拔樣區廢耕造林則傾向增加輕質部與團粒內有機物碳含量、重質部無顯著變化，推測可能是受到氣候、有效性氮含量與土壤質地的綜合影響。比較耕犁農地與廢耕造林地的輕質部的13C核磁共振圖譜，暗示當耕犁農地廢耕造林時，其含氧/氮脂肪族碳 (O/N-alkyl-C) 會減少，脂肪族碳 (alkyl-C) 與羧基碳 (carboxyl-C) 比例則增加。此外，固態13C核磁共振圖譜驗證了團粒內有機物應比輕質部分解程度高而更難分解。不同的農業管理也會影響有機碳儲存量及成分，梅峰、武陵、清境及瑞穗樣區無耕犁農地因施加有機質肥料，所以SOC濃度、SOC儲存量與輕質部碳含量較耕犁農地多，甚至多於廢耕造林地；輕質部也因此具較多的芳香基碳 (aromatic-C) 與含氧/氮脂肪族碳 (O/N-alkyl-C)。總而言之，農地的廢耕造林不僅改變土壤有機碳濃度及儲存量，影響也可展現於有機物劃分的碳含量與碳13核磁共振圖譜。

Afforestation or abandonment of arable fields has been proposed as a way to increase terrestrial carbon storage, offset anthropengenic carbon emission, and mitigate climate change. When the arable fields are afforested or abandoned, the accumulation in soil organic carbon (SOC) is a key pool to sequestrate carbon. In this study, a total of eleven sites with afforested/abandoned age over 15 years and elevation ranging from 14 to 2,056 m were investigated. We examined the increments of SOC by comparing with the adjacent tilled (e.g. croplands) and no-till (e.g. tea plantations or orchards) fields in two sampling layers, 0 - 10 and 10 - 20 cm in depth. In addition to total SOC measurements, a density fractionation of soil organic matter was also conducted in order to differentiate SOC into light fraction (LF), intra-aggregate fraction (IAF), and heavy fraction (HF) to gain more information about the mechanism of SOC storage. Our results indicated that SOC concentration and stock varied with elevation, land use management, and soil depth. According to the results of nested ANOVA, both SOC concentration and SOC stock in tilled and no-till fields would increase significantly after abandonment/afforestation. For the sites with elevation below 500 m, the SOC stock in the abandoned/afforested fields were 15.90-25.86 ton C ha-1 higher than the adjacent tilled fields, and 7.97-25.38 ton C ha-1 higher than the adjacent no-till fields for the 0-20 cm layer. For the sites with middle elevation (1,015-2,056 m), the SOC concentration in the abandoned/afforested arable fields were 22.8 ± 12.8 g C kg-1 higher than the adjacent tilled fields, but the SOC stock might not be different due to high stone content in abandoned/afforested field. Moreover, the SOC concentration and stock in abandoned/afforested field were not different or even less than no-till fields where organic amendments were frequently applied. The change of SOC in three fractions varied from site to site. For the sites with low elevation, carbon content of all of three fractions increased due to warmer climate, higher nitrogen availability and larger proportion of silt and clay; however, for the site with higher elevation, the carbon content might not increase in heavy fraction (the relative stable fraction). The 13C nuclear magnetic resonance spectroscopy (NMR) confirmed that the intra-aggregate fractions were more decomposed organic matter than the light fractions. Compared to the tilled fields, the LF of afforested/abandoned field presented stronger signal in carboxyl-C and alkyl-C region, but weaker signal in aromatic-C and O/N-alkyl-C. This might indicate that the organic matter in tilled fields could be decomposed easilier. For those no-till fields with organic amendment application, the results would display stronger signal in aromatic-C and O/N-alkyl-C. In conclusion, afforestation or abandonment on arable field could not only change the quantity of soil organic carbon stock and the soil organic matter fractions, but also the quality of soil organic carbon.