土地利用改變對台灣農地土壤有機碳存量之影響評估

Abstract

This study is to evaluate the effect of the shifts of land use on soil organic carbon storage of cultivated land in Taiwan. The soil fertility survey data were used to evaluate soil organic carbon storage of cultivated land in Taiwan, and soil organic carbon contents were compared among different types of land use. On the base of soil survey data during 1978- 1982, the storage of soil organic carbon on the surface soil (0-20 cm) of cultivated land in Taiwan is 27.5 Tg, calculated by 850 thousand hectares of cultivated land in 1998, assuming that the majority of cultivated land has reached to equilibrium under the same long-term cultivation. This amount is 93 % of carbon released from energy source in 1990. The organic carbon storage of surface soil (0-20 公分)is about 30-40 % of the organic carbon storage to a depth of 100 cm. The survey data shows that paddy field obviously can accumulate more organic matter; therefore, the storage of surface soil (0-20 公分) organic carbon can be added 4.7 Tg if the current upland field is shifted to paddy field. But there is 5.5 Tg carbon released from soil if paddy field is shifted to upland field, showing that the surface soil (0-20 公分) organic carbon of cultivated land in Taiwan is between 22.0 to 32.2 Tg. According to the result of long-term experiments, the time for equilibrium on shifts of land use is 30 years. Nevertheless, the carbon amount stored or released by upland field shifted to paddy field or paddy field shifted to upland field is about 0.5 % and 0.6 % of carbon released from energy source. 本研究之目的係利用土壤肥力調查資料估算台灣農地土壤有機碳存量，並評估土地利用改變對農地土壤有機碳存量之影響，藉此評估土壤有機碳存量之變動量佔能源部門釋出碳量之百分比，以了解土壤有機碳之變動量對溫室氣體減量之重要性。依據1959-1967年間全省農田肥力測定所調查之78萬公頃耕地的土壤有機質分析結果，估算全省農地表土（0-20公分）的有機碳存量爲21.7 Tg (太克，1012g)，此碳量爲1990年能源部門釋出碳量(29.5 Tg)的74 %。若以1978-1982年間之調查資料爲基礎，又假設大部分已達平衡，以1998年的耕地面積85萬公頃，計算台灣農地土壤的表土（0-20公分）有機碳存量爲27.5 Tg，此碳量爲1990年能源部門釋出碳量(29.5 Tg)的93%。表土（0- 20公分）有機碳存量，約佔土壤一公尺深存量的30-40%左右。依調查資料顯示水田明顯可累積較多的有機質，因此若使現有的旱田轉作水田，則全省表土（0-20公分）有機碳存量可再增加4.7 Tg碳量，但若將水田轉作旱田則表土（0-20公分）有機碳可再釋出5.5 Tg之碳量，顯示台灣農地表土（0- 20公分）有機碳約在22.0-32.2 Tg之間。根據長期試驗結果顯示，土地利用改變所需的平衡時間約30年，因此，旱田轉作水田或水田轉作旱田，每年表土（0-20公分）所貯存或釋出的碳量，約是能源部門釋出碳量的0.5%與0.6%。