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Study on the Sources of Soil CO2 Flux in Guandaushi Forest Ecosystem
|關鍵字:||Forest ecosystem;森林生態系;Soil carbon;Soil carbon dioxide flux;Turnover rate of soil organic matter;Stable carbon isotope;Taiwan long-term ecological research;土壤碳;土壤二氧化碳通量;土壤有機質周轉率;穩定碳同位素;台灣長期生態研究||出版社:||土壤環境科學系||摘要:||
在穿越集水區三種林分的人行步道上劃設的採樣點，分別現場量測土壤表面二氧化碳的釋放速率，並取樣帶回土壤A層分析其中有機碳與氮。落葉層總有機碳含量從221到546 g C m－2，土壤層則為18.2到41.4 kg C m－2；總有機氮含量則各為4.5~13.9與1.62~3.74 kg N m－2；落葉層碳氮比為32~79，土壤層為11~16。
撥開土壤表面落葉層後，以動態氣室與紅外線氣體檢測儀現場操作測定二氧化碳釋放率。土壤二氧化碳釋放率隨晝夜與季節變化，在1995年6月到1997年4月間，試區土壤二氧化碳釋放率範圍從9到50 mol CO2 m－2 month－1。長期觀測各樣點土壤呼吸量的結果推算，土壤碳的年釋放量為1.24到2.35 kg C m－2 yr－1。
將杉木林樣區土壤剖面160公分深分成許多層次，依序取樣並測量其土壤碳、氮含量及其13C PDB、15NAir值。土壤碳素經由表層之土壤呼吸年釋放量為1.24 kg C m－2 year－1，整個土層貯存有29.8 kg C m－2的碳。各土層有機碳含量為0.55至3.55%，上層較多，向下遞減。剖面各層土壤13C PDB值範圍從-24.0 到-27.3 ‰，與現生植被根系的13C PDB值接近，表現C-3植物有機物的特徵。13C PDB值與土層深度之關係顯示中段部分土層有13C的富集現象，且土壤與森林植物根的13C PDB值有顯著差異。
從截根處理結果顯示，杉木林植物根呼吸佔土壤二氧化碳釋放量的25至50 ％。由於根系僅被截斷到表層30公分，可能低估根呼吸對二氧化碳通量之貢獻。扣除根系的生態系自營性呼吸量，估計異營性呼吸量佔總呼吸量以50%計算，杉木林及闊葉林之土壤有機質的更新率為19~51年。另從13C PDB值分析，土層40cm處之根呼吸佔有其土壤二氧化碳產量的85%；配合根系與植被碳化物之13C PDB值顯示土壤表層60 cm內應是二氧化碳生產較活躍之處。
The organic carbon and nitrogen contents, values of 13CPDB and 15NAir in soils and litter, and soil respiration were monitored in the Guandaushi Forest Ecosystem, one of the long-term ecological research (LTER) stations in Taiwan. The elevation of station ranges from 700 to 1700 m height above sea level. The primary hardwoods, secondary hardwoods and 35-years-old China-fir are contribution to the stands of watershed. The 13CPDB, 15NAir values of soil, 13CPDB of soil carbon dioxide in coniferous pedon were analyzed. The soil respiration rates and amounts of carbon and nitrogen in soils and litter in three stands were investigated respectively.
The sampling sites were located at route cross the watershed of Guandaushi Creek. The amounts of organic carbon were analyzed in laboratory and ranges from 221 to 546 g C m-2 in litter layer, and 18.2 to 41.4 kg C m-2 in soil pedon. The amounts of the organic nitrogen are 4.5~13.9 and 1.62~3.74 kg N m-2 in litter and soil layer, respectively. The C/N ratios of litter and soil layer are 32~79 and 11.3~15.7.
The soil respiration rates were monitored in situ by an infrared gas analyzer (IRGA) with dynamic chamber and varied with seasons. The rates of soil respiration in watershed ranged from 9 to 50 mole CO2 m－2 month－1. The annual soil carbon dioxide efflux ranges from 1.24 to 2.35 kg C m-2 yr-1 by long term monitoring.
The pedon of 160 cm depth in China-fir stands sites were sampled in levels and analyzed in soil carbon, nitrogen, 13CPDB and 15NAir. The amounts of soil organic carbon were 0.55 to 3.55% in the pedon levels, and decreasingly with the depth. The organic carbon content was 29.8 kg C m-2 in the whole pedon. The soil carbon dioxide efflux was 1.24 kg C m-2 yr-1. The 13CPDB values were from -27.4 to -24.0 ‰ in the horizons, and typical 13C of C-3 plants, near the values of root samples. The relationship between 13CPDB values and depth suggested that the enriched 13C of soils were different from that of root samples in middle levels.
The soil respiration is contribution of root metabolism (autotrophic) and microbial decomposition (heterotrophic). The heterotrophic respiration was assessed by the root-exceed process and compared with bared soil respiration rates. The turned over rates of soil carbon stock ranged from 19 to 51 years in the pedon with assumption of 50% of heterotrophic respiration in the steady state. The 85 % of soil carbon dioxide production in 40 cm depth was contributed from root respiration by analysis with 13CPDB values. The analyses of 13CPDB values in root and charcoal samples suggested the carbon dioxide produced actively in the pedon above 60 cm depth.
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