施用原始與氮改質銀合歡生物炭對土壤碳封存與植物之影響

Abstract

生物炭是在低氧條件下透過生物質熱解產生的材料，作為土壤改良劑已有長久的歷史，但其減少土壤碳排和增強碳封存的潛力直到最近幾十年間才引起人們的重視。有鑑於全球淨零碳排放之趨勢以及碳捕獲、利用和封存（CCUS）技術的發展，研究土壤中加入生物炭來緩解全球暖化的可行性是相當重要的。 在這項研究中，我們利用外來入侵種植物銀合歡，在700 ℃下製備原始生物炭（BC），並用一種新型的單步驟氮摻雜法進行了氮改質（ABC）。兩種類型的生物炭先經過物理和化學性質分析，隨後進行二氧化碳吸附測試、31天的培養實驗和植物種植的微系統實驗，以模擬生物炭施加後土壤的減碳與農業效益。 CO2吸附等溫線的分析表明，BC和ABC在0 ℃時表現出符合Freundlich模型的吸附行為，而在25 ℃和50 ℃時，會過渡到類似於Langmuir模式之吸附。此外，ABC在較高溫下具有比BC更高的吸附能力，顯示出 N 摻雜對於 CO2 吸附的助益。 在培養實驗中，雖然BC導致CO2的累積排放量增加24%，但最終仍使土壤具有較高的土壤總碳和有機碳含量，且未觀察到明顯的氮開採現象。隨後對 BC 中的土壤微生物分析，顯示出其具有較高的放線菌含量，代表BC造成的啟動效應屬於微生物群體週轉導致的〝明顯〞啟動效應。相反的，ABC 會造成負啟動效應，其累積CO2排放量減少了39%，且土壤孔隙中二氧化碳濃度增加了8%。此外，ABC還使土壤總碳和有機碳分別增加了216%和55.6%，顯示其具有優越的固碳潛力。生物炭施加所增加的固碳能力可能肇因於：(1) 生物炭的高pH值提高對酸性CO2分子吸收，與促進微生物群體變化；(2)生物炭中的有毒熱解副產物抑制特定微生物生長；(3)生物炭吸附CO2使其保留在土壤孔隙中；(4)芳香胺的吸附導致環境中養分有效性的降低，抑制特定微生物生長。 在微系統實驗中，BC 和 ABC 分別使累積CO2排放量減少了23%和40%。然而，與對照組相比，除了土壤總碳含量外，大部分BC 和ABC的數據之間皆未顯示出統計上的顯著性。這種現象主要是由於植物在土壤碳循環中占主導地位，從而降低了生物炭對於CO2固定的相對顯著性。但是從植物-土壤系統的總有機碳含量可發現到，添加BC和ABC後可顯著地增加總有機碳 (分別為19%與10%)，顯示出添加銀合歡生物炭對碳封存的整體助益。另外值得注意的是，由於 ABC往往具有較高的總碳含量和較低的碳排放量，因此ABC與BC相比不具顯著性的有機碳數據可能是因為不適用的測量方法所造成，而這尚須進一步的研究來釐清。

Biochar is a material produced through biomass pyrolysis under low-oxygen conditions, historically used as a soil amendment. Its potential for mitigating soil carbon emissions and enhancing carbon sequestration has gained attention only in recent decades. Given the global trends toward net-zero carbon emissions and the development of carbon capture, utilization, and storage (CCUS) technologies, it is crucial to explore the applicability of biochar in soil as a mitigation solution. In this study, pristine biochar (BC) was derived from an invasive plant, Leucaena leucocephala, at 700 ℃. The biomass was also modified with a novel one-step N-doping method to produce N-containing biochar (ABC). Both types of biochar underwent physical and chemical property analysis, followed by CO2 adsorption tests, a 31-day incubation experiment, and a microcosm experiment with plant involvement to assess their benefits in simulated agricultural practice. The simulation of CO2 adsorption isotherms suggested that both BC and ABC exhibited adsorption behavior corresponding to the Freundlich model at 0 ℃ and transitioning to the Langmuir model at 25 ℃ and 50 ℃. Additionally, ABC had higher adsorption capacities at elevated temperatures than those of BC, suggesting the benefits of N-doping for CO2 adsorption. In the incubation experiment, BC caused a 24% increase in cumulative CO2 emission. Nevertheless, the BC amendment still increased soil total and organic carbon content without the evident N-mining observed. The subsequent soil microbial analysis of BC amendment indicated a higher content of Actinobacteria, suggesting an apparent priming effect due to microbial turnover. Conversely, the ABC amendment led to a negative priming effect with a 39% decrease in CO2 emissions and an 8% higher CO2 concentration in soil pores compared with Control. Moreover, the ABC amendment significantly increased soil total and organic carbon by 216% and 55.6%, respectively, indicating superior carbon sequestration potential. The mechanisms behind these results may include: (1) the high pH of the biochar enhances capture of acidic CO2 molecules, and stimulate the microbial turnover, (2) toxic pyrolytic by-products in the biochar inhibit the growth of specific microorganisms, (3) retention of CO2 in soil pores was enhanced due to biochar adsorption, and (4) a nutrient-deficient environment was resulted due to the sorption of nutrient by aromatic amines in ABC. During the microcosm experiment, BC and ABC resulted in a 23% and 40% decrease in CO2 emissions, respectively. Apart from soil total carbon content, most of the data and subsequent analysis did not show statistical significance compared to the control. This phenomenon was primarily due to the dominant role of plants in the soil carbon cycle, which diminished the effectiveness of the biochar. However, analyzing the total organic carbon content of the plant-soil system revealed significant increases in total organic carbon content with the addition of both BC (19%) and ABC (10%), indicating the overall benefits of biochar addition for carbon sequestration. It is also notable that the lack of significant difference between BC and ABC might be due to the inapplicability of the measurement method, as ABC tends to have higher total carbon content and lower carbon emissions, which require further study to clarify its effect.