Methane emission induced by short-chain organic acids in lowland soil

ABSTRACT Methane (CH4) is the second major greenhouse gas after CO2, exerting a significant influence on the climate and the chemistry of the atmosphere. In lowland soil, acetate and H2/CO2 are the most important precursors of CH4 and formed from organic matter fermentation in an anaerobic environment, giving rise to short-chain organic acids (ethanoic, propanoic, and butanoic), depending on the type of crop residue and the soil management system. Ethanoic acid can be directly converted to CH4 by methanogenic microorganisms, but propanoic and butanoic acids must be converted to acetate before being converted to CH4. This study aimed to quantify, in isolation, the dynamics and CH4 emission potential of the three short-chain organic acids found in flooded lowland soils with rice crops. The study was carried out in a controlled environment using four standard carbon doses (0, 90, 180, and 270 mg kg−1) of ethanoic, propanoic, and butanoic acids. The dynamics and the potential emission of CH4 from soil were investigated when the acids were applied to flooded soil previously incubated for 20 days. The CH4 emission dynamics were altered with the application of the three short-chain organic acids to the soil, even using an equal amount of carbon. The faster and more intense emission was achieved with the ethanoic acid application in relation to the other two acids application, while butanoic acid presents slower, delayed, and prolonged dynamics of CH4 emission. Propanoic acid resulted in the lowest CH4 emission due to its own stoichiometry and the temperature condition in which the experiment was conducted, which were unfavorable to the hydrogenotrophic bacteria. The addition of short-chain organic acids promoted a priming effect in the soil with conversion values of C to CH4 above the calculated theoretical values.

摘要：甲烷（CH₄）是仅次于二氧化碳（CO₂）的第二大温室气体，对全球气候及大气化学过程均具有显著影响。在低地土壤中，乙酸与H₂/CO₂是甲烷生成的最重要前体物质，它们由厌氧环境下的有机质发酵生成，可产生短链有机酸（short-chain organic acids，包括乙酸、丙酸与丁酸），其组分比例取决于作物残体类型与土壤管理模式。乙酸可直接被产甲烷微生物（methanogenic microorganisms）转化为甲烷，而丙酸与丁酸则需先转化为乙酸，后续才能进一步生成甲烷。
本研究旨在单独量化水稻种植淹水低地土壤中三种短链有机酸的动态变化特征及其甲烷排放潜能。实验在可控环境中开展，设置乙酸、丙酸与丁酸三种有机酸的四种标准碳剂量（0、90、180及270 mg·kg⁻¹），并探究了将上述有机酸施加至预培养20天的淹水土壤后，土壤的甲烷动态变化与排放潜能。
研究结果表明，即便施加的碳量一致，三种短链有机酸的施加均会改变土壤的甲烷排放动态。相较于另外两种有机酸处理，施加乙酸可获得更快且更强的甲烷排放；而丁酸处理则呈现出更为缓慢、滞后且持续时间更长的甲烷排放动态。丙酸处理的甲烷排放量最低，这源于其自身的化学计量特性，以及实验所处的温度条件对氢营养型细菌（hydrogenotrophic bacteria）的生长活动不利。施加短链有机酸可引发土壤激发效应（priming effect），其碳向甲烷的转化效率高于理论计算值。