不饱和土壤CH_4的吸收与氧化

Methane uptake and oxidation by unsaturated soil

不饱和土壤是已知唯一的 CH4 生物壑。综述了不饱和土壤 CH4 的吸收、氧化过程及其影响因素。不饱和土壤中 CH4 氧化的临界浓度低 ,因而甲烷氧化菌可氧化大气 CH4 并将其当作唯一的碳源和能源。土壤 CH4 吸收率与土壤湿度通常呈负相关关系。土壤湿度过高 ,大气 CH4 和 O2 向土壤中扩散受阻 ;或土壤湿度过低引起水分胁迫均导致甲烷氧化菌活性下降。NH+ 4对土壤中 CH4 氧化的抑制作用可归结为 NH3和 CH4 在甲烷单氧酶水平上的竞争、由氧化作用向硝化作用的转移以及 NH+ 4氧化生成的 NO- 2 的毒性。NH+ 4对 CH4 氧化的抑制作用与土壤有效氮含量成正比。各类氮肥对 CH4 氧化抑制作用 :化肥 >有机肥 ;铵态氮肥 >尿素。 NO- 3对 CH4 氧化没有抑制效应。阳离子代换量 (CEC)高的土壤 NH+ 4对 CH4 氧化的抑制作用轻。 CH4 氧化菌对大气 CH4 的高亲和力及 CH4 氧化所需较低的活化能导致其温度系数 Q1 0 较小。地温较低时 ,土壤氧化 CH4 的能力随温度升高而升高。当地温高于 CH4 氧化的最佳温度时 ,CH4 氧化菌难以与硝化细菌及其它微生物竞争利用土壤空气中的 O2 ,导致其活性降低。甲烷氧化菌对 p H值变化不敏感。团粒结构较好的壤土可保护 CH4 氧化菌免受干扰。未受干扰的森林土壤 CH4

Unsaturated soils are the only known biogenic sinks of CH_4. In this paper we summarized the CH_4 oxidizing process in unsaturated soil and its influent factors. Methanotrophs can use atmospheric methane as the only source of carbon and energy because of the low critical concentration for methane oxidation. In General, CH_4 absorption rate is inversely proportional to soil moisture. The activity of methanotrophs becomes small if the transfer of CH_4 and O_2 from atmosphere to soils is prevented when soil moisture is over high, or water stress occurs at low soil moisture conditions. The inhibitory effect of NH^+_4 on CH_4 oxidation can be attributed to its competition with CH_4 for O_2 by the methane monooxygenases, the transfer of oxidation to nitrification and the toxicity of NO^-_2 produced. The inhibitory effect of NH^+_4 on CH_4 oxidation is proportional to available nitrogen. Chemical fertilizers are stronger than manures, and ammonium are stronger than urea in inhibiting CH_4 oxidation. NO^-_3 has no inhibitory effect on CH_4 oxidation. High cation exchange capacity (CEC) can alleviate the inhibitory effect of NH^+_4 on CH_4 oxidation. The high affinity of methanotrophs to atmospheric CH_4 and low activation energy for CH_4 oxidation leads to a small coefficient of temperature (Q_(10)). When soil temperature is low, the ability of CH_4 oxidation by soils is proportional to soil temperature. When soil temperature is higher than the optimal value, the activity of methanotrophs will decrease because it is difficult for methanotrophs to compete with nitrifiers and other microbes for the limited O_2 in soil air. Methanotrophs are very insensitive to pH value. Good granular structure of soil favors the activity of CH_4-oxidizing bacteria from being disturbed. The peak of CH_4-oxidizing rate in the undisturbed forest soil is usually in sub-topsoil (5～10cm). In farm soil, CH_4-oxidizing rate increases obviously under the depth of farming layer (25cm) because tillage destroys the structure of topsoil. The average CH_4 absorption rates of farmland, grassland and forest soil are 0.28, 0.52 and 1.51mg/(m^2·d), respectively. The absorption of N by plants can alleviate the inhibitory effect of NH^+_4 on CH_4-oxidizing bacteria. No-tillage, the use of nitrate fertilizer and (or) organic fertilizer favor CH_4 oxidization in farm soil.