森林土壤甲烷吸收的主控因子及其对增氮的响应研究进展

森林土壤甲烷(CH4)吸收在生态系统碳、氮循环和碳平衡研究中具有重要作用。论述了森林土壤CH4的产生和消耗过程及其主控因子,有效氮不同的森林土壤CH4吸收对氮素输入的响应差异及其驱动机制,并且明确了现有研究的不足和未来研究的重点。研究表明:大气氮沉降输入倾向于抑制富氮森林土壤的CH4吸收,而对贫氮森林土壤CH4吸收具有显著的促进作用,其内在的氮素调控机制至今尚不明确。主要的原因是过去通过高剂量施氮试验所得出的理论难以准确地解释低水平氮沉降情景下森林土壤CH4吸收过程,有关森林土壤CH4吸收对大气氮沉降响应的微生物学机理也缺乏系统性研究。未来研究的重点是探讨森林土壤CH4物理扩散和净吸收过程对施氮类型、剂量的短期与长期响应,量化深层土壤CH4累积和消耗对表层土壤CH4吸收的贡献,揭示森林土壤CH4吸收对增氮响应的物理学与生物化学机制。另外,研究森林土壤甲烷氧化菌群落活性、结构对施氮类型和剂量的响应,阐明土壤CH4吸收与甲烷氧化菌群落组成的内在联系,有助于深入揭示森林土壤CH4吸收对增氮响应的微生物学机制。

The primary factors controlling methane uptake from forest soils and their responses to increased atmospheric nitrogen deposition: a review

Methane (CH₄) is the second largest greenhouse gas in the atmosphere and its warming potential is 23 times that of CO₂ on a century time scale. The contribution of atmospheric CH₄ to the current level of global warming is up to 20%. Uptake of CH₄ by forest soils plays an important role in the carbon (C) and nitrogen (N) cycles and C budget of forest ecosystems, accounting for 80% of the total biological CH₄ sink. In this paper, the primary factors affecting CH₄ generation and consumption processes in forest soils are reviewed. The mechanisms that determine the CH₄ uptake rate in forest soils with different available N status to N addition are also reviewed. The inadequacies of existing research are discussed and future research priorities are proposed. CH₄ uptake from forest soils depends on the balance between CH₄ generation and oxidation in soils, and is affected by environmental factors such as available soil substrate, soil temperature, soil moisture, soil pH, nutrient availability, and vegetation types. Atmospheric N deposition tends to inhibit the CH₄ uptake in N-rich forest soils, but obviously promotes the CH₄ uptake in N-poor forest soils. Moreover, it is found that smaller amounts of N tend to stimulate CH₄ uptake and larger amounts tend to inhibit CH₄ uptake by the soil. When all other variables are accounted for, the switch occurs at 100 kg N/hm². However, the mechanisms that determine the effect of N concentration on CH₄ uptake in forest soils are not clear. Evidence from high level N addition experiments cannot accurately explain the CH₄ uptake from forest soils under the scenario of chronic atmospheric N deposition. On the other hand, systematic research into the microbiological mechanisms that affect forest soil CH₄ uptake responses to increasing N deposition is scarce. Presently, the mechanisms responsible for the inhibiting effect of N addition on CH₄ uptake from water unsaturated soils are relatively clear and include the following four aspects: 1) The competition for methane monooxygenase between CH₄ and NH₃ in soils; 2) The physiological water shortage of soil microbes caused by osmotic pressure and added salt ions; 3) The toxic effects of metabolites such as nitrite and hydroxylamine; and 4) The inhibition caused by soil nitrogen turnover. In the future, research focusing on this area should explore the short term and long term responses of CH₄ physical diffusion and net uptake in forest soils to added N of different types and levels. This, along with quantification of the contribution of CH₄ accumulation and consumption in deep soils to the net CH₄ uptake flux at the soil surface, could reveal the physical and biochemical mechanisms of CH₄ uptake from forest soils responding to N addition. Furthermore, it is necessary to study the short term and long term responses of methane oxidizing bacteria activity and community structure to added N of different types and levels, and to clarify the internal relations between soil CH₄ uptake and community composition of methane oxidizing bacteria. This research could help to determine the microbiological mechanisms of CH₄ uptake from forest soils in response to N addition.