大气CO_2浓度升高对稻田根际土壤甲烷氧化细菌丰度的影响

Effect of elevated CO_2 on methanotrophs in the rhizosphere of rice plant

甲烷氧化细菌是目前已知的稻田甲烷氧化唯一生物,在减少稻田甲烷排放、降低大气甲烷浓度方面发挥着重要作用。利用中国稻/麦轮作FACE(Free Air Carbon-dioxide Enrichment)试验平台,采用实时荧光定量PCR技术,研究了大气CO2浓度升高下,典型水稻生长期根际土壤甲烷氧化细菌数量的变化规律,及其对不同施肥处理(高氮HN和常氮LN)的响应。2009和2010连续2a的观测结果表明,大气CO2浓度升高促进了2009年秧苗期和分蘖期,2010年秧苗期、拔节期和灌浆期甲烷氧化细菌的生长;并可能对2010年常氮条件下成熟期甲烷氧化细菌产生了较显著(P<0.1)抑制;进一步针对甲烷氧化细菌主要类群的分析表明,高氮条件下大气CO2浓度升高提高了稻田根际土壤中Ⅰ型甲烷氧化细菌的丰度。

Methane （CH4） is the second most important greenhouse gas. Flooded rice fields are a major source of atmospheric CH4, accounting for 5%-19% of total global emissions. Methanotrophs are the only known microorganisms that catalyze the oxidation of CH4 produced in the rice fields and thus play an important role in attenuating the emissions of CH4 into the atmosphere. Atmospheric CO2 concentration has increased drastically and is predicted to double by the end of this century. This dramatic change in atmospheric CO2 may have profound ecological consequence on biogeochemical cycles of key elements such as methane oxidation in rice fields. Free air CO2 enrichment （FACE） is an ideal technique to assess ecosystem responses to the projected atmospheric CO2 elevation by 2100 because it provides undisturbed field conditions and more reliable measurements than closed chambers with elevated CO2. The effect of elevated atmospheric CO2 on microbial methane oxidation has been investigated mostly by traditional techniques such as most probable number （MPN）, which may underestimate the changes in methanotrophic changes under elevated CO2. China-FACE was established in 2004 to investigate agricultural ecosystem response to simulated global change of rising atmospheric CO2, and could approach the in situ conditions as much as possible for investigating the microbial mechanisms underlying methane oxidation changes under elevated CO2. The rhizospheric soils at different rice growing seasons were collected to quantify the changes in methanotrophic populations using real-time quantitative PCR of pmoA genes of type Ia, type Ib and total methanotrophs. Pairwise comparison demonstrated positive responses of methanotrophic population size to elevated CO2 at most stages of rice growing seasons. Total methanotrophs abundance were stimulated at the seedling and tillering stages in 2009, and seedling, jointing and grain filling stages in 2010 regardless of the application rates of nitrogenous fertilizer. In 2009, the total methanotrophs abundance was elevated at the ripening stage and declined at the flowering stage under high application rate of nitrogenous fertilizer, while the reverse trend was observed with regard to the low application rate of nitrogenous fertilizer. This implies that the availability of nitrogenous fertilizers might have intensified the response of methanotrophic populations to elevated CO2 in rice field. A significant decline （P〈0.1） in total methanotrophic population was observed at the ripening stage in 2010,suggesting that the elevated CO2 inhibits the growth of methanotrophs under certain conditions. The consistent response to elevated CO2 was observed for both type Ia and total methanotrophic population during rice growing seasons except for the tillering stage under low application rate of nitrogenous fertilizer in 2009. Type Ⅰb methanotrophic abundance was increased at most rice growing stages except for the grain filling stage under high application rate of nitrogenous fertilizer and the ripening stage under low application rate of nitrogenous fertilizer in 2010. Furthermore, high application rate of nitrogenous fertilizer further stimulated the abundance of type Ⅰ methanotrophs under elevated CO2. This result implies that intensive fertilization appeared to favor the growth of type Ⅰ rather than type Ⅱ methanotrophs under elevated CO2 condition. The results of this study suggested that the rising CO2 concentration in the atmosphere may affect the community structure of methanotrophs, which could ultimately alter the biogeochemical methane cycling in rice fields.