Mechanism of Methane Transport from the Rhizosphere to the Atmosphere through Rice Plants

Abstract

To clarify the mechanisms of methane transport from the rhizosphere into the atmosphere through rice plants (Oryza sativa L.), the methane emission rate was measured from a shoot whose roots had been kept in a culture solution with a high methane concentration or exposed to methane gas in the gas phase by using a cylindrical chamber. No clear correlation was observed between change in the transpiration rate and that in the methane emission rate. Methane was mostly released from the culm, which is an aggregation of leaf sheaths, but not from the leaf blade. Micropores which are different from stomata were newly found at the abaxial epidermis of the leaf sheath by scanning electron microscopy. The measured methane emission rate was much higher than the calculated methane emission rate that would result from transpiration and the methane concentration in the culture solution. Rice roots could absorb methane gas in the gas phase without water uptake. These results suggest that methane dissolved in the soil water surrounding the roots diffuses into the cell-wall water of the root cells, gasifies in the root cortex, and then is mostly released through the micropores in the leaf sheaths.Recent studies of ancient air trapped in polar ice cores (7,17) have shown that the concentration of atmospheric methane has more than doubled during the past 200 years and that during the last decade atmospheric methane has increased approximately 1% per year (4). Because methane is one of the so-called greenhouse gases, in addition to C02, N20, 03, and chlorofluorocarbons, the increase in atmospheric methane may cause an increase in the globally averaged surface temperature (19,25). About 80% of methane emissions are produced biologically by methanogenic bacteria in flooded soils and in the intestines ofdomestic animals (9). Rice (Oryza sativa L.) paddy fields are known to be a major source of methane (5) and the area of rice paddy fields in the world averaged over the last 35 years has increased 1.6% per year (13). Although a full explanation of increasing atmospheric methane concentration remains uncertain, the increasing area of rice paddy fields in the world is considered to be an important cause of the recent shifts in the atmospheric methane balance. Studies have found that methane emission from vegetated plots in rice paddy fields were much higher than from unvegetated plots (6, 14). Therefore, Cicerone and Shetter (6) proposed that methane emitted to the atmosphere from rice paddy fields is transported mostly through rice plants and not across the water-air interface via bubbles or molecular diffusion.In rice and other hydrophytes, it is well known that atmospheric 02 is transported to the submerged organs from the leaf parts above water through the aerenchyma and intercellular gas space systems by diffusion (2, 10, 15, 24) or by mass flow (8). Since these internal air spaces in rice plants are particularly well developed in the culm (1) and roots (16), the ventilation system in rice plants plays an important role in t...

Applying stable isotope probing of phospholipid fatty acids and rRNA in a Chinese rice field to study activity and composition of the methanotrophic bacterial communities in situ

“…Such technical approach is suitable for microcosm experiments, but can hardly be used for field studies. Since rice plants allow the transport of O 2 from the atmosphere into the soil and of CH 4 from the soil into atmosphere via their gas vascular system and aerenchyma tissue in roots and shoots by a diffusional process (Nouchi et al, 1990;Schü tz et al, 1991), we inferred that transport of 13 C-labelled CH 4 from the atmosphere into the rhizosphere is also possible and that rhizospheric methanotrophs can be labelled in this way. The transport of 13 C-labelled CH 4 from the atmosphere into the rhizosphere was indeed operating as shown by the incorporation of 13 C into microbial PLFAs.…”

“…The total emission of CH 4 from Chinese rice fields was estimated to be in the range of 8.05 ± 3.69 Tg CH 4 per year, depending on the type of rice paddy field (Cai, 1997). In these ecosystems, the main CH 4 emission to the atmosphere occurs through the aerenchyma system of the rice plants (Nouchi et al, 1990;Schü tz et al, 1991). In turn, oxygen from the atmosphere is transported to the rice roots enabling aerobic and microaerophilic conditions in the rhizosphere .…”

Combination of photoacoustic detector with gas diffusion probes for the measurement of methane concentration gradients in submerged paddy soil

“…In flooded rice fields the methane produced in deeper layers is mainly emitted by vascular transport through the rice plant and by bubbling to the atmosphere (Holzapfel-Pschom et al, 1985;Nouchi et al, 1990). Since the vascular system of rice plants also transports oxygen into the soil, the rice field soil becomes a complex structure containing oxic and anoxic zones (Armstrong, 1979;Frenzel et al, 1992).…”

Aspects of methane flow from sediment through emergent cattail (Typha latifolia) plants

“…Cyperus papyrus (Li & Jones, 1995) and Peltandra virginica (Frye, Mills & Odum, 1994). A different ecological pattern occurs in J. effusus (this study) and rice (Nouchi, Mariko & Aoki, 1990) with greater methane concentrations in leafy shoots during the day than at night.…”

“…For example, Nouchi et al (1990) demonstrated that methane leaks from the culm (aggregation of leaf sheaths) of rice through small micropores that do not undergo opening and closing like stomata. This observation helps explain why methane does not accumulate within rice plants overnight.…”