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...

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We investigated the carbon dynamics and budget in a grassland of Miscanthus sinensis, which is widely distributed in Japan, over a 2‐year period (2000–2001). Plant biomass began to increase from May and peaked in September, then decreased towards the end of the growing season (October). Soil respiration rates also exhibited seasonal fluctuations that reflected seasonal changes in soil temperature and root respiration. The contribution of root respiration to total soil respiration was 22–41% in spring and summer, but increased to 52–53% in September. To determine the net ecosystem production (carbon budget), we estimated annual net primary production, soil respiration, and root respiration. Net primary production was 1207 and 1140 g C m−2 in 2000 and 2001, respectively. Annual soil respiration was 1387 g C m−2 in 2000 and 1408 g C m−2 in 2001; root respiration was 649 and 695 g C m−2 in 2000 and 2001, respectively. Moreover, some of the carbon fixed as net production (457–459 g C m−2) is removed by mowing in autumn in this grassland. Therefore, the annual carbon budget was estimated to be −56 g C m−2 in 2000 and − 100 g C m−2 in 2001. These results suggest that the Miscanthus sinensis grassland in Japan can act as a source of CO2.

We measured diurnal and wintertime changes in CO2 fluxes from soil and snow surfaces in a Japanese cool‐temperate Quercus/Betula forest between December 1994 and May 1995. To evaluate the relationship between these winter fluxes and temperature, flux measurements were made with the open‐flow infrared gas analyzer (IRGA) method rather than with the more commonly used closed chamber method or the snow CO2 profile method. The open‐flow IRGA method proved to be more successful in measurements of winter CO2 fluxes than the two standard methods. Despite colder air temperatures, soil temperature profiles were greater than 0°C because of the thermal insulation effect of deep snowpack. This reveals that soil temperature is satisfactory for microbial respiration throughout the winter. Unfrozen soils under the snowpack showed neither diurnal nor wintertime trends in CO2 fluxes or in soil surface temperature, although there was a daily snow surface CO2 flux of 0.18–0.32 g m−2. By combining this with other reference data, Japanese cool‐temperate forest soils in snowy regions can be estimated to emit < 100 g m−2 carbon over an entire winter, and this value accounts for < 15% of the annual emission. In the present study, when data for all winter fluxes were taken together, fluxes were most highly correlated with deep soil temperatures rather than the soil surface temperature. Such a high correlation can be attributed to the relatively increased respiration of the deep soil where the temperature was higher than the soil surface temperature. Thus, deeper soil temperature is a better predictor of winter CO2 fluxes in cold and snowy ecosystems.