CH4 emission rates have been measured in an Italian rice paddy between 1984 and 1986, covering three vegetation periods. For these measurements a fully automated, computerized sampling and analyzing system was developed which allowed the simultaneous determination of CH4 emission rates at 16 different field plots. CH4 emission rates showed strong diurnal and seasonal variations. Diurnal changes correlated with changes in soil temperature. During the season, CH4 emission rates showed a first maximum in May–June before tillering and a second maximum in July during the reproductive stage of the rice plants. In 1985 and 1986 two maxima were observed during summer in addition to the first maximum in the rate of CH4 emission during spring. Application of mineral and/or organic fertilizer strongly influenced the CH4 emission rates, depending on the type, rate, and mode of fertilizer application. Thus the rates decreased by at most 40% and 60% after fertilization by deep incorporation with 200 kg N/ha urea and 200 kg N/ha ammonium sulfate, respectively. Application of 200 kg N/ha calcium cyanamide led to a reduction of the first maximum of CH4 emission but caused the second maximum to increase, the overall result being that the seasonally averaged CH4 emission rate was comparable to that observed in unfertilized fields. Application of rice straw at a rate of 12 t/ha enhanced the rate of CH4 emission by a factor of 2 compared with the control. Higher application rates of rice straw did not cause a further increase in CH4 emission. The complete records of CH4 emissions over three vegetation periods indicate an average seasonal CH4 emission rate from unfertilized fields of 0.28 g CH4/m2 d, with a range of 0.16–0.38 g CH4/m2 d. Based on this value and applying the observed temperature dependence of the CH4 emission rates, the global annual CH4 emission from rice paddies is estimated to be in the range of 50–150 Tg, with a likely average of 100 Tg. This figure represents between 19% and 25% of the global CH4 emission, indicating that rice paddies are one of the most important individual sources of atmospheric CH4.

Abstract.The seasonal change of the rates of production and emission of methane were determined under in-situ conditions in an Italian rice paddy in 1985 and 1986. The contribution to total emission of CH, of plant-mediated transport, ebullition, and diffusion through the flooding water was quantified by cutting the plants and by trapping emerging gas bubbles with funnels. Both production and emission of CH, increased during the season and reached a maximum in August. However, the numbers of methanogenic bacteria did not change. As the rice plants grew and the contribution of plant-mediated CH, emission increased, the percentage of the produced CH, which was reoxidized and thus, was not emitted, also increased. At its maximum, about 300 ml CH, were produced per m* per hour. However, only about 6% were emitted and this was by about 96% via plant-mediated transport. Radiotracer experiments showed that CH, was produced from H,/C02 (30 -50%) and from acetate. The pool concentration of acetate was in the range of 6 -10 mM. The turnover time of acetate was 12 -16 h. Part of the acetate pool appeared to be not available for production of CH, or CO,.

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The shift of incubation temperature in anoxic paddy soil from 30°C to 15°C resulted in a reversible decrease of the methane production rate and of the H2 steady state partial pressure. Only at 30°C but not at 17°C, total CH4 production rates were enhanced by the addition of H2, acetate, or cellulose compared to the control. Apparent activation energies which were calculated from the temperature dependence of CH4 production were higher in presence than in absence of excess H2. Decrease of temperature caused a decrease of the H2 turnover rate constant and of the Gibbs free energy of H2‐dependent methanogenesis, and also resulted in a smaller contribution of H2 to total methanogenesis. However, H2‐dependent methanogenesis was significantly stimulated by excess H2 and slightly inhibited by acetate at low as well as high temperature. The results show that H2‐producing bacteria were limited by temperature to a greater extent than the methanogens so that the methanogenic microbial community in paddy soil was limited by the supply of H2. At low as well as high temperatures, excess H2 apparently enabled part of the methanogenic community to shift from acetate‐dependent to H2‐dependent CH4 production. At low temperature, excess H2 had only this effect, but with increasing temperature, excess H2 additionally stimulated total methanogenic activity and eventually even growth.

The effect of temperature on CH4 production, turnover of dissolved H2, and enrichment of H2‐utilizing anaerobic bacteria was studied in anoxic paddy soil and sediment of Lake Constance. When anoxic paddy soil was incubated under an atmosphere of H2/CO2, rates of CH4 production increased 25°C, but decreased at temperatures lower than 20°C. Chloroform completely inhibited methano‐genesis in anoxic paddy soil and lake sediment, but did not or only partially inhibit the turnover of dissolved H2, especially at low incubation temperatures. Cultures with H2 as energy source resulted in the enrichment of chemolithotrophic homoacetogenic bacteria whenever incubation temperatures were lower than 20°C. Hydrogenotrophic methanogens could only be enriched at 30°C from anoxic paddy soil. A homoacetogen