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.

SummaryMethane emission rates from rice-vegetated paddy fields followed a seasonal pattern different to that of weed-covered or unvegetated fields. Presence of rice plants stimulated the emission of CH 4 both in the laboratory and in the field. In unvegetated paddy fields CH 4 was emitted almost exclusively by ebullition. By contrast, in rice-vegetated fields more than 90% of the CH, emission was due to plant-mediated transport. Rice plants stimulated methanogenesis in the submerged soil, but also enhanced the CH 4 oxidation rates within the rhizosphere so that only 23% of the produced CH4 was emitted. Gas bubbles in vegetated paddy soils contained lower CH4 mixing ratios than in unvegetated fields. Weed plants were also efficient in mediating gas exchnage between submerged soil and atmosphere, but did not stimulate methanogenesis. Weed plants caused a relatively high redox potential in the submerged soil so that 95% of the produced CH4 was oxidized and did not reach the atmosphere. The emission of CH 4 was stimulated, however, when the cultures were incubated under gas atmospheres containing acetylene or consisting of O 5 -free nitrogen.

Methane release rates from rice paddies have been measured in Andalusia, Spain, during almost a complete vegetation period in 1982 using the static box system. The release rates ranged between 2 and 14 mg/m2/h and exhibited a strong seasonal variation with low values during the tillering stage and shortly before harvest, while maximum values were observed at the end of the flowering stage. The CH4 release rate, averaged over the complete vegetation period, accounted for 4 mg/m2/h which results in a worldwide CH 4 emission from rice paddies of 35-59 x 10 t2 g/yr if we assume that the observed CH 4 release rates are representative of global conditions. The CH, release rates showed diurnal variations with higher values late in the afternoon which were most likely caused by temperature variations within the upper layers of the paddy soils. Approximately 95% of the CH 4 emitted into the atmosphere by rice paddies was due to transport through the rice plants. Transport by bubbles or diffusion through the paddy water was of minor importance. Incubation experiments showed that CH, was neither produced nor consumed in the paddy water. The relase of CH, from rice paddies caused a diurnal variation of CH 4 in ambient air within the rice-growing area with maximum values of up to 2.3 ppmv during the early morning, compared to average daytime values of 1.75 ppmv.

CH4 release rates from rice paddies were measured in Vercelli, Italy, in 1983 during a complete vegetation period, using a static box system. The rice paddies were found to be a source of atmospheric methane during the time of flooding. The CH4 release rates range between a few milligrams of CH4 per square meter per hour and 51 mg CH4 m−2 h−1 and show a seasonal variation with maximum emission rates between tillering and flowering. Averaged over the complete vegetation period, the CH4 release rate accounts for 16 mg m−2 h−1. Significant differences in the CH4 release rates of unfertilized field plots and plots fertilized with mineral fertilizer (CaCN2) were not observed. The CH4 release rates show strong diurnal variations, with highest values in the late afternoon and lowest values in the early morning, which coincides with the temperature variation in the upper soil layer (1–10 cm). These variations must be taken into account in estimating reliable global CH4 emission rates from rice paddies. CH4 is almost exclusively emitted into the atmosphere by gas bubbles during the first 6 weeks after flooding the rice paddies, that is, on fields without vegetation. Afterwards, 80% of the observed CH4 transport from the paddy soil into the atmosphere was due to diffusion through the stems of the rice plants. Transport by diffusion through the paddy water appeared to be of minor importance. The global annual CH4 emission from rice paddies is estimated to be of the order of 39–94 Tg yr−1 (Tg = 1012 g) for 1940 conditions and 70–170 Tg yr−1 for 1979 conditions, indicating a secular trend of the CH4 emission by rice paddies of about 1.6% per year during the last 35 years.

Production and emission of methane from submerged paddy soil was studied in laboratory rice cultures and in Italian paddy fields. Up to 80% of the CH4 produced in the paddy soil did not reach the atmosphere but was apparently oxidized in the rhizosphere. CH4 emission through the rice plants was inhibited by an atmosphere of pure O2 but was stimulated by an atmosphere of pure N2 or an atmosphere containing 5% acetylene. Gas bubbles taken from the submerged soil contained up to 60% CH4, but only < 1% CH4 after the bubbles had passed the soil‐water interface or had entered the intercellular gas space system of the rice plants. CH4 oxidation activities were detected in the oxic surface layer of the submerged paddy soil. Flooding the paddy soil with water containing > 0.15% sea salt (0.01% sulfate) resulted in a strong inhibition of the rates of methanogenesis and a decrease in the rates of CH4 emission. This result explains the observation of relatively low CH4 emission rates in rice paddy areas flooded with brackish water.