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Methane emissions at different rice productivity levels were observed from Texas rice paddy soils during the years 1991--95. Analysis of field measurements showed that seasonal methane emission (E) was strongly dependent on soil, cultivar, and rice grain yield. The relationship can be quantitatively described by E (g m-2) = 0.048 xSI x VI x GY. SI is a soil index to characterize the relative effectof soil texture on emission and is linked with soil sand percentage.VI is a variety index to identify the intervarietal difference in methane emission and is related to the amount of methane emission per unit grain yield. GY is grain yield (g m-2). Constant 0.048was derived from the measurements of 10 cultivars planted in 1993. Computed emission applying the relationship is well matched with measured data. The comparison of computed with measured seasonal methane emission over an 80-day period using a total of 32 data sets yields a correlation coefficient r2 of 0.800. In addition, the ratio of seasonal methane emission to net primary productivity was calculated on a carbon to carbon basis, which produces an average value of 2.8%, ranging from 1.2% to 5.4%. A further investigation indicated that the ratio is soil and variety dependent and can be quantitatively explained by C[CH4]/C[NPP] (%) = 3.21 x SI x VI + 0.12 (r2 = 0.738, n = 32). Under the condition of 30% soil sand, this ratio is (approx.) 3% for the majority of cultivars. [ABSTRACT FROM AUTHOR]

Measurements focused on seasonal contribution of rice productivity to methane emission were made in three experiments conducted in Texas flooded paddy soils during 1994 and 1995 growing seasons. A total of five rice cultivars representing two distinct groups in methane emission were involved. Over a 10-week period after permanent flooding, total seasonal methane emission was positively correlated with rice above-ground biomass (r2 = 0.845, n = 11). A very strong dependence of daily methane emission on above-ground vegetative biomass (r2 = 0.887, n = 93) and on root biomass (r2 = 0.816, n = 33) was also observed. Calculation from three developmentalperiods (vegetative, reproductive and ripening) of rice plant indicated that more than 75% of total seasonal methane was emitted during the last 5-week period in concert with reproductive and ripening stages, while rice biomass production during the same period-amounted to (approx.) 50% of the seasonal total. According to the correlation of cumulative methane emission with above-ground biomass increment between every two-week interval (r2 = 0.490, n = 93, P = 0.000),the carbon released as methane is approximately equivalent to 3% and4.5% of photosynthetically fixed carbon in the biomass for low and high emission cultivars, respectively. A further investigation showed that these fractions are related to plant growth and development. Thecarbon ratio of methane emitted to net photosynthetic production during vegetative, reproductive, and ripening periods averaged 0.9%, 3.6% and 7.9%, respectively, for low emission cultivars, and 2.0%, 5.0% and 8.3%, respectively, for high emission cultivars. Moreover, the ratio was strongly dependent on plant biomass, resulting in r2 values from 0.775 to 0.907. [ABSTRACT FROM AUTHOR]