Methane emission from Texas rice paddy soils. 2. Seasonal contribution of rice biomass production to CH4 emission

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 ( r 2 = 0.845, n = 11). A very strong dependence of daily methane emission on above‐ground vegetative biomass ( r 2 = 0.887, n = 93) and on root biomass ( r 2 = 0.816, n = 33) was also observed. Calculation from three developmental periods (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 ≈ 50% of the seasonal total. According to the correlation of cumulative methane emission with above‐ground biomass increment between every two‐week interval ( r 2 = 0.490, n = 93, P = 0.000), the carbon released as methane is approximately equivalent to 3% and 4.5% 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. The carbon 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 r 2 values from 0.775 to 0.907.