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 hoo ded paddy soils during 1994 and 1995 growing seasons. A total of five rice cultivars representing two distinct groups in methane emission we re involved. Over a 10-week period after permanent flooding, total sea sonal methane emission was positively correlated with rice above-groun d biomass (r(2) = 0.845, n = 11). A very strong dependence of daily me thane emission on above-ground vegetative biomass (r(2) = 0.887, n = 9 3) and on root biomass (r(2) = 0.816, n = 33) was also observed. Calcu lation 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 rep roductive and ripening stages, while rice biomass production during th e same period amounted to approximate to 50% Of the seasonal total. Ac cording to the correlation of cumulative methane emission with above-g round biomass increment between every two-week interval (r(2) = 0.490, n 93, P = 0.000), the carbon released as methane is approximately equ ivalent to 3% and 4.5% of photosynthetically fixed carbon in the bioma ss for low and high emission cultivars, respectively. A further invest igation showed that these fractions are related to plant growth and de velopment. The carbon ratio of methane emitted to net photosynthetic p roduction during vegetative, reproductive, and ripening periods averag ed 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. Moreov er, the ratio was strongly dependent on plant biomass, resulting in r( 2) values from 0.775 to 0.907.