Methane emission from irrigated rice cultivation: Quantities, models and practice

Three experiments focused on the contribution of rice productivity to methane emission were conducted in Texas flooded rice paddy soils during 1994-95 growing seasons. Measurements of methane emission from different rice paddy soils during 1991-92 growing seasons (Sass et al., 1994) and from ten different cultivars in 1993 growing season (Willis, 1995; Sass and Fisher, 1995) were cited to quantify the relationships of methane emission with soil, rice cultivar and grain yield. Under the similar soil sand content and agronomic management regime, total seasonal methane emission was positively correlated with rice grain yield and aboveground biomass at harvest. Linear relationships of daily methane emission with aboveground vegetative biomass and root biomass were also observed. On a carbon to carbon basis, the ratio of methane emission to rice net primary productivity was dependent on soil and rice variety, and increased with rice plant development. Models emphasized the contributions of rice plants to the processes of methane production, oxidation and emission and also the influence of environmental factors were developed to predict methane emission from flooded rice fields. Relative effects of soil texture, soil temperature and rice variety on methane production/emission were quantified by three dimensionless indices: soil index, temperature index and variety index, respectively. Model validation against observations from various regions of the world, including Italy, China, Indonesia, Philippines and USA demonstrated that methane emission can be predicted from rice growth and development, cultivar character, soil texture and temperature, and organic matter amendments. Of these, rice growth and development is a principal parameter governing the processes of methane production, oxidation and emission in irrigated rice paddies. Model estimates suggest that annual amount of methane emitted from Chinese rice fields ranges from 7.03 to 13.32 Tg CH$\sb4$ yr$\sp{-1}$ with an average value of 9.45 Tg CH$\sb4$ yr$\sp{-1}$ under permanent irrigation and the majority of methane was emitted in the region located at latitude between 25$\sp\circ$ and 32$\sp\circ$ N. Comparisons of estimated with the observed emissions show that the estimates were in general close to the measurements at most locations