161 Using a crop/soil simulation model and GIS techniques to assess methane emissions from rice fields in Asia. II. Model validation and sensitivity analysis R.B. Matthews 1 , R. Wassmann 2,3 , L.V. Buendia 2 & J.W. Knox 1 1 Institute of Water and Environment, Cranfield University, Silsoe, Bedfordshire MK45 4DT, United Kingdom; 2 International Rice Research Institute, MCPO Box 3127, Makati City 1271, Philippines; 3 Fraunhofer Institute for Atmospheric Environmental Research, Garmisch-Partenkirchen, Germany Key words: methane, rice, Oryza sativa, anaerobic, model, simulation, carbon dynamics Abstract The MERES (Methane Emissions from Rice EcoSystems) simulation model was tested using experimental data from IRRI and Maligaya in the Philippines and from Hangzhou in China. There was good agreement between simulated and observed values of total aboveground biomass, root weight, grain yield, and seasonal methane (CH 4 ) emissions. The importance of the contribution of the rice crop to CH 4 emissions was highlighted. Rhizodeposition (root exudation and root death) was predicted to contribute about 380 kg C ha -1 of methanogenic substrate over the season, representing 37% of the total methanogenic substrate from all sources when no organic amendments were added. A further 225 kg C ha -1 (22%) was predicted to come from previous crop residues, giv- ing a total of around 60% originating from the rice crop, with the remaining 41% coming from the humic fraction of the soil organic matter (SOM). Sensitivity analysis suggested that the parameter representing transmissivity to gaseous transfer per unit root length (λ r ) was important in determining seasonal CH 4 emissions. As this transmissivity increased, more O 2 was able to diffuse to the rhizosphere, so that CH 4 production by methanogens was reduced and more CH 4 was oxidized by methanotrophs. These effects outweighed the opposing influence of increased rate of transport of CH 4 through the plant, so that the overall effect was to reduce the amount of CH 4 emitted over the season. Varying the root-shoot ratio of the crop was predicted to have little effect on seasonal emissions, the increased rates of rhizodeposition being counteracted by the increased rates of O 2 diffusion to the rhizosphere. Increasing the length of a midseason drainage period reduced CH 4 emissions significantly, but periods longer than 6-7 d also decreased rice yields. Organic amendments with low C/N were predicted to be more beneficial, both in terms of enhancing crop yields and reducing CH 4 emissions, even when the same amount of C was applied. This was due to higher rates of immobilization of C into microbial biomass, removing it temporarily as a methanogenic substrate. Introduction Methane (CH 4 ) is one of the principal greenhouse gases and has been estimated to account for 15-20% of cur- rent radiative forcing. Rice soils, characterized by O 2 depletion, high moisture, and relatively high organic substrate levels, offer an ideal environment for the ac- tivity of methanogenic bacteria and are one of the ma- jor anthropogenic CH 4 sources. Precise estimates of source size have been difficult because of the large spa- tial and temporal variability in CH 4 emission rates meas- ured at different sites due to differences in climate, soils, rice cultivars used, and crop management practices. Representation and integration of these factors within a geographical information system, coupled with the development of mechanistic models describing the processes involved in CH 4 production and emission, is a logical way forward. Nutrient Cycling in Agroecosystems 58: 161–177, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.