Using a crop/soil simulation model and GIS techniques to assess methane emissions from rice fields in Asia. I. Model development

The development of the MERES (Methane Emissions in Rice EcoSystems) model f or simulating methane (CH4) emissions from rice fields is described. The CE RES-Rice crop simulation model was used as a basis, employing the existing routines simulating soil organic matter (SOM) decomposition to predict the amount of subsrate available for methanogenesis. This was linked to an exis ting submodel, described elsewhere in this volume (Arah & Kirk, 2000), whic h calculates steady-state fluxes and concentrations of CH4 and O-2 in flood ed soils. Extra routines were also incorporated to simulate the influence o f the combined pool of alternative electron acceptors in the soil (i.e., NO 3-, Mn4+, Fe3+, SO42-) on CH4 production. The rate of substrate supply is c alculated in the SOM routines of the CERES-Rice model from (a) the rate of decomposition of soil organic material including that left from the previou s crop and any additions of organic matter, (b) root exudates (modified fro m the original CERES-Rice model using recent laboratory data), and (c) the decomposition of dead roots from the current crop. A fraction of this rate of substrate supply, determined by the concentration of the oxidized form o f the alternative electron acceptor pool, is converted to CO2 by bacteria w hich outcompete the methanogenic bacteria, thereby suppressing CH4 producti on. Any remaining fraction of the substrate supply rate is assumed to be po tentially available for methanogenesis. The CH4 dynamics submodel uses this potetial methanogenesis rate, along with a description of the root length distribution in the soil profile supplied by the crop model, to calculate t he steady-state concentrations and fluxes of O-2 and CH4. The reduced form of the alternative electron acceptor pool is allowed to reoxidize when soil pores fill with air if the field is drained. The MERES model was able to e xplain well the seasonal patterns of CH4 emissions in an experiment involvi ng mid- and end-season drainage and additions of organic material at IRRI i n the Philippines.