Using a crop/soil simulation model and GIS techniques to assess methane emissions from rice fields in Asia. II. Model validation and sensitivity analysis
Rb. Matthews et al., Using a crop/soil simulation model and GIS techniques to assess methane emissions from rice fields in Asia. II. Model validation and sensitivity analysis, NUTR CYCL A, 58(1-3), 2000, pp. 161-177
The MERES (Methane Emissions from Rice EcoSystems) simulation model was tes
ted using experimental data from IRRI and Maligaya in the Philippines and f
rom Hangzhou in China. There was good agreement between simulated and obser
ved values of total aboveground biomass, root weight, grain yield, and seas
onal methane (CH4) emissions. The importance of the contribution of the ric
e crop to CH4 emissions was highlighted. Rhizodeposition (root exudation an
d root death) was predicted to contribute about 380 kg C ha(-1) of methanog
enic 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, g
iving a total of around 60% originating from the rice crop, with the remain
ing 41% coming from the humic fraction of the soil organic matter (SOM). Se
nsitivity analysis suggested that the parameter representing transmissivity
to gaseous transfer per unit root length (lambda (r)) was important in det
ermining seasonal CH4 emissions. As this transmissivity increased, more O-2
was able to diffuse to the rhizosphere, so that CH4 production by methanog
ens was reduced and more CH4 was oxidized by methanotrophs. These effects o
utweighed the opposing influence of increased rate of transport of CH4 thro
ugh the plant, so that the overall effect was to reduce the amount of CH4 e
mitted over the season. Varying the root-shoot ratio of the crop was predic
ted to have little effect on seasonal emissions, the increased rates of rhi
zodeposition being counteracted by the increased rates of O-2 diffusion to
the rhizosphere. Increasing the length of a midseason drainage period reduc
ed CH4 emissions significantly, but periods longer than 6-7 d also decrease
d rice yields. Organic amendments with low C/N were predicted to be more be
neficial, both in terms of enhancing crop yields and reducing CH4 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.