Revising a process‐based biogeochemistry model (DNDC) to simulate methane emission from rice paddy fields under various residue management and fertilizer regimes

Abstract

A comprehensive biogeochemistry model, DNDC, was revised to simulate crop growth and soil processes more explicitly and improve its ability to estimate methane (CH 4) emission from rice paddy fields under a wide range of climatic and agronomic conditions. The revised model simulates rice growth by tracking photosynthesis, respiration, C allocation, tillering, and release of organic C and O 2 from roots. For anaerobic soil processes, it quantifies the production of electron donors [H 2 and dissolved organic carbon (DOC)] by decomposition and rice root exudation, and simulates CH 4 production and other reductive reactions based on the availability of electron donors and acceptors (NO 3−, Mn 4+, Fe 3+, and SO 42−). Methane emission through rice is simulated by a diffusion routine based on the conductance of tillers and the CH 4 concentration in soil water. The revised DNDC was tested against observations at three rice paddy sites in Japan and China with varying rice residue management and fertilization, and produced estimates consistent with observations for the variation in CH 4 emission as a function of residue management. It also successfully predicted the negative effect of (NH 4)2 SO 4 on CH 4 emission, which the current model missed. Predicted CH 4 emission was highly sensitive to the content of reducible soil Fe 3+, which is the dominant electron acceptor in anaerobic soils. The revised DNDC generally gave acceptable predictions of seasonal CH 4 emission, but not of daily CH 4 fluxes, suggesting the model's immaturity in describing soil heterogeneity or rice cultivar‐specific characteristics of CH 4 transport. It also overestimated CH 4 emission at one site in a year with low temperatures, suggesting uncertainty in root biomass estimates due to the model's failure to consider the temperature dependence of leaf area development. Nevertheless, the revised DNDC explicitly reflects the effects of soil electron donors and acceptors, and can be used to quantitatively estimate CH 4 emissions from rice fields under a range of conditions.