|
Tamon Fumoto, National Institute for Agro-Environmental Sciences (Japan)
Tetsuji Yanagihara, Hokkaido Kamikawa Agricultural Experiment Station (Japan)
Takashi Saito, Fukushima Agricultural Technology Centre (Japan)
Kazuyuki Yagi, National Institute for Agro-Environmental Sciences (Japan)
|
|
Agriculture is considered to be one of major anthropogenic sources of greenhouse gases (GHG). Particularly, rice production is a main source of CH4 which contributes to global warming second only to CO2. Since CH4 is produced by anaerobic soil microorganisms in flooded rice field, deliberate draining of the field is expected to be a practical measure for reducing CH4 emission as well as maintaining rice yields. In this study, a biogeochemical process model (DNDC-Rice) was developed, tested and applied to assess GHG mitigation potentials of alternative water regimes of rice fields. DNDC-Rice simulates biogeochemical processes of soil-rice system based on inputs of soil properties, daily weather and farming practice. It simulates rice growth by tracking photosynthesis, respiration, carbon allocation, as well as release of organic carbon from roots. For anaerobic soil processes, DNDC-Rice calculates production of electron donors (H2 and dissolved organic carbon) by decomposition of organic matters and root exudation. Rates of CH4 production and other reductive reactions are calculated based on availability of electron donors and acceptors (N, Mn, Fe and S). When the field is drained, DNDC-Rice simulates O2 diffusion into soil and oxidation of CH4 and other reduced species. Using observed data from different sites of rice fields, DNDC-Rice was calibrated and validated with respect to CH4 and CO2 emissions under various conditions of soil, climate and farming practice. For assessment of GHG mitigation potentials, we ran DNDC-Rice with a GIS database of 1 km1 km mesh scale, containing rice field area, soil properties, daily weather and farming practice of each mesh. Currently, the database covered 2.6 % of total rice field area in Hokkaido region of Japan. Methane and CO2 emissions were estimated assuming alternative water regimes in rice growing season, i.e., double drainage for 1 week (1W-1W), a single drainage for 2 weeks (2W), and double drainage for 2 weeks and 1 week (2W-1W), as well as conventional water regime (CWR). Resulting estimates are summarized as follows: (1) Mitigation of CH4 emission was achieved better with longer continuous drained period: reduction in average CH4 emission, as compared to that with CWR, was 10 % with 1W-1W, but rose to 27 and 30 % with 2W and 2W-1W, respectively. (2) Alternative water regimes enhanced average CO2 emission by 0.1-0.2 Mg ha-1 y-1, but increase in CO2 emission was well cancelled by reduction in CH4 emission (0.4-1.2 Mg CO2 ha-1 y-1). Thus, alternative water regimes achieved mitigation of GHG emission by up to 1.0 Mg CO2 ha-1 y-1. If this estimate is simply expanded to total rice field in Japan (1.7 M ha), it becomes 1.7 Tg CO2 y-1, accounting for 0.12 % of total GHG emission from Japan. However, as effects of water regimes are likely to depend on soil properties and climate of rice fields, construction of regional databases on those factors is required for reliable national-scale assessments.
|
