[1] Validations of the DeNitrification-DeComposition (DNDC) model against field data sets of trace gases (CH 4 , N 2 O, and NO) emitted from cropping systems in Japan, China, and Thailand were conducted. The model-simulated results were in agreement with seasonal N 2 O emissions from a lowland soil in Japan from 1995 to 2000 and seasonal CH 4 emissions from rice fields in China, but failed to simulate N 2 O and NO emissions from an Andisol in Japan as well as NO emissions from the lowland soil. Seasonal CH 4 emissions from rice cropping systems in Thailand were poorly simulated because of site-specific soil conditions and rice variety. For all of the simulated cases, the model satisfactorily simulated annual variations of greenhouse gas emissions from cropping systems and effects of land management. However, discrepancies existed between the modeled and observed seasonal patterns of CH 4 and N 2 O emissions. By incorporating modifications based on the local soil properties and management, DNDC model could become a powerful tool for estimating greenhouse gas emissions from terrestrial ecosystems.

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[1] The seasonal courses of methane (CH 4 ) and nitrous oxide (N 2 O) fluxes were simultaneously monitored in a paddy field using a closed chamber system with automated gas sampling and analyzing equipment. Water management and fertilizer application practices followed Japanese conventional ones. CH 4 flux gradually increased after the first flood irrigation of the field and reached $150 mg CH 4 m À2 d À1 at the beginning of July. After the first summer drainage, however, CH 4 flux dropped rapidly to almost zero within a few days. CH 4 flux then gradually increased again according to intermittent flood irrigations, but was much less than that before the first drainage. Immediately after the first flood irrigation, N 2 O flux rapidly increased, although its temporal peak lasted only within a few days. During the subsequent continuously and intermittently flooded periods, N 2 O flux remained at almost zero until the final drainage, except for slight and temporal peaks just after the top-dress application of supplemental fertilizer. About 1 week after the final drainage in autumn, N 2 O flux gradually increased, and the most significant high peak of N 2 O flux was observed after the harvest of rice plants, which lasted for about 2 weeks. The amounts of cumulative CH 4 and N 2 O emissions throughout the whole year of 2002 were 3128 mg CH 4 m À2 and 60.2 mg N m À2 , respectively. Both the amounts of cumulative CH 4 and N 2 O emissions during the rice cultivation period were low compared with those reported in previous studies. These results suggest the advantage of Japanese conventional water management and fertilizer application for reducing the combined effect for global warming by CH 4 and N 2 O emissions from paddy fields, since the practices of drainage and intermittent flood irrigation in summer markedly lessen CH 4 emission in the latter half of the rice cultivation period with little enhancement of N 2 O emission.

Nitrous oxide (N2O) emission and methane (CH4) uptake were measured in an experimental long‐term tillage field (Andosol) in Hokkaido, northern Japan, to assess their contributions to net global warming, associated with arable crop production. From May 2001 to August 2002, the field was cultivated with winter wheat, adzuki bean, sugar beet, potato, and cabbage, where the total N applied was 110, 40, 150, 60, and 220 kg N ha−1 yr−1, respectively. Under conventional tillage (CT) cropping systems, basal N fertilization and plowing for residue incorporation had little effect on N2O fluxes, but vigorous N2O emission was observed when rotary harrowing was used for incorporating N‐rich cabbage residues into soil in summer. Also, high N2O emissions occurred when there was heavy rainfall after a large amount of N fertilizer had been applied to sugar beet and also when there was thawing of frozen soil and snow in the winter wheat treatment. Despite the differing N2O flux patterns among the crops, the annual N2O emissions from each crop were positively correlated with the total N applied as fertilizer. Under CT systems, across all five crops, the mean N2O emission factor (the percent ratio of N2O‐N emitted out of total N applied as fertilizer) was 0.36%. Under reduced tillage (RT) cropping systems, where crop residues were left on the ground over winter, large quantities of N2O were emitted from adzuki bean and sugar beet residues when the frozen soil and snow thawed. Therefore, total N2O emissions from adzuki bean and sugar beet cultivated under RT systems were much greater than under CT systems. The rates of CH4 uptake by arable soils were less sensitive to crop type, field management practices, and fertilizer application rates, but the rates were strongly influenced by long‐term tillage management. For fallow, winter wheat, adzuki bean, and sugar beet treatments, the CH4 uptake rates in the CT soils (1.36 kg CH4 ha−1 yr−1), which had a 20‐year history of intensive plowing, were lower than those in the RT soils (2.40 kg CH4 ha−1 yr−1). Thus RT production systems improved CH4 uptake by arable soils, although they adversely affected N2O emissions for adzuki bean and sugar beet production.

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We measured nitrous oxide (N 2 O) and nitric oxide (NO) emissions during the snow-free season (AprilNovember) over a 3-year period (1998)(1999)(2000) from a Silandic Andosols cultivated with maize (Zea mays L.) in central Hokkaido, Japan. In May, before furrowing, composted cattle manure was broadcast onto the field at a rate of 3.0 g N m . An impermeable layer lay 1.3 m below ground level. As a result, after heavy rains and during the snow-melting period, the groundwater table rose to near the ground surface. The N 2 O and NO emission rates ranged from 0.0 to 6.4 and from 0.00 to 0.94 mg N m, respectively. The highest N 2 O emission was observed after heavy rain in summer and autumn. The magnitude and seasonal pattern of N 2 O emissions from the inter row were similar to those from the row itself, although chemical fertilizer had not been applied to the inter row. In contrast, an increase in NO emissions was observed only from the row. Seasonal fluctuations in soil and concentrations and the emission ratio N 2 O-N/NO-N suggested that N 2 O and NO emitted after fertilizer application (May to early July) were produced mainly by nitrification, whereas N 2 O emitted after heavy rains (after mid-July) was produced mainly by denitrification. Total N 2 O and NO emissions during the snow-free season ranged from 0.7 to 2.8 and from 0.0 to 0.7 g N m -2 , respectively, over a 3-year period. The N 2 O and NO emissions from our field were relatively high compared with those reported worldwide. In contrast, reported N 2 O emission rates from agricultural Andosols in Japan are typically lower than those from other agricultural soils in Japan and around the world. Therefore, the results of the present study suggest that high N 2 O emissions may occur from Japanese agricultural Andosols that are poorly drained.

To assess their impacts on net global warming, total greenhouse gas emissions (mainly CO2, N2O and CH4) from agricultural production in arable land cropping systems in the Tokachi region of Hokkaido, Japan, were estimated using life cycle inventory (LCI) analysis. The LCI data included CO2 emissions from on‐farm and off‐farm fossil fuel consumption, soil CO2 emissions induced by the decomposition of soil organic matter, direct and indirect N2O emissions from arable lands and CH4 uptake by soils, which were then aggregated in CO2‐equivalents. Under plow‐based conventional tillage (CT) cropping systems for winter wheat, sugar beet, adzuki bean, potato and cabbage, on‐farm CO2 emissions from fuel‐consuming operations such as tractor‐based field operations, truck transportation and mechanical grain drying ranged from 0.424 Mg CO2 ha−1 year−1 for adzuki bean to 0.826 Mg CO2 ha−1 year−1 for winter wheat. Off‐farm CO2 emissions resulting from the use of agricultural materials such as chemical fertilizers, biocides (pesticides and herbicides) and agricultural machines were estimated by input–output tables to range from 0.800 Mg CO2 ha−1 year−1 for winter wheat to 1.724 Mg CO2 ha−1 year−1 for sugar beet. Direct N2O emissions previously measured in an Andosol field of this region showed a positive correlation with N fertilizer application rates. These emissions, expressed in CO2‐equivalents, ranged from 0.041 Mg CO2 ha−1 year−1 for potato to 0.382 Mg CO2 ha−1 year−1 for cabbage. Indirect N2O emissions resulting from N leaching and surface runoff were estimated to range from 0.069 Mg CO2 ha−1 year−1 for adzuki bean to 0.381 Mg CO2 ha−1 year−1 for cabbage. The rates of CH4 removal from the atmosphere by soil uptake were equivalent to only 0.020–0.042 Mg CO2 ha−1 year−1. From the difference in the total soil C pools (0–20 cm depth) between 1981 and 2001, annual CO2 emissions from the CT and reduced tillage (RT) soils were estimated to be 4.91 and 3.81 Mg CO2 ha−1 year−1, respectively. In total, CO2‐equivalent greenhouse gas emissions under CT cropping systems in the Tokachi region of Hokkaido amounted to 6.97, 7.62, 6.44, 6.64 and 7.49 Mg CO2 ha−1 year−1 for winter wheat, sugar beet, adzuki bean, potato and cabbage production, respectively. Overall, soil‐derived CO2 emissions accounted for a large proportion (64–76%) of the total greenhouse gas emissions. This illustrates that soil management practices that enhance C sequestration in soil may be an effective means to mitigate large greenhouse gas emissions from arable land cropping systems such as those in the Tokachi region of northern Japan. Under RT cropping systems, plowing after harvesting was omitted, and total greenhouse gas emissions from winter wheat, sugar beet and adzuki bean could be reduced by 18%, 4% and 18%, respectively, mainly as a result of a lower soil organic matter decomposition rate in the RT soil and a saving on the fuels used for plowing.