Since rotation of paddy rice-upland crop cultivations is a major land use management widely conducted in Japan, effect of change in land use on greenhouse gas emissions from drained paddy fields are required to be clarified. In this study, methane (CH₄) and nitrous oxide (N₂O) emissions in both paddy fields (PR) and drained fields under upland crop cultivations (UR: single cropping of upland rice, SW: double cropping of soybean/wheat) were measured over two and a half years in the experimental fields with an automated flux monitoring system.

CH₄ emission in the PR plots in 2003 was 5.8 times higher than that in 2002, mainly because of a prolonged period of continuous submergence in 2003. N₂O emissions in the PR plots were low during the paddy rice cultivation periods.

In the UR and SW plots, CH₄ emissions were stopped. On the other hand, significantly higher N₂O emissions were often observed during the flowering to ripening stages of the cultivated upland crops, and other temporal increases in N₂O fluxes were also occasionally found after heavy rainfall or crop harvesting. Cumulative N₂O emissions in the UR and SW plots were three- to fivefold higher than that in the PR plots.

The combined net global warming potentials (net GWP) by the CH₄ and N₂O emissions were 100 to 442, 102 to 110, and 79 to 146 g CO₂ m^–2 in the PR, UR, and SW plots, respectively. Net GWP in the PR plots in 2003 was extremely high compared with the other values, owing to the high levels of CH₄ emission.

These results suggest that drainage of paddy soils for upland crop cultivation is effective for reducing net GWP in paddy fields with high potential for CH₄ emission, such as those that are poorly drained or have low soil iron content.